Next Article in Journal
Synthesis and Optimization of Cs2B′B″X6 Double Perovskite for Efficient and Sustainable Solar Cells
Next Article in Special Issue
Fluconazole Analogs and Derivatives: An Overview of Synthesis, Chemical Transformations, and Biological Activity
Previous Article in Journal
Rafting on the Evidence for Lipid Raft-like Domains as Hubs Triggering Environmental Toxicants’ Cellular Effects
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues

by
Arminas Urbonavičius
1,2,
Sonata Krikštolaitytė
1,
Aurimas Bieliauskas
2,
Vytas Martynaitis
1,
Joana Solovjova
1,
Asta Žukauskaitė
1,3,
Eglė Arbačiauskienė
1,* and
Algirdas Šačkus
2,*
1
Department of Organic Chemistry, Kaunas University of Technology, Radvilėnų pl. 19, LT-50254 Kaunas, Lithuania
2
Institute of Synthetic Chemistry, Kaunas University of Technology, K. Baršausko g. 59, LT-51423 Kaunas, Lithuania
3
Department of Chemical Biology, Palacký University, Šlechtitelů 27, CZ-78371 Olomouc, Czech Republic
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(18), 6599; https://doi.org/10.3390/molecules28186599
Submission received: 12 July 2023 / Revised: 1 September 2023 / Accepted: 9 September 2023 / Published: 13 September 2023

Abstract

:
In this paper, an efficient synthetic route from pyrazole-chalcones to novel 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones as 3-hydroxyflavone analogues is described. The methylation of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with methyl iodide in the presence of a base yielded a compound containing a 5-methoxy group, while the analogous reaction of 5-hydroxy-2-phenyl-6-(pyridin-4-yl)pyrano[2,3-c]pyrazol-4(2H)-one led to the zwitterionic 6-(N-methylpyridinium)pyrano[2,3-c]pyrazol derivative. The treatment of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with triflic anhydride afforded a 5-trifloylsubstituted compound, which was further used in carbon–carbon bond forming Pd-catalyzed coupling reactions to yield 5-(hetero)aryl- and 5-carbo-functionalized pyrano[2,3-c]pyrazoles. The excited-state intramolecular proton transfer (ESIPT) reaction of 5-hydroxypyrano[2,3-c]pyrazoles from the 5-hydroxy moiety to the carbonyl group in polar protic, polar aprotic, and nonpolar solvents was observed, resulting in well-resolved two-band fluorescence. The structures of the novel heterocyclic compounds were confirmed by 1H-, 13C-, 15N-, and 19F-NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.

Graphical Abstract

1. Introduction

Fused pyrazole derivatives represent an important class of organic compounds as they are found in a large number of biologically and chemically active compounds [1]. These compounds are known for their anticancer [2], antimicrobial [3], antiviral [4], and anti-coagulant properties [5], and for their activity against CNS disorders [6]. Some of the fused pyrazole moieties are present in marketed drugs, such as apixaban, sildenafil, indiplon, zaleplon, etazolate, cartazolate, allopurinol, and futibatinib, which was recently approved by the U.S. Food and Drug Administration (FDA) [7].
Among other fused systems, pyrano[2,3-c]pyrazoles have been investigated for analgesic and anti-inflammatory [8], antimicrobial [9,10], and anticancer [11] activities. Recently, Sun et al. described the nano-formulation and anticancer activity of a 6-amino-4-(2-hydroxyphenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile via the blocking of the cell cycle through a p53-independent pathway [12], while Nguyen et al. reported a four-component sulfonated amorphous carbon and eosin Y-catalyzed synthesis and the molecular docking of 6-amino-1,4- or 2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles as inhibitors of p38 MAP kinase [13]. In our previous studies, we reported the synthesis, characterization, and biological evaluation of several pyrano[2,3-c]pyrazole derivatives [14,15]. However, 5-hydroxy-2,6-diarylpyrano[2,3-c]pyrazol-4(2H)-ones, which can serve as potential analogues of 3-hydroxyflavone, are still understudied.
3-Hydroxyflavone I (Figure 1) is known as the backbone of all flavonols. Flavonols are a class of the flavonoid family, a group of naturally occurring substances with variable phenolic structures, found in fruits, vegetables, grains, bark, roots, stems, flowers, tea, and wine [16,17,18]. Quercetin II and kaempferol III (Figure 1) are the most prevalent in plants and are among the flavonols that have been most investigated and reviewed for beneficial health properties, such as antioxidant, antimicrobial, hepatoprotective, and anti-inflammatory properties, and other effects [19,20]. Synthetic and semisynthetic flavonol derivatives have been reported in the literature in an attempt to improve the biochemical and pharmacological properties of their corresponding natural compounds. For example, synthesis and anti-Leishmania activity were reported for benzothiophene-flavonols [21]. A series of spirochromone-flavonols [22] and thiophene-pyrazole-flavonols [23] was synthesized and tested as antimicrobial agents. In addition, flavonols containing an isothiazolidine ring have been found to be effective inhibitors of cyclin-dependent kinase 2 (CDK2) [24].
3-Hydroxyflavones are known as fluorescent dyes because of their typical excited-state intramolecular photon transfer (ESIPT). ESIPT is one type of proton transfer reaction that has been the subject of considerable interest and a number of investigations in recent decades [25,26]. 3-Hydroxyflavones have been investigated as therapeutic imaging agents, including as fluorescence sensors and probes for the detection of the microenvironment, metal ions, and structures of proteins and DNA [27,28,29,30]. For example, Jiang et al. reported the application of 3-hydroxyflavone-based ESIPT fluorescent dyes for the dynamic imaging of lipid droplets with cells and tissues [31]. In a study by Kamariza et al., a 3-hydroxychromone derivative, 2-[7-(diethylamino)-9,9-dimethyl-9H-fluoren-2-yl]-3-hydroxy-4H-chromen-4-one, was conjugated to trehalose and a bright solvatochromic dye was obtained that detects Mycobacterium tuberculosis in a matter of minutes [32].
The O-methylation of 3-hydroxyflavones with reagents such as diazomethane, methyl iodide, dimethyl sulfate, or dimethyl carbonate proceeded to give O-methylated flavonoids, which exhibited a variety of biological activities [33,34,35]. For example, Ohtani et al. investigated the effect of 3-methoxyflavone derivatives, such as those of compound IV (Figure 1), on P-glycoprotein by measuring the potentiation of cellular accumulation and growth inhibition [36]. Juvale et al. reported the inhibitory activity of 3-methoxyflavones against a breast cancer resistance protein (BVRP/ABCG2) [37]. Furthermore, 3-hydroxyflavone treated with p-TsCl in the presence of a base afforded a corresponding flavone tosylate V, which was used in a Suzuki–Miyaura reaction for cross coupling with various phenyl boronic acids to give 2,3-diarylbenzopyrans [38,39]. Flavone-like 2,3-diarylbenzopyrans, such as compound VI (Figure 1), have been synthesized as novel selective inhibitors of cyclooxygenase-2 [40,41].
In the continuation of our research on the development of novel fused heterocyclic pyrazole-containing systems, we report here the synthesis, structural elucidation, and optical properties of novel 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one derivatives as analogues of 3-hydroxyflavones. The ESIPT reaction of 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones from the 5-hydroxy moiety to the carbonyl group in MeOH and polar aprotic and non-polar solvents was also investigated. The obtained 5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones were further functionalized by methylation as well as the Pd-catalyzed Suzuki, Heck, and Sonogashira coupling reactions of intermediate 5-triflate.

2. Results and Discussion

2.1. Chemistry

The synthesis of 6-(hetero)aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones 3ah was carried out as depicted in Scheme 1. 1-Phenyl-1H-pyrazol-3-ol 1 was obtained using a previously reported method [42,43] and subjected to a Claisen–Schmidt condensation reaction with variously 4′-substituted (hetero)aryl aldehydes in the presence of ethanolic sodium hydroxide, as we have described [44]. Heating the reaction mixture at 55 °C for 3 to 5 h afforded (E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-ones 2ah in poor to excellent yields (36–95%).
In a subsequent step, an Algar–Flynn–Oyamada (AFO) synthetic approach was applied for the formation of novel pyrano[2,3-c]pyrazol-4(2H)-ones 3ah. The AFO reaction is a stepwise process whereby chalcones undergo an oxidative cyclization to form flavones in the presence of alkaline hydrogen peroxide [45]. The AFO reaction outcome is dependent on the choice of the base; therefore, chalcone 2a was used as a model compound for the fine-tuning of the reaction conditions. Several organic and inorganic bases (NaOH, KOH, NaOAc, TEA, and NaHCO3) were screened in different mixtures of ethanol/water as a solvent and a divergent amount of hydrogen peroxide (Table S1). The best result was obtained when using NaOH in EtOH and employing 5 eq of H2O2. Stirring chalcones 2ah with hydrogen peroxide in an alkaline ethanolic solution at −25 °C for 2 h and at room temperature overnight afforded the flavonol analogues 3ah in poor to good yields (30–67%). The pyrano[2,3-c]pyrazol-4(2H)-ones 3e and 3g were obtained in lower yields (30–32%) when chalcones bearing naphtalen-2-yl or furan-3-yl substituents (2e and 2g, respectively) were used as starting materials in the AFO reaction. Unfortunately, the AFO reaction of (E)-3-(4-fluorophenyl)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-one or its 3-(4-nitrophenyl) counterpart gave only traces of targeted pyrano[2,3-c]pyrazol-4(2H)-ones.
According to the mechanistic studies reported in the literature [45,46,47], the formation of 6-(hetero)aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones 3 from (E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-ones 2, employing AFO reaction conditions could proceed according to two different pathways, as depicted in Figure 2, using the transformation of 2a to 3a as an example. According to the approach suggested by Shen et al., first epoxide A (Figure 2, path A) is formed; then it is subsequently cyclized to 5-hydroxy-2,6-diphenyl-5,6-dihydropyrano[2,3-c]pyrazol-4(2H)-one B and oxidized to target 5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one 3a [45]. Alternatively, as suggested by Ferreira et al., pyrazole-chalcone 2a might first undergo a cyclization forming a 2,6-diphenyl-2,6-dihydropyrano[2,3-c]pyrazol-4-olate C (Figure 2, path B), followed by an attack of hydrogen peroxide and subsequent oxidation to form 3a [47].
For further modification of the obtained flavanol analogue 3a, O-alkylation reaction conditions were applied. As a result, treating 3a with methyl iodide in the presence of cesium carbonate in dioxane at 40 °C gave O-methylated compound 4 in a 79% yield (Scheme 2).
Subsequently the methylation of compound 3h containing both the hydroxyl group and the pyridin-4-yl substituent was investigated (Scheme 3). With the alkylation reaction conditions described above (MeI, Cs2CO3, dioxane, 40 °C), a formation of zwitterionic pyrano[2,3-c]pyrazol derivative 5 as the main product was observed.
The proposed mechanism for the formation of compound 5 is shown in Scheme 3. Presumably, first, as a result of the reaction of pyridinyl-containing compound 3h with methyl iodide, methylpyridinium iodide 6 was formed. This was also demonstrated when alkylating compound 3h in the absence of a base as salt 6 was obtained in a 78% yield. The subsequent treatment of methylpyridinium iodide 6 with a base led to the formation of methylpyridinium hydroxide Y, which, upon the removal of the water molecule, led to the formation of the corresponding structure 5 as a resonance hybrid with the two contributing forms A and B, zwitterionic and neutral molecular structures, respectively. Pat et al. investigated the two-photon absorption (TPA) processes in a class of 4-quinopyran chromophores. The neutral molecular structure with a quinoid geometry is the molecular ground state, while the zwitterionic configuration with a benzenoid structure contributes significantly. The bond connecting the donor and acceptor phenylene fragments is a double bond when the molecule is neutral, while it is a single bond for the zwitterionic structure [48].
Further functionalization of pyrano[2,3-c]pyrazol-4(2H)-ones was accomplished via O-triflate intermediate 7, which was synthesized from 5-hydroxy-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (3a) following a standard procedure using Tf2O in the presence of TEA (Scheme 4). The obtained 4-oxo-2,4-dihydropyrano[2,3-c]pyrazol-5-yl trifluoromethanesulfonate 7 was subjected to Suzuki, Heck, and Sonogashira reactions to examine the employment of Pd-catalyzed coupling reactions for the functionalization of pyrano[2,3-c]pyrazol-4(2H)-ones. Triflate 7 underwent Suzuki-type cross coupling with (hetero)aryl boronic acids to give compounds 8ae in fair to excellent yields (44–95%). In the course of this coupling, standard conditions were applied, i.e., Pd(PPh3)4 was used as a catalyst and anhydrous K3PO4 as a base in dioxane at 90 °C. The reaction was carried out in the presence of KBr, which is known to suppress the decomposition of the palladium catalyst transition state by converting phosphonium salts to palladium bromide [49]. The Suzuki reaction yield was lower (44%) when 4-chlorophenylboronic acid was used for the cross coupling.
The Heck reaction of triflate 7 and tert-butyl acrylate under the standard conditions (Pd(PPh3)2Cl2, TEA, DMF, 100 °C) gave a poor yield (24%) of tert-butyl (E)-3-(4-oxo-2,4-dihydropyrano[2,3-c]pyrazol-5-yl)acrylate 8f, while the Sonogashira cross-coupling reaction of compound 7 with phenylacetylene under the usual conditions (Pd(PPh3)2Cl2, CuI, TEA, DMF, 65 °C) afforded alkyne 8g in a good yield (71%). A similar approach of flavonol functionalization employing Pd-catalyzed reactions of O-triflates was also reported by Kumar et al. in their study on the synthesis of 3,4-diarylpyrazoles and 4,5-diarylpyrimidines, starting with triarylbismuth as a three-fold arylating reagent and 3-trifloxychromones [50]. Dahlén et al. reported a synthetic strategy to form 2,3,6,8-tetrasubstituted chromone derivatives employing a Stille coupling reaction for the functionalization of the third position of the ring via intermediate 4-oxo-4H-chromen-3-yl trifluoromethanesulfonates [51]. Notably, it was observed that the latter compounds were not active under Heck reaction conditions. In addition, Akwari et al. demonstrated effective 3-arylation of flavones via a Suzuki cross-coupling reaction of 3-(trifluorosulphonyloxy)flavone [52].

2.2. NMR Spectroscopic Investigations

The formation of 6-(hetero)aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones 3ah and their derivatives 4, 5, 6, 7, and 8ag was confirmed through detailed analysis of their spectroscopic data. Key information for structure elucidation was obtained from NMR spectral data using a combination of standard and advanced NMR spectroscopy techniques, such as 1H-13C HMBC, 1H-13C LR-HSQMBC, 1H-15N HMBC, 1H-13C HSQC, 1H-13C H2BC, 1H-1H COSY, 1H-1H TOCSY, 1H-1H NOESY, and 1,1-ADEQUATE experiments. Since popular NMR prediction programs such as CSEARCH, ACD C+H predictor, as well as NMR chemical shift databases for structural dereplication depend on high-quality data with unambiguously assigned resonances [53], we carried out NMR studies with the obtained compounds to fully map all the 1H, 13C and 15N NMR signals as accurately as possible. The corresponding NMR data for the selected representatives of the aforementioned new ring systems are displayed in Figure 3 and Figure 4.
An initial comparison of the 1H NMR spectra between chalcone 2a and compound 3a, which was isolated as the sole product, clearly indicated the disappearance of characteristic olefinic protons (δ 7.63 and 7.75 ppm) from the prop-2-en-1-one moiety. Furthermore, the 13C NMR and DEPT, along with the 1H-13C HSQC spectroscopic data of 3a, revealed the presence of two new quaternary carbons (δ 139.16 and 144.4 ppm) in the absence of two olefinic methine carbons, clearly indicating a successful oxidative cyclization to flavanol. The structure of the pyrano[2,3-c]pyrazol-4(2H)-one ring system 3a bearing phenyl substituents at sites N-2 and C-6 was further elucidated via the connectivities based on the through-space correlations from the 1H-1H NOESY spectrum. In this case, distinct NOEs were exhibited between the pyrazole ring proton 3-H (singlet, δ 9.38 ppm) and the neighboring phenyl group 2′(6′)-H protons (δ 8.01–8.03 ppm), which confirms their proximity in space. The pyrazole 3-H proton was easily distinguished as it exhibited not only long-range HMBC correlations with neighboring N-2 “pyrrole-like” (δ −167.7 ppm) and N-1 “pyridine-like” (δ −117.0 ppm) nitrogen atoms, but also HMBC correlations with the quaternary carbons C-3a (δ 108.3 ppm) and C-7a (δ 161.2 ppm), respectively. The quaternary carbons C-5 (δ 139.16 ppm) and C-6 (δ 144.4 ppm) were assigned by comparing the long-range correlations obtained from the 1H-13C HMBC and 1H-13C LR-HSQMBC spectra. The most downfield and significantly broadened 1H signal resonating at δ 9.44 ppm was attributed to the hydroxyl group as it lacked correlations in the HSQC spectra. Finally, by process of elimination, the most downfield 13C signal resonating at δ 171.8 ppm was confidently assigned to the carbonyl carbon, thus completing our assignment of the pyrano[2,3-c]pyrazol-4(2H)-one ring system. An in-depth analysis of NMR spectral data showed that the chemical shift values were highly consistent within the flavonol analogues 3ah, thus validating the shifts for each position (Table S2).
The presence of a hydroxyl group at site 5 was further confirmed by the conversion of 3a to O-methylated and O-triflated derivatives 4 and 7, respectively. While the structural elucidation of O-methylated compound 4 was straightforward and followed the same logical approach as in the case of compounds 3ah, additional distinct NOEs were observed between the methoxy group protons (δ 3.77 ppm) and the neighboring phenyl group 2″(6″)-H protons (δ 7.96–7.98 ppm). The formation of O-triflated derivative 7 was clearly distinguished from the 13C NMR spectrum, where the CF3 group was observed as a quartet at δ 118.2 ppm (q, 1JCF = 320.8 Hz). Moreover, the 19F NMR spectrum revealed a chemical shift of the CF3 group at δ −74.0 ppm, which is in good agreement with the data reported in the literature [54,55]. The triflate intermediate 7 underwent Pd-catalyzed coupling reactions to give derivatives 8ag, whose structures were also unambiguously elucidated. For instance, compound 8f was obtained as an E-isomer. The magnitude of the vicinal coupling between the olefinic protons Ha (δ 7.35 ppm) and Hb (δ 7.31 ppm), which exhibited an AB-spin system and appeared as two sets of doublets (3JHa,Hb = 15.9 Hz), unquestionably confirmed E-configuration at the C=C double bond. Lastly, the olefinic protons were easily discriminated as only the proton Ha exhibited long-range 1H-13C HMBC correlations with neighboring C-4, C-5, and C-6 quaternary carbons (Figure 3).
The NMR spectral data of compound 3h with a pyridine moiety at site 6 revealed similar chemical shifts in the pyrano[2,3-c]pyrazol-4(2H)-one ring system compared with 3a–g. The 1H-15N HMBC spectrum revealed a new downfield 15N signal resonating at δ −62.2 ppm in addition to N-2 “pyrrole-like” (δ −166.9 ppm) and N-1 “pyridine-like” (δ −117.1 ppm) nitrogen atoms from the pyrazole moiety. The formation of methylpyridinium iodide 6 via the alkylation of 3h was unambiguously confirmed from the 1H-15N HMBC and 1H-1H NOESY spectral data. For instance, distinct NOEs were observed between the methyl group protons (δ 4.39 ppm) and the neighboring pyridinium 2″(6″)-H protons (δ 9.03 ppm). The aforementioned protons revealed strong long-range correlations with the methylpyridinium nitrogen at δ −183.3 ppm, which is in good agreement with the data reported in the literature [56].
In the case of compound 5, which can exist in two resonant forms, the 1H-15N HMBC spectrum revealed a new upfield 15N signal resonating at δ −214.4 ppm. Furthermore, distinct 1H and 13C signals, which appeared to be broadened, were also observed (sites 2″, 3″, 5″, and 6″). Additionally, the key information for the structure elucidation of compounds 3h, 5, and 6 was obtained after an in-depth analysis of the long-range correlations in the 1H-13C HMBC, 1H-13C H2BC, and 1H-13C LR-HSQMBC spectra (Figure 4) [57].
Figure 4. Relevant 1H-13C HMBC, 1H-13C LR-HSQMBC, 1H-13C H2BC, 1H-15N HMBC, 1H-1H NOESY, and 1,1-ADEQUATE correlations, as well as 1H NMR (italics), 13C NMR, and 15N NMR (bold) chemical shifts of compounds 3h (DMSO-d6), 5 (DMSO-d6), and 6 (DMSO-d6).
Figure 4. Relevant 1H-13C HMBC, 1H-13C LR-HSQMBC, 1H-13C H2BC, 1H-15N HMBC, 1H-1H NOESY, and 1,1-ADEQUATE correlations, as well as 1H NMR (italics), 13C NMR, and 15N NMR (bold) chemical shifts of compounds 3h (DMSO-d6), 5 (DMSO-d6), and 6 (DMSO-d6).
Molecules 28 06599 g004
Then, we carried out NMR studies of compounds 3h and 5 at 25 °C in TFA-d solutions (Scheme 5) to convert them to pyridinium and methylpyridinium trifluoroacetates 9 and 10, respectively. The 15N NMR spectral data confirmed that it was easily achieved as “pyridinium-like” 15N signals comparable to compound 6 resonating at δ −189.2 ppm and δ −183.2 ppm were observed. Moreover, in the case of compound 10, which was obtained from compound 5, the broadening of the 1H and 13C signals was absent.

2.3. Single-Crystal X-ray Diffraction Analysis

The asymmetric molecular structure of compound 5 is shown in Figure 5a. The single crystal is composed of compound 5 solvated with molecules of methanol. The methanol formed hydrogen bonds in the monocrystal of 5, including the hydrogen link to the O(15) (the HO length is 1.917 Å) (Table S6). The intramolecular hydrogen bond is also observed between the O(15) enolate oxygen and the C(17)–H(17) hydrogen atom (the HO length is 2.207 Å). The main core of compound 5 consists of the planar pyrano[2,3-c]pyrazole ring system, which possesses phenyl and the pyridin-4-yl substituents at N(2) and C(6), respectively. These substituents are slightly distorted from the pyrano[2,3-c]pyrazole plane. The phenyl ring is turned approx. 10° and the pyridinyl ring for approx. 6° counterclockwise when looking outward from the core. The N(19)–C(22) bond length of the N-methylpyridinium moiety is 1.4737(14) Å (Table S9), and the C(17)–C(18) and C(20)–C(21) bond lengths are 1.3721(15) and 1.3633(16) Å, respectively, and agree with the known bond lengths of the N-methylpyridynium salts [58]. All atoms of the pyridine moiety are located in the same plane in agreement with the data reported in the literature [59].
The selected bond lengths and angles of the pyrano[2,3-c]pyrazole ring are shown in Table 1 and Table 2. The C=O bond length of the pyran-4-one moiety is 1.2265(14) Å which is characteristic of ketone [60]. The C(5)–O(15) bond length [1.2723(13) Å] is shorter than the typical C–O single bond (~1.43 Å) [61], but longer than the typical C=O double bond (~1.23 Å) [62]. It is notable that the C(6)-O(7) bond length [1.4135(12) Å] is longer than that in the O(7)–C(7a) [1.3387(12) Å]. The N(1)–N(2) and N(2)–C(3) bond lengths are 1.3839(12) and 1.3458(14) Å, respectively, and agree with the known bond lengths of pyrazole compounds [63,64,65,66]. The sum of the angles between the covalent bonds around the N(2) atom is 360°, which indicates that a trigonal planar geometry exists at the sp2-hybridized nitrogen atom. The molecules in the crystal are located in columns made up of asymmetric units held by hydrogen bonds (Figure 5b).

2.4. Optical Investigations

The optical properties of 5-hydroxy-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-ones 3ah in various solvents, such as polar protic (MeOH), polar aprotic (THF and DMF), and non-polar (toluene), were investigated by UV–vis spectroscopy; the compounds were also subjected to fluorimetric measurements. The UV–vis electronic absorption spectra of compounds 3a and 3b in MeOH showed the absorption maximum in the 337 and 341 nm, respectively (Figure 6a, Table 3, entries 1, 2). The presence of electron-donating substituents on the phenyl ring of compounds 3c,d resulted in a bathochromic shift of the longest wavelength absorption transition. The presence of the 4-methoxyphenyl substituent of structure 3c shifted λmax upward by 18 nm (Table 3, entry 3), and the presence of the 3,4-dimethoxyphenyl substituent in structure 3d shifted λmax upward by 24 nm (Table 3, entry 4) compared to 3a, respectively. The bathochromic effect of λmax at 353 nm is also observed in the UV spectra of the naphthalene ring containing compound 3e, with a significant delocalization of 10-π electrons (Table 3, entry 5). Moreover, the replacement of the phenyl ring in the molecular structure of the products by heterocyclic rings, thiophen-2-yl, furan-3-yl, and pyridin-4-yl moieties induced a significant bathochromic shift of the near-ultraviolet band compared to that of compound 3a. Specifically, the spectra of the compounds 3f, 3g, and 3h contained intense absorption bands with λmax at 365, 360 and 355 nm, respectively (Table 3, entries 6, 7, 8).
The fluorescence spectra of compounds 3ah in the MeOH solution contained two well-separated fluorescence bands at around 440 and 590 nm (Figure 6b, Table 3). It is well known that the fluorescence spectra of 3-hydroxyflavone exhibit double emission due to excited-state intramolecular proton transfer (ESIPT) [67,68,69,70,71,72,73,74,75]. Similarly, in compounds 3ah, the proton transfer process (ESIPT) can occur, resulting in the formation of two forms in the excited state: the normal (N*) and tautomeric (ESIPT product, T*) forms. For example, the excitation of form N-3a leads to the excited state N*, which passes into the product T* by means of proton transfer (Figure 7). The T* form then relaxes to the ground state T form and emits fluorescence at a much longer wavelength compared to normal absorption [28,44]. Therefore, the form N* of the normal emission has a Stokes shift of 8927 cm−1, while the tautomeric product T* has a Stokes shift of 12491 cm−1.
Measurements of the intensity ratio of the N* and T* bands, IN*/IT*, in ESIPT compounds are used for ratiometric detection [70]. A strong effect of group substitution in the compounds 3ah was observed on the IN*/IT* fluorescence intensity ratio. 4-Chlorophenyl-substituted compound 3b, compared to the corresponding unsubstituted compound 3a, showed a dramatically decreased IN*/IT ratio of ~11-fold (Table 3, entry 2). Conversely, the 4-methoxyphenyl-substituted compound 3c and 3,4-dimethoxyphenyl-substituted compound 3d, compared to the corresponding compound 3a, showed increased IN*/IT* ratios by ~1.6- and ~4-fold, respectively (Table 3, entries 3,4). In addition, it was found that the corresponding compounds 3eh, containing the naphthalen-2-yl, thiophen-2-yl, furan-3-yl, and pyridin-4-yl groups replacing the phenyl group in compound 3a, caused a decrease in IN*/IT* ratios of ~2–3-fold (Table 3, entries 5–8).
The fluorescence quantum yield (Φf) of the solutions was estimated using the integrating sphere method. It appeared that the fluorescence quantum yield was sensitive to the structure of compounds 3ah. For unsubstituted compound 3a, a high Φf value was observed at 59.3%. The fluorescence quantum yield of 4-methoxyphenyl-group-containing compound 3c was low and did not exceed 14%. The highest Φf value (76.1%) was measured for naphthalen-2-yl-group-containing compound 3e; the thiophen-2-yl, furan-3-yl, and pyridin-4-yl groups of compounds 3f, 3g, and 3h emitted fluorescence with the observed Φf values of 55.8%, 42.6%, and 13.1%, respectively. It is notable that 3-hydroxyflavone had low quantum yield values in methanol (Φf = 3%) and DMF (Φf = 1.3%) [70].
Next, the UV–vis electronic absorption spectra of compounds 3ah in a polar aprotic solvent, THF, showed the absorption maximum in the 339–362 nm range (Figure 8a, Table 4, entries 1–8). The fluorescence spectra (*λex = 380 nm) of compounds 3ah in the THF solution showed two fluorescence bands at around 441 nm and 591 nm (Figure 8b, Table 4, entries 1–8), which were similar to the bands in MeOH. The inhibition of the ESIPT reaction by protic solvents in 3-hydroxyflavones is associated with the formation of intermolecular H-bonds, which weaken the intramolecular H-bond necessary for the ESIPT reaction [69,74]. Therefore, the relative intensity of the N* band was very weak compared to that of the T* band for compounds 3a in THF as an aprotic solvent. Compounds 3cg, especially the ones containing methoxyphenyl groups, presented dramatically decreased IN*/IT* ratios in the THF solutions compared to those in MeOH, but the 4-chlorophenyl substituent possessing compound 3b retained similar IN*/IT* ratios. However, compound 3h containing the pyridinyl substituent possessed reversed IN*/IT* ratios of 0.221 from 0.031 in MeOH. It is possible that the molecule transfered the corresponding proton to pyridine instead of to the carbonyl group. In this case, the pyridin-4-yl substituent inhibits the proton transfer process (ESIPT).
The fluorescence spectra of compound 3a in polar aprotic solvent, DMF, contained two fluorescence bands in the regions of 428 and 589 nm and showed an IN*/IT* ratio of 0.009 (Figure 8b, Table 4, entry 9), while compound 3a in toluene contained two fluorescence bands in the region of 430 and 589 nm and showed an IN*/IT* ratio of 0.004 (Figure 8b, Table 4, entry 10).
Derivative 4 with the 5-MeO substituent had an electron spectrum very close to its analog 3a (absorption maximum 306 nm and 302 nm, respectively) (Figure S1a). In the fluorescence spectrum of compound 4 in THF solution, two bands were observed at 475 and 582 nm with insignificant fluorescence (Φf < 0.1%) (Figure S1b, Table S12, entry 1). Ormson et al. reported that the fluorescence quantum yield for 3-hydroxyflavone is much greater than for the corresponding methoxy compounds and their fluorescence lifetimes are longer [76].
The UV–vis absorption and fluorescence emission spectra of 5-substituted 2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-ones 8a,f,g in THF were also investigated (Figure S1a, Table S12). The absorption maximum of compounds 8a,f,g was in the range from 297 to 302 nm (near-ultraviolet region). None of the investigated compounds 8a,f,g exhibited an absorption in the visible portion of the electronic spectrum. Upon excitation at 340 nm in THF solution, compounds 8a,f,g showed fluorescence emission maxima (λem) at around 593–603 nm, although fluorescence was weak (Figure S1b, Table S12). Compound 8a derived from 2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one containing 5-phenyl substituent produced a low quantum yield (Φf = 1%) (Table S12, entry 2); for compounds 8f and 8g, the observed Φf had a negligible value of only <0.1% (Table S12, entries 3, 4). All compounds 8a,f,g possessed very high values of Stokes shift of ~Δυ = 16000 nm.
Finally, we investigated the UV–vis spectra of compounds 5 and 6. In a polar aprotic solvent, THF, both compounds showed the same absorption maximum in the 528 nm (Figure 9, Table 5). The versatility of derivatives of zwitterionic chromophore, including pyran compounds, in synthetic and material applications, has been well documented [77,78,79].

3. Materials and Methods

3.1. General

All the chemicals and solvents were purchased from common commercial suppliers. Diffraction data were collected on a Rigaku, XtaLAB Synergy, Dualflex, HyPix diffractometer (Rigaku Corporation, Tokyo, Japan). The crystals were kept at 150.0(1) K while collecting the data. Using Olex2, the structure was solved with the ShelXT structure solution program using intrinsic phasing and refined with the olex2.refine refinement package using Gauss–Newton minimization. The 1H, 13C, and 15N NMR spectra were recorded in CDCl3 or DMSO-d6 at 25 °C on a Bruker Avance III 700 (700 MHz for 1H, 176 MHz for 13C, and 71 MHz for 15N) spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) equipped with a 5 mm TCI 1H-13C/15N/D z-gradient cryoprobe. The chemical shifts were referenced to tetramethylsilane (TMS) and expressed in ppm. The 15N NMR spectra were referenced against neat, external nitromethane (coaxial capillary). 19F NMR spectrum (376 MHz) was obtained on a Bruker Avance III 400 instrument (Bruker BioSpin AG, Faellanden, Switzerland) with absolute referencing via δ ratio. The FT-IR spectra were recorded by ATR method on either a Bruker Vertex 70v spectrometer (Bruker Optik GmbH, Ettlingen, Germany) with an integrated Platinum ATR accessory or on a Bruker Tensor 27 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) using KBr pellets. The melting points of the crystalline compounds were measured in open capillary tubes with a Buchi M 565 apparatus and are uncorrected. Mass spectra were obtained using a Shimadzu LCMS-2020 (ESI+) spectrometer (Shimadzu Corporation, Kyoto, Japan). High-resolution mass spectra (HRMS) were measured using a Bruker MicrOTOF-Q III (ESI+) apparatus (Bruker Daltonik GmbH, Bremen, Germany). All the reactions were performed in oven-dried glassware with magnetic stirring. The reaction progress was monitored by TLC analysis on Macherey-Nagel™ ALUGRAM® Xtra SIL G/UV254 plates (Macherey-Nagel GmbH & Co. KG, Düren, Germany) which were visualized by UV light (254 and 365 nm wavelengths). The compounds were purified by flash chromatography in glass columns (stationary phase of silica gel, high-purity grade of 9385, pore size of 60 Å, and particle size of 230–400 mesh, supplied by Sigma-Aldrich; Merck KGaA, Darmstadt, Germany). The UV–vis spectra were recorded on a Shimadzu 2600 UV/vis spectrometer (Shimadzu Corporation, Japan). The fluorescence spectra were recorded on an FL920 fluorescence spectrometer from Edinburgh Instruments (Edinburgh Analytical Instruments Limited, Edinburgh, UK). The PL quantum yields were determined from dilute solutions by an absolute method using the Edinburgh Instruments integrating sphere excited with a Xe lamp. The optical densities of the sample solutions were ensured to be below 0.1 to avoid reabsorption effects. All the optical measurements were performed at room temperature under ambient conditions. The following abbreviations are used in reporting the NMR data: Ph, phenyl; Pyr, pyridine; Pz, pyrazole; Naph, naphtalene; and Th, thiophene. The 1H, 13C, and 1H-15N HMBC NMR spectra, as well as the HRMS data of the new compounds, are provided in Figures S2–S81 of the Supplementary Material. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre with CCDC reference number 2287991 for 6-(1-methylpyridin-1-ium-4-yl)-4-oxo-2-phenyl-2,4-dihydropyrano[2,3-c]pyrazol-5-olate (5); formula C19H17N3O4; unit cell parameters: a 10.39395(17) b 12.15508(19) c 13.08074(18), space group P21/c.

3.2. Synthetic Procedures

Compounds 2ac and 2eh were synthesized in accordance with the procedure described in ref. [44].

3.2.1. (2E)-3-(3,4-Dimethoxyphenyl)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-one (2d)

The compound was synthesized in accordance with the procedure described in ref. [44] using 3,4-dimethoxybenzaldehyde. Orange solid. Yield 67% (2349 mg); m.p. 225–226 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 3.82 (s, 3H, 4-OCH3), 3.84 (s, 3H, 3-OCH3), 7.05 (d, J = 8.1 Hz, 1H, CPh 5-H), 7.33 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.34–7.37 (m, 2H, CPh 2,6-H), 7.52 (t, J = 7.9 Hz, 2H, NPh 3,5-H), 7.57 (d, J = 15.6 Hz, 1H, C(O)CHCH), 7.66 (d, J = 15.6 Hz, 1H, C(O)CHCH), 7.84 (d, J = 7.8 Hz, 2H, NPh 2,6-H), 9.12 (s, 1H, Pz 5-H), 11.07 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 55.6 (4-OCH3), 55.7 (3-OCH3), 111.0 (Pz C-4), 111.3 (CPh C-2), 111.8 (CPh C-5), 118.1 (NPh C-2,6), 121.8 (C(O)CHCH), 122.7 (CPh C-6), 126.6 (NPh C-4), 127.5 (CPh C-1), 129.6 (NPh C-3,5), 131.5 (Pz C-5), 138.9 (NPh C-1), 142.0 (C(O)CHCH), 149.0 (CPh C-3), 151.1 (CPh C-4), 162.0 (Pz C-3), 182.9 (C=O). 15N NMR (71 MHz, DMSO-d6): δN ppm −182.3 (Pz N-1), −118.3 (Pz N-2). IR (νmax, cm−1): 3118, 2932, 1652 (C=O), 1586, 1510, 1451, 1218, 1026, 977, 742, 679. HRMS (ESI+) for C20H18N2NaO4 ([M + Na]+) calcd 373.1159, found 373.1162.

3.2.2. General Procedure for the Synthesis of 3a–h

To a solution of 2ah (1 mmol) in EtOH (5 mL) at −10 °C, aq. NaOH (20%, 1 mL, 5 mmol) was added; the reaction mixture was cooled down to −25 °C and H2O2 30% (0.51 mL, 5 mmol) was added dropwise. The reaction mixture was stirred for 2 h and at room temperature for another 16 h. The solids were filtered off, washed with warm water, cold MeOH, and ether, and dried. The product was recrystallized from ACN.
  • 5-Hydroxy-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (3a). Off white solid; yield 58% (177 mg); m.p. 183–184 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 7.46 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.49 (t, J = 7.4 Hz, 1H, 6-CPh 4-H), 7.55–7.58 (m, 2H, 6-CPh 3,5-H), 7.58–7.61 (m, 2H, NPh 3,5-H), 8.01–8.03 (m, 2H, NPh 2,6-H), 8.13–8.14 (m, 2H, 6-CPh 2,6-H), 9.38 (s, 1H, 3-H), 9.44 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 108.3 (C-3a), 119.9 (NPh C-2,6), 126.6 (C-3), 127.9 (6-CPh C-2,6), 128.5 (NPh C-4), 129.0 (6-CPh C-3,5), 130.0 (6-CPh C-4), 130.2 (NPh C-3,5), 131.8 (6-CPh C-1), 139.16 (C-5), 139.19 (NPh C-1), 144.4 (C-6), 161.2 (C-7a), 171.8 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −167.7 (N-2), −117.0 (N-1). IR (νmax, cm−1): 3110, 3062, 2920, 2850, 1679 (C=O), 1568, 1489, 1199, 1110, 913, 832, 761, 696. HRMS (ESI+) for C18H12N2NaO3 ([M + Na]+) calcd 327.0740, found 327.0740.
  • 6-(4-Chlorophenyl)-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one (3b). Light yellow solid; yield 63% (213 mg); m.p. 262–263 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 7.45 (t, J = 7.3 Hz, 1H, NPh 4-H), 7.58 (t, J = 7.8 Hz, 2H, NPh 3,5-H), 7.62 (d, J = 8.6 Hz, 2H, 6-CPh 3,5-H), 8.01 (d, J = 7.9 Hz, 2H, NPh 2,6-H), 8.15 (d, J = 8.6 Hz, 2H, 6-CPh 2,6-H), 9.38 (s, 1H, 3-H), 9.70 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 107.9 (C-3a), 119.5 (NPh C-2,6), 126.3 (C-3), 128.1 (NPh C-4), 128.6 (6-CPh C-3,5), 129.1 (6-CPh C-2,6), 129.8 (NPh C-3,5), 130.2 (6-CPh C-1), 134.1 (6-CPh C-4), 138.7 (NPh C-1), 139.1 (C-5), 142.7 (C-6), 160.6 (C-7a), 171.3 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −167.5 (N-2), −117.2 (N-1). IR (νmax, cm−1): 3348, 3289, 3100, 1645 (C=O), 1576, 1495, 1442, 1098, 825, 752, 679. HRMS (ESI+) for C18H11ClN2NaO3 ([M + Na]+) calcd 361.0350, found 361.0350.
  • 5-Hydroxy-6-(4-methoxyphenyl)-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one (3c). Yellow solid; yield 51% (171 mg); m.p. 263–264 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 3.85 (s, 3H, CH3), 7.12–7.13 (m, 2H, 6-CPh 3,5-H), 7.45 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.57–7.60 (m, 2H, NPh 3,5-H), 8.00–8.02 (m, 2H, NPh 2,6-H), 8.09–8.12 (m, 2H, 6-CPh 2,6-H), 9.27 (s, 1H, 3-H), 9.36 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 55.3 (CH3), 107.9 (C-3a), 114.2 (6-CPh C-3,5), 119.4 (NPh C-2,6), 123.6 (6-CPh C-1), 126.0 (C-3), 128.0 (NPh C-4), 129.2 (6-CPh C-2,6), 129.8 (NPh C-3,5), 137.8 (C-5), 138.8 (NPh C-1), 144.4 (C-6), 160.2 (6-CPh C-4), 160.7 (C-7a), 171.2 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −168.3 (N-2). IR (νmax, cm−1): 3286, 3134, 1642 (C=O), 1580, 1509, 1441, 1256, 1108, 821, 748, 680. HRMS (ESI+) for C19H14N2NaO4 ([M + Na]+) calcd 357.0846, found 357.0841.
  • 6-(3,4-Dimethoxyphenyl)-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one (3d). Orange solid; yield 67% (245 mg); m.p. 252–253 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 3.84 (s, 3H, 6-CPh 3-OCH3), 3.85 (s, 3H, 6-CPh 4-OCH3), 7.15 (d, J = 8.7 Hz, 1H, 6-CPh 5-H), 7.45 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.58 (t, J = 8.0 Hz, 2H, NPh 3,5-H), 7.71 (d, J = 2.1 Hz, 1H, 6-CPh 2-H), 7.78 (dd, J = 8.6, 2.1 Hz, 1H, 6-CPh 6-H), 8.02 (d, J = 7.7 Hz, 2H, NPh 2,6-H), 9.28 (s, 1H, OH), 9.35 (s, 1H, 3-H). 13C NMR (176 MHz, DMSO-d6): δC ppm 55.6 (6-CPh 3,4-OCH3), 107.8 (C-3a), 110.7 (6-CPh C-2), 111.5 (6-CPh C-5), 119.4 (NPh C-2,6), 121.3 (6-CPh C-6), 123.7 (6-CPh C-1), 126.0 (C-3), 128.0 (NPh C-4), 129.8 (NPh C-3,5), 137.9 (C-5), 138.8 (NPh C-1), 144.3 (C-6), 148.3 (6-CPh C-3), 150.0 (6-CPh C-4), 160.6 (C-7a), 171.1 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −168.2 (N-2), −117.3 (N-1). IR (νmax, cm−1): 3281, 2963, 1632 (C=O), 1583, 1515, 1439, 1106, 754, 657. HRMS (ESI+) for C20H16N2NaO5 ([M + Na]+) calcd 387.0951, found 387.0953.
  • 5-Hydroxy-6-(naphthalen-2-yl)-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one (3e). Yellow solid; yield 32% (113 mg); m.p. 256–257 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 7.46 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.59–7.63 (m, 4H, NPh 3,5-H and Naph 6,7-H), 7.99 (d, J = 7.8 Hz, 1H, Naph 5-H), 8.04 (d, J = 7.9 Hz, 2H, NPh 2,6-H), 8.06–8.09 (m, 1H, Naph 4-H), 8.09–8.10 (m, 1H, Naph 8-H), 8.27 (dd, J = 8.7, 1.8 Hz, 1H, Naph 3-H), 8.71 (s, 1H, Naph 1-H), 9.41 (s, 1H, 3-H), 9.59 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 107.8 (C-3a), 119.3 (NPh C-2,6), 124.3 (Naph C-3), 126.1 (C-3), 126.7 (Naph C-7), 127.38 (Naph C-1 and Naph C-6), 127.44 (Naph C-5), 127.82 (Naph C-4), 128.01 (NPh C-4), 128.76 (Naph C-8), 128.79 (Naph C-2), 129.7 (NPh C-3,5), 132.4 (Naph C-8a), 132.9 (Naph C-4a), 138.7 (NPh C-1), 139.0 (C-5), 143.8 (C-6), 160.7 (C-7a), 171.2 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −167.7 (N-2), −117.4 (N-1). IR (νmax, cm−1): 3240, 1629 (C=O), 1576, 1564, 1441, 1386, 1216, 1096, 753, 685. HRMS (ESI+) for C22H14N2NaO3 ([M + Na]+) calcd 377.0897, found 377.0908.
  • 5-Hydroxy-2-phenyl-6-(thiophen-2-yl)pyrano[2,3-c]pyrazol-4(2H)-one (3f). Yellow solid; yield 62% (193 mg); m.p. 187–188 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 7.29 (dd, J = 5.0, 3.8 Hz, 1H, Th 5-H), 7.44–7.46 (m, 1H, NPh 4-H), 7.57–7.60 (m, 2H, NPh 3,5-H), 7.87 (dd, J = 3.8, 1.2 Hz, 1H, Th 3-H), 7.88 (dd, J = 5.0, 1.2 Hz, 1H, Th 4-H), 8.00–8.02 (m, 2H, NPh 2,6-H), 9.35 (s, 1H, 3-H), 10.12 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 108.1 (C-3a), 119.3 (NPh C-2,6), 126.1 (C-3), 127.66 (Th C-3), 127.70 (Th C-5), 127.9 (NPh C-4), 129.6 (NPh C-3,5), 130.4 (Th C-4), 132.3 (Th C-2), 136.5 (C-5), 138.6 (NPh C-1), 141.9 (C-6), 160.2 (C-7a), 170.4 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −168.6 (N-2), −117.2 (N-1). IR (νmax, cm−1): 3259, 3113, 1629 (C=O), 1575, 1503, 1217, 1103, 826, 753, 685. HRMS (ESI+) for C16H10N2NaO3S ([M + Na]+) calcd 333.0304, found 333.0309.
  • 6-(Furan-3-yl)-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one (3g). Beige solid; yield 30% (89 mg); m.p. 228–229 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 6.78 (dd, J = 3.4, 1.7 Hz, 1H, Furanyl 5-H), 7.23 (d, J = 3.4 Hz, 1H, Furanyl 4-H), 7.45 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.59 (t, J = 7.9 Hz, 2H, NPh 3,5-H), 8.00 (d, J = 7.8 Hz, 2H, NPh 2,6-H), 8.02 (d, J = 1.0 Hz, 1H, Furanyl 2-H), 9.36 (s, 1H, 3-H), 9.86 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6): δC ppm 108.2 (C-3a), 112.7 (Furanyl C-5), 114.5 (Furanyl C-4), 119.3 (NPh C-2,6), 126.1 (C-3), 127.9 (NPh C-4), 129.7 (NPh C-3,5), 136.9 (C-5), 138.0 (Furanyl C-3), 138.6 (NPh C-1), 144.0 (C-6), 144.9 (Furanyl C-2), 160.2 (C-7a), 170.3 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −168.4 (N-2), −117.1 (N-1). IR (νmax, cm−1): 3246, 3138, 2957, 2856, 1633 (C=O), 1576, 1483, 1221, 1124, 934, 845, 755, 681. HRMS (ESI+) for C16H10N2NaO4 ([M + Na]+) calcd 317.0533, found 317.0534.
  • 5-Hydroxy-2-phenyl-6-(pyridin-4-yl)pyrano[2,3-c]pyrazol-4(2H)-one (3h). Yellow solid; yield 53% (163 mg); m.p. 298–299 °C. 1H NMR (700 MHz, DMSO-d6) δ 7.45–7.48 (m, 1H, Ph 4-H), 7.56–7.61 (m, 2H, Ph 3,5-H), 8.00–8.03 (m, 2H, Ph 2,6-H), 8.04–8.07 (m, 2H, Pyr 3,5-H), 8.74–8.77 (m, 2H, Pyr 2,4-H), 9.41 (s, 1H, 3-H), 10.13 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6) δ 107.8 (C-3a), 119.4 (Ph C-2,6), 120.6 (Pyr C-3,5), 126.4 (C-3), 128.1 (Ph C-4), 129.7 (Ph C-3,5), 138.45 (Pyr C-4), 138.53 (Ph C-1), 140.5 (C-6), 140.9 (C-5), 150.0 (Pyr C-2,6), 160.5 (C-7a), 171.2 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −166.9 (N-2), −117.1 (N-1), −62.2 (Pyr N). IR (νmax, cm−1): 3112, 3087, 1648 (C=O), 1571, 1500, 1442, 1228, 1026, 834, 754, 629. HRMS (ESI+) for C17H11N3O3 ([M + H]+) calcd 306.0873, found 306.0871.

3.2.3. Procedure for the Synthesis of 5-Methoxy-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (4)

To a solution of 3a (304 mg, 1 mmol) in dioxane (30 mL) Cs2CO3, (0.65 g, 2 mmol) and MeI (0.07 mL, 1.1 mmol) were added. The reaction mixture was stirred at 40 °C for 3 h, neutralized with aq. KHSO4, and purified via column chromatography (SiO2, eluent: methanol/dichloromethane, 1:9, v/v). White solid; yield 79% (251 mg); m.p. 220–221 °C. 1H NMR (700 MHz, DMSO-d6): δH ppm 3.77 (s, 3H, CH3), 7.45 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.56–7.61 (m, 5H, NPh 3,5-H and 6-CPh 3,4,5-H), 7.96–7.98 (m, 2H, 6-CPh 2,6-H), 8.00 (d, J = 8.0 Hz, 2H, NPh 2,6-H), 9.34 (s, 1H, 3-H). 13C NMR (176 MHz, DMSO-d6): δC ppm 60.1 (CH3), 109.8 (C-3a), 119.5 (NPh C-2,6), 126.5 (C-3), 128.1 (NPh C-4), 128.2 (6-CPh C-2,6), 128.7 (6-CPh C-3,5), 129.8 (NPh C-3,5), 130.4 (6-CPh C-1), 130.7 (6-CPh C-4), 138.7 (NPh C-1), 140.8 (C-5), 154.1 (C-6), 160.8 (C-7a), 172.1 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −168.0 (N-2), −115.9 (N-1). IR (νmax, cm−1): 3101, 2936, 1640 (C=O), 1578, 1554, 1443, 1351, 1134, 764, 753, 682. HRMS (ESI+) for C19H14N2NaO3 ([M + Na]+) calcd 341.0897, found 341.0899.

3.2.4. Procedure for the Synthesis of 6-(1-Methylpyridin-1-ium-4-yl)-4-oxo-2-phenyl-2,4-dihydropyrano[2,3-c]pyrazol-5-olate (5)

To a solution of 3a (304 mg, 1 mmol) in dioxane (30 mL), Cs2CO3 (0.65 g, 2 mmol) and MeI (0.07 mL, 1.1 mmol) were added. The reaction mixture was stirred at 40 °C for 3 h, neutralized with aq. KHSO4, and purified via column chromatography (SiO2, eluent: methanol/dichloromethane, 1:9, v/v). Red solid; yield 59% (264 mg); decomposition 240–241 °C. 1H NMR (700 MHz, DMSO-d6) δ 3.95 (CH3), 7.42 (m, 1H, Ph 4-H), 7.56 (m, 2H, Ph 3,5-H), 7.98 (m, 2H, Ph 2,6-H), 8.12–8.20 (m, 2H, Pyr 2,6-H), 9.14–9.20 (m, 2H, Pyr 3,5-H), 9.41 (s, 1H, 3-H). 13C NMR (176 MHz, DMSO-d6) δ 44.5 (CH3), 109.4 (C-3a), 112.9 (Pyr C-5), 114.6 (Pyr C-3), 119.1 (Ph C-2,6), 126.7 (C-3), 127.6 (Ph C-4), 129.6 (Ph C-3,5), 134.8 (C-6), 138.7 (Ph C-1), 140.9 (Pyr C-6), 141.6 (Pyr C-2), 143.3 (Pyr C-4), 161.2 (C-7a), 164.1 (C-5), 176.1 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −214.4 (Pyr N); −170.1 (N-2), −120.2 (N-1). IR (νmax, cm−1): 3089, 2920, 2852, 1629 (C=O), 1569, 1488, 1465, 1382, 1189, 756, 689. HRMS (ESI+) for C18H14N3O3 ([M + Na]+) calcd 342.0849, found 342.0852.

3.2.5. Procedure for the Synthesis of 4-(5-Hydroxy-4-oxo-2-phenyl-2,4-dihydropyrano[2,3-c]pyrazol-6-yl)-1-methylpyridin-1-ium Iodide (6)

To a solution of 3a (305 mg, 1 mmol) in ACN (15 mL), MeI (1 mL, 16.1 mmol) was added. The reaction mixture was stirred at 40 °C for 2 h and diluted with DMF (15 mL); the solution was slowly dripped into cold diethyl ether. The formed crystals were filtrated and washed with a small amount of MeOH and diethyl ether. Orange solid; yield 78% (348 mg); decomposition 277–278 °C. 1H NMR (700 MHz, DMSO-d6) δ 4.39 (CH3), 7.47 (m, 1H, Ph 4-H), 7.58 (m, 2H, Ph 3,5-H), 7.97 (m, 2H, Ph 2,6-H), 8.63 (m, 2H, Pyr 3,5-H), 9.03 (m, 2H, Pyr 2,6-H), 9.45 (s, 1H, 3-H), 11.49 (s, 1H, OH). 13C NMR (176 MHz, DMSO-d6) δ 48.0 (CH3), 108.4 (C-3a), 119.9 (Ph C-2,6), 123.8 (Pyr C-3,5), 127.4 (C-3), 128.9 (Ph C-4), 130.3 (Ph C-3,5), 146.1 (Pyr C-4), 138.8 (Ph C-1), 137.5 (C-6), 145.4 (C-5), 145.9 (Pyr C-2,6), 160.7 (C-7a), 171.4 (C-4). 15N NMR (71 MHz, DMSO-d6): δN ppm −183.2 (Pyr N); −165.7 (N-2), −117.1 (N-1). IR (νmax, cm−1): 3135, 3066, 1646 (C=O), 1575, 1497, 1441, 1388, 1232, 1197, 1109, 761. HRMS (ESI+) for C18H14N3O3 (M+) calcd 320.1030, found 320.1032.

3.2.6. Procedure for the Synthesis of 4-Oxo-2,6-diphenyl-2,4-dihydropyrano[2,3-c]pyrazol-5-yl Trifluoromethanesulfonate (7)

To a solution of 3a (304 mg, 1 mmol) in DCM (30 mL), at 0 °C, TEA (0.7 mL, 5 mmol) and Tf2O (0.34 mL, 2 mmol) were added dropwise. The reaction mixture was stirred at 24 °C for 16 h, diluted with DCM (100 mL), washed with brine (100 mL), and purified via column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:6, v/v). Beige solid; yield 74% (323 mg); m.p. 200–201 °C. 1H NMR (700 MHz, CDCl3): δH ppm 7.46 (t, J = 7.3 Hz, 1H, NPh 4-H), 7.55–7.58 (m, 4H, NPh 3,5-H and 6-CPh 3,5-H), 7.62 (t, J = 7.3 Hz, 1H, 6-CPh 4-H), 7.78 (d, J = 8.0 Hz, 2H, NPh 2,6-H), 7.87 (d, J = 7.5 Hz, 2H, 6-CPh 2,6-H), 8.59 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 109.4 (C-3a), 118.2 (q, 1JC,F = 320.8 Hz, CF3), 120.3 (NPh C-2,6), 125.5 (C-3), 128.4 (6-CPh C-1), 129.0 (6-CPh C-3,5), 129.1 (NPh C-4), 129.2 (6-CPh C-2,6), 130.1 (NPh C-3,5), 132.6 (6-CPh C-4), 134.6 (C-5), 138.9 (NPh C-1), 158.2 (C-6), 161.3 (C-7a), 168.8 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −165.8 (N-2), −112.7 (N-1). 19F NMR (376 MHz, CDCl3): δF ppm –74.0 (CF3). IR (νmax, cm−1): 3105, 2918, 1658 (C=O), 1594, 1553, 1425, 1208, 1134, 1019, 897, 757, 686, 600. HRMS (ESI+) for C19H11F3N2NaO5S ([M + Na]+) calcd 459.0233, found 459.0235.

3.2.7. General Procedure for the Synthesis of 5-(Hetero)aryl-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-ones 8a–e

To a solution of 7 (436 mg, 1 mmol) in dioxane (15 mL), K3PO4 (634 mg, 3 mmol), KBr (131 mg, 1.1 mmol), appropriate (hetero)arylboronic acid (2.5 mmol), and Pd(PPh3)4 (69 mg, 0.06 mmol) were added. The reaction mixture was stirred at 90 °C for 16 h, diluted with H2O (80 mL), extracted with DCM (3 × 15 mL), and purified via column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:6, v/v).
  • 2,5,6-Triphenylpyrano[2,3-c]pyrazol-4(2H)-one (8a). Yellow solid; yield 95% (346 mg); m.p. 265–266 °C. 1H NMR (700 MHz, CDCl3): δH ppm 7.20–7.21 (m, 2H, 5-CPh 2,6-H), 7.24–7.25 (m, 2H, 6-CPh 3,5-H), 7.28–7.33 (m, 4H, 5-CPh 3,4,5-H and 6-CPh 4-H), 7.39 (d, J = 7.6 Hz, 2H, 6-CPh 2,6-H), 7.42 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.53 (t, J = 7.9 Hz, 2H, NPh 3,5-H), 7.80 (d, J = 8.0 Hz, 2H, NPh 2,6-H), 8.55 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 109.7 (C-3a), 119.9 (NPh C-2,6), 123.0 (C-5), 124.7 (C-3), 127.7 (5-CPh C-4), 128.0 (6-CPh C-3,5), 128.24 (NPh C-4), 128.29 (5-CPh C-3,5), 129.78 (6-CPh C-2,6), 129.83 (NPh C-3,5), 130.0 (6-CPh C-4), 131.4 (5-CPh C-2,6), 132.8 (5-CPh C-1), 133.0 (6-CPh C-1), 139.2 (NPh C-1), 160.8 (C-6), 162.4 (C-7a), 175.5 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −169.5 (N-2), −115.3 (N-1). IR (νmax, cm−1): 3093, 2922, 1642 (C=O), 1578, 1561, 1493, 1349, 1224, 1056, 755, 730, 694, 683. HRMS (ESI+) for C24H16N2NaO2 ([M + Na]+) calcd 387.1104, found 387.1107.
  • 5-(4-Methylphenyl)-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (8b). White solid; yield 62% (235 mg); m.p. 256–257 °C. 1H NMR (700 MHz, CDCl3): δH ppm 2.34 (s, 3H, CH3), 7.08 (m, 2H, 5-CPh 2,6-H), 7.11 (m, 2H, 5-CPh 3,5-H), 7.24–7.27 (m, 2H, 6-CPh 3,5-H), 7.32 (m, 2H, 6-CPh 4-H), 7.40–7.43 (m, 3H, 6-CPh 2,6-H, NPh 4-H), 7.53 (m, 2H, NPh 3,5-H), 7.78–7.81 (m, 2H, NPh 2,6-H), 8.55 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 21.3 (CH3), 109.7 (C-3a), 119.8 (NPh C-2,6), 122.9 (C-5), 124.6 (C-3), 128.0 (6-CPh C-3,5), 128.2 (NPh C-4), 129.1 (5-CPh C-3,5), 129.6 (5-CPh C-1), 129.7 (6-CPh C-2,6), 129.8 (NPh C-3,5), 129.9 (6-Ph C-4), 131.2 (5-CPh C-2,6), 133.1 (6-CPh C-1), 137.4 (5-CPh C-4), 139.2 (NPh C-1), 160.5 (C-6), 162.3 (C-7a), 175.7 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −169.7 (N-2), −115.3 (N-1). IR (νmax, cm−1): 3098, 3023, 1649 (C=O), 1578, 1565, 1348, 1181, 1021, 755, 742, 732, 683. HRMS (ESI+) for C25H18N2O2 ([M + Na]+) calcd 401.1260, found 401.1262.
  • 5-(4-Methoxyphenyl)-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (8c). White solid; yield 77% (304 mg); m.p. 236–237 °C. 1H NMR (700 MHz, CDCl3): δH ppm 3.80 (s, 3H, CH3), 6.85 (m, 2H, 5-CPh 3,5-H), 7.12 (m, 2H, 5-CPh 2,6-H), 7.24–7.28 (m, 2H, 6-CPh 3,5-H), 7.32 (m, 2H, 6-CPh 4-H), 7.40–7.44 (m, 3H, 6-CPh 2,6-H, NPh 4-H), 7.54 (m, 2H, NPh 3,5-H), 7.80 (m, 2H, NPh 2,6-H), 8.54 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 55.2 (CH3), 109.7 (C-3a), 113.9 (5-CPh C-3,5), 119.8 (NPh C-2,6), 122.5 (C-5), 124.6 (C-3), 124.8 (5-CPh C-1), 128.0 (6-CPh C-3,5), 128.2 (NPh C-4), 129.7 (6-CPh C-2,6), 129.8 (NPh C-3,5), 129.9 (6-Ph C-4), 132.5 (5-CPh C-2,6), 133.2 (6-CPh C-1), 139.2 (NPh C-1), 159.1 (5-CPh C-4), 160.5 (C-6), 162.3 (C-7a), 175.8 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −169.7 (N-2), −115.6 (N-1). IR (νmax, cm−1): 3102, 3024, 1650 (C=O), 1598, 1567, 1335, 1241, 1167, 1023, 748, 686, 549. HRMS (ESI+) for C25H18N2O3 ([M + Na]+) calcd 417.1210, found 417.1208.
  • 5-(4-Chlorophenyl)-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (8d). White solid; yield 44% (176 mg); m.p. 255–256 °C. 1H NMR (700 MHz, CDCl3): δH ppm 7.14 (m, 2H, 5-CPh 2,6-H), 7.27–7.31 (m, 4H, 5-CPh 3,5-H, 6-CPh 3,5-H), 7.35 (m, 2H, 6-CPh 4-H), 7.39 (m, 2H, 6-CPh 2,6-H), 7.43 (m, 1H, NPh 4-H), 7.54 (m, 2H, NPh 3,5-H), 7.80 (m, 2H, NPh 2,6-H), 8.55 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 109.5 (C-3a), 119.9 (NPh C-2,6), 121.8 (C-5), 124.7 (C-3), 128.2 (6-CPh C-3,5), 128.4 (NPh C-4), 128.6 (5-CPh C-3,5), 129.7 (6-CPh C-2,6), 129.8 (NPh C-3,5), 130.3 (6-Ph C-4), 131.3 (5-CPh C-1), 132.7 (6-CPh C-1), 132.8 (5-CPh C-2,6), 133.7 (5-CPh C-4), 139.1 (NPh C-1), 161.0 (C-6), 162.32 (C-7a), 175.2 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −169.1 (N-2), −115.0 (N-1). IR (νmax, cm−1): 3206, 3105, 1650 (C=O), 1568, 1422, 1348, 1211, 1135, 757, 731, 686. HRMS (ESI+) for C24H15ClN2O2 ([M + Na]+) calcd 421.0714, found 421.0711.
  • 2,6-Diphenyl-5-(thiophen-3-yl)pyrano[2,3-c]pyrazol-4(2H)-one (8e). Beige solid; yield 80% (297 mg); m.p. 265–266 °C. 1H NMR (700 MHz, CDCl3): δH ppm 6.88 (m, 1H, Th 4-H), 7.20 (m, 1H, Th 2-H), 7.24 (m, 1H, Th 5-H), 7.31 (m, 2H, 6-CPh 3,5-H), 7.37 (m, 2H, 6-CPh 4-H), 7.42 (m, 1H, NPh 4-H), 7.44 (m, 2H, 6-CPh 2,6-H), 7.54 (m, 2H, NPh 3,5-H), 7.80 (m, 2H, NPh 2,6-H), 8.54 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 109.6 (C-3a), 118.1 (C-5), 119.9 (NPh C-2,6), 124.6 (C-3), 124.7 (Th C-5), 126.4 (Th C-2), 128.1 (6-CPh C-3,5), 128.3 (NPh C-4), 129.8 (Th C-4), 129.5 (6-CPh C-2,6), 129.8 (NPh C-3,5), 130.2 (6-Ph C-4), 131.9 (Th C-3), 132.2 (6-CPh C-1), 139.1 (NPh C-1), 160.8 (C-6), 162.2 (C-7a), 175.3 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −169.4 (N-2), −115.2 (N-1). IR (νmax, cm−1): 3100, 1644 (C=O), 1577, 1564, 1441, 1328, 1218, 753, 739, 685. HRMS (ESI+) for C22H14N2O2S ([M + H]+) calcd 393.0668, found 393.0669.

3.2.8. Procedure for the Synthesis of tert-Butyl (2E)-3-(4-oxo-2,6-diphenyl-2,4-dihydropyrano[2,3-c]pyrazol-5-yl)prop-2-enoate (8f)

To a solution of 7 (436 mg, 1 mmol) in dry DMF (10 mL), TEA (0.28 mL, 2 mmol), tert-butyl acrylate (0.29 mL, 2 mmol), and Pd(PPh3)2Cl2 (35 mg, 0.05 mmol) were added. The reaction mixture was stirred at 100 °C for 72 h, diluted with H2O (100 mL), extracted with EtOAc (3 × 50 mL), washed with brine (100 mL), and purified via column chromatography (SiO2, eluent: dichloromethane). White solid; yield 24% (99 mg); decomposition 312 °C. 1H NMR (700 MHz, CDCl3): δH ppm 1.48 (s, 9H, C(CH3)3), 7.31 (d, J = 15.9 Hz, 1H, CHCHCOOC(CH3)3), 7.35 (d, J = 15.8 Hz, 1H, CHCHCOOC(CH3)3), 7.42 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.52–7.56 (m, 5H, NPh 3,5-H and 6-CPh 3,4,5-H), 7.68 (d, J = 6.8 Hz, 2H, 6-CPh 2,6-H), 7.79 (d, J = 8.0 Hz, 2H, NPh 2,6-H), 8.54 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 28.2 (C(CH3)3), 80.3 (C(CH3)3), 110.0 (C-3a), 115.9 (C-5), 120.0 (NPh C-2,6), 125.0 (C-5), 125.5 (CHCHCOOC(CH3)3), 128.6 (NPh C-4), 128.8 (6-CPh C-3,5), 130.0 (NPh C-3,5), 130.2 (6-CPh C-2,6), 131.6 (6-CPh C-4), 132.1 (6-CPh C-1), 135.3 (CHCHCOOC(CH3)3), 139.2 (NPh C-1), 161.7 (C-7a), 166.0 (C-6), 167.1 (CHCHCOOC(CH3)3), 175.2 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −168.8 (N-2), −114.4 (N-1). IR (νmax, cm−1): 3106, 2971, 1647 (C=O), 1584, 1554, 1445, 1290, 1149, 753, 688. HRMS (ESI+) for C25H22N2NaO4 ([M + Na]+) calcd 437.1472, found 437.1473.

3.2.9. Procedure for the Synthesis of 2,6-Diphenyl-5-(phenylethynyl)pyrano[2,3-c]pyrazol-4(2H)-one (8g)

To a solution of 7 (436 mg, 1 mmol) in dry DMF (10 mL), TEA (0.28 mL, 2 mmol), CuI (19 mg, 0.1 mmol), phenylacetylene (0.16 mL, 1.5 mmol), and Pd(PPh3)2Cl2 (42 mg, 0.06 mmol) were added. The reaction mixture was stirred at 65 °C for 1 h, diluted with H2O (100 mL), extracted with EtOAc (3 × 50 mL), washed with brine (100 mL), and purified via column chromatography, (SiO2, eluent: dichloromethane). White solid; yield 71% (276 mg); m.p. 222–223 °C. 1H NMR (700 MHz, CDCl3): δH ppm 7.31–7.34 (m, 3H, C≡CPh 3,4,5-H), 7.42 (t, J = 7.4 Hz, 1H, NPh 4-H), 7.49–7.50 (m, 2H, C≡CPh 2,6-H), 7.52–7.57 (m, 5H, NPh 3,5-H and 6-CPh 3,4,5-H), 7.79 (d, J = 8.1 Hz, 2H, NPh 2,6-H), 8.25 (d, J = 7.9 Hz, 2H, 6-CPh 2,6-H), 8.55 (s, 1H, 3-H). 13C NMR (176 MHz, CDCl3): δC ppm 82.0 (C≡CPh), 97.9 (C≡CPh), 107.7 (C-5), 108.8 (C-3a), 120.0 (NPh C-2,6), 123.3 (C≡CPh C-1), 124.8 (C-3), 128.3 (6-CPh C-3,5), 128.4 (C≡CPh C-3,5), 128.6 (C≡CPh C-4 and NPh C-4), 129.4 (6-CPh C-2,6), 130.0 (NPh C-3,5), 131.66 (6-CPh C-4), 131.71 (C≡CPh C-2,6), 132.3 (6-CPh C-1), 139.1 (NPh C-1), 161.9 (C-7a), 165.1 (C-6), 174.2 (C-4). 15N NMR (71 MHz, CDCl3): δN ppm −168.6 (N-2), −114.3 (N-1). IR (νmax, cm−1): 3094, 3057, 1649 (C=O), 1577, 1542, 1442, 1361, 1264, 1118, 750, 684. HRMS (ESI+) for C26H16N2NaO2 ([M + Na]+) calcd 411.1104, found 411.1101.

4. Conclusions

In conclusion, we showed that the diverse 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-one derivatives as analogues of 3-hydroxyflavones can be conveniently synthesized from appropriate (E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)prop-2-en-1-ones employing Algar–Flynn–Oyamada reaction conditions. Further functionalization of the 5-position of the pyrano[2,3-c]pyrazol-4(2H)-one ring was achieved by employing various Pd-catalyzed coupling reactions of the intermediate 5-triflate. Extensive NMR spectroscopic studies were undertaken using standard and advanced methods to unambiguously determine the structure and configuration of the synthesized compounds. The synthesized 3-hydroxyflavone analogues were characterized by good quantum yields and large Stokes shifts. In addition, the excited-state intramolecular proton transfer (ESIPT) reaction of 5-hydroxypyrano[2,3-c]pyrazol-4(2H)-one from the 5-hydroxy moiety to the carbonyl group in polar protic, polar aprotic, and non-polar solvents was observed, resulting in a well-resolved two-band fluorescence.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28186599/s1. Table S1: Optimization of AFO reaction conditions of (E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)-3-phenylprop-2-en-1-one (2a) to get 5-hydroxy-2,6-diphenylpyrano[2,3-c]pyrazol-4(2H)-one (3a); Table S2: Relevant 1H and 13C NMR spectral data of 6-(hetero)aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones 3ah in DMSO-d6 (δ in ppm); Tables S3–S11: Data for X-ray analysis of compound 5: Table S3: Experimental parameters and CCDC-2287991; Table S4: Sample and crystal data of compound 5; Table S5: Data collection and structure refinement of compound 5; Table S6: Methanol formed and other selected hydrogen bonds in monocrystal of compound 5; Table S7: Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2×103) for compound 5; Table S8: Anisotropic displacement parameters (Å2×103) for compound 5; Table S9: Bond lengths for compound 5; Table S10: Bond angles for compound 5; Table S11: Hydrogen atom coordinates (Å×104) and isotropic displacement parameters (Å2×103) for compound 5; Figure S1: (a) UV absorption spectra of compounds 4 and 8a,f,g in THF; (b) fluorescence emission spectra (λex = 340 nm) of compounds 4 and 8a,f,g in THF; Table S12: Absorption (λabs absorption maxima and ε), fluorescence emission (λem and quantum yield Φf), and Stokes shift parameters for 4 and 8a,f,g in THF (λex = 340 nm); Figures S2–S81: 1H, 13C, 1H-15N HMBC NMR, and HRMS (ESI) spectra of compounds 2d, 3ah, 4–7, 8ag. References [80,81,82] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, A.Š.; methodology, A.Š., E.A. and S.K.; formal analysis, A.Š. and E.A.; investigation, A.U. and A.B.; resources, A.Š. and E.A.; data curation, A.Š., A.U., A.B. and E.A.; writing—original draft preparation, A.Š., E.A., A.B., V.M. and A.U.; writing—review and editing, A.Ž. and J.S.; visualization, A.Š. and A.U.; supervision, S.K., E.A. and A.Š.; funding acquisition, A.Š. and E.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Research Council of Lithuania (No. S-MIP-23-51).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors are grateful to S. Belyakov (Latvian Institute of Organic Synthesis, Riga, Latvia) for performing the X-ray analysis.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not available.

References

  1. Li, M.-M.; Huang, H.; Pu, Y.; Tian, W.; Deng, Y.; Lu, J. A close look into the biological and synthetic aspects of fused pyrazole derivatives. Eur. J. Med. Chem. 2022, 243, 114739. [Google Scholar] [CrossRef] [PubMed]
  2. Hassan, A.Y.; Mohamed, M.A.; Abdel-Aziem, A.; Hussain, A.O. Synthesis and Anticancer Activity of Some Fused Heterocyclic Compounds Containing Pyrazole Ring. Polycycl. Aromat. Compd. 2020, 40, 1280–1290. [Google Scholar] [CrossRef]
  3. Bondock, S.; Fadaly, W.; Metwally, M.A. Synthesis and antimicrobial activity of some new thiazole, thiophene and pyrazole derivatives containing benzothiazole moiety. Eur. J. Med. Chem. 2010, 45, 3692–3701. [Google Scholar] [CrossRef]
  4. Han, C.; Guo, Y.-C.; Wang, D.-D.; Dai, X.-Y.; Wu, F.-J.; Liu, H.-F.; Dai, G.-F.; Tao, J.-C. Novel pyrazole fused heterocyclic ligands: Synthesis, characterization, DNA binding/cleavage activity and anti-BVDV activity. Chin. Chem. Lett. 2015, 26, 534–538. [Google Scholar] [CrossRef]
  5. Pinto, D.J.P.; Orwat, M.J.; Koch, S.; Rossi, K.A.; Alexander, R.S.; Smallwood, A.; Wong, P.C.; Rendina, A.R.; Luettgen, J.M.; Knabb, R.M.; et al. Discovery of 1-(4-Methoxyphenyl)-7-oxo-6-(4-(2-oxopiperidin-1-yl)phenyl)-4,5,6,7-tetrahydro-1H-pyrazolo[3,4-c]pyridine-3-carboxamide (Apixaban, BMS-562247), a Highly Potent, Selective, Efficacious, and Orally Bioavailable Inhibitor of Blood Coagulation Factor Xa. J. Med. Chem. 2007, 50, 5339–5356. [Google Scholar] [PubMed]
  6. Xu, Y.; Zhang, Z.; Jiang, X.; Chen, X.; Wang, Z.; Alsulami, H.; Qin, H.-L.; Tang, W. Discovery of δ-sultone-fused pyrazoles for treating Alzheimer’s disease: Design, synthesis, biological evaluation and SAR studies. Eur. J. Med. Chem. 2019, 181, 111598. [Google Scholar] [CrossRef]
  7. Syed, Y.Y. Futibatinib: First Approval. Drugs 2022, 82, 1737–1743. [Google Scholar] [CrossRef]
  8. Kumar, A.; Lohan, P.; Aneja, D.K.; Gupta, G.K.; Kaushik, D.; Prakash, O. Design, synthesis, computational and biological evaluation of some new hydrazino derivatives of DHA and pyranopyrazoles. Eur. J. Med. Chem. 2012, 50, 81–89. [Google Scholar] [CrossRef]
  9. Parikh, P.H.; Timaniya, J.B.; Patel, M.J.; Patel, K.P. Microwave-assisted synthesis of pyrano[2,3-c]-pyrazole derivatives and their anti-microbial, anti-malarial, anti-tubercular, and anti-cancer activities. J. Mol. Struct. 2022, 1249, 131605. [Google Scholar] [CrossRef]
  10. Parshad, M.; Verma, V.; Kumar, D. Iodine-mediated efficient synthesis of pyrano[2,3-c]pyrazoles and their antimicrobial activity. Monatsh. Chem. 2014, 145, 1857–1865. [Google Scholar] [CrossRef]
  11. Wang, J.; Liu, D.; Zheng, Z.; Shan, S.; Han, X.; Srinivasula, S.M.; Croce, C.M.; Alnemri, E.S.; Huang, Z. Structure-based discovery of an organic compound that binds Bcl-2 protein and induces apoptosis of tumor cells. Proc. Natl. Acad. Sci. USA 2000, 97, 7124–7129. [Google Scholar] [CrossRef] [PubMed]
  12. Sun, X.; Zhang, L.; Gao, M.; Que, X.; Zhou, C.; Zhu, D.; Cai, Y. Nanoformulation of a Novel Pyrano[2,3-c]Pyrazole Heterocyclic Compound AMDPC Exhibits Anti-Cancer Activity via Blocking the Cell Cycle through a P53-Independent Pathway. Molecules 2019, 24, 624. [Google Scholar] [CrossRef] [PubMed]
  13. Nguyen, H.T.; Truong, M.-N.H.; Le, T.V.; Vo, N.T.; Nguyen, H.D.; Tran, P.H. A New Pathway for the Preparation of Pyrano[2,3-c]pyrazoles and molecular Docking as Inhibitors of p38 MAP Kinase. ACS Omega 2022, 7, 17432–17443. [Google Scholar] [CrossRef] [PubMed]
  14. Bieliauskas, A.; Krikštolaitytė, S.; Holzer, W.; Šačkus, A. Ring-closing metathesis as a key step to construct 2,6-dihydropyrano[2,3-c]pyrazole ring system. Arkivoc 2018, 2018, 296–307. [Google Scholar] [CrossRef]
  15. Milišiūnaitė, V.; Kadlecová, A.; Žukauskaitė, A.; Doležal, K.; Strnad, M.; Voller, J.; Arbačiauskienė, E.; Holzer, W.; Šačkus, A. Synthesis and Anthelmintic Activity of Benzopyrano[2,3-c]Pyrazol-4(2H)-One Derivatives. Mol. Divers. 2020, 24, 1025–1042. [Google Scholar] [CrossRef]
  16. Al-Khayri, J.M.; Sahana, G.R.; Nagella, P.; Joseph, B.V.; Alessa, F.M.; Al-Mssallem, M.Q. Flavonoids as Potential Anti-Inflammatory. Molecules 2022, 27, 2901. [Google Scholar] [CrossRef]
  17. Panche, A.; Diwan, A.; Chandra, S. Flavonoids: An overview. J. Nutr. Sci. 2016, 5, E47. [Google Scholar] [CrossRef]
  18. Ullah, A.; Munir, S.; Badshah, S.L.; Khan, N.; Ghani, L.; Poulson, B.G.; Mews, A.-H.; Jaremko, M. Important Flavonoids and Their Role as a Therapeutic Agent. Molecules 2020, 25, 5243. [Google Scholar] [CrossRef]
  19. Jan, R.; Khan, M.; Asaf, S.; Lubna; Asif, S.; Kim, K.-M. Bioactivity and Therapeutic Potential of Kaempferol and Quercetin: New Insights for Plant and Human Health. Plants 2022, 11, 2623. [Google Scholar] [CrossRef]
  20. Nejabati, H.R.; Roshangar, L. Kaempferol: A potential agent in the prevention of colorectal cancer. Physiol. Rep. 2022, 10, e15488. [Google Scholar] [CrossRef]
  21. Borsari, C.; Jiménez-Antón, M.D.; Eick, J.; Bifeld, E.; Torrado, J.J.; Olías-Molero, A.I.; Corral, M.J.; Santarem, N.; Baptista, C.; Severi, L.; et al. Discovery of a benzothiophene-flavonol halting miltefosine and antimonial drug resistance in Leishmania parasites through the application of medicinal chemistry, screening and genomics. Eur. J. Med. Chem. 2019, 183, 111676. [Google Scholar] [CrossRef]
  22. Kishore, N.R.; Ashok, D.; Sarasija, M.; Murthy, N.Y.S. One-pot synthesis of spirochromanone-based 3-hydroxy-4H-chromen-4-ones by a modified Algar–Flynn–Oyamada reaction and evaluation of their antimicrobial activity. Chem. Heterocycl. Compd. 2017, 53, 1187–1191. [Google Scholar] [CrossRef]
  23. Ashok, D.; Kifah, M.A.; Lakshmi, B.V.; Sarasija, M.; Adam, S. Microwave-assisted one-pot synthesis of some new flavonols by modified Algar–Flynn–Oyamada reaction and their antimicrobial activity. Chem. Heterocycl. Compd. 2016, 52, 172–176. [Google Scholar] [CrossRef]
  24. Lee, J.; Park, T.; Jeong, S.; Kim, K.-H.; Hong, C. 3-Hydroxychromones as cyclin-dependent kinase inhibitors: Synthesis and biological evaluation. Bioorg. Med. Chem. Lett. 2007, 17, 1284–1287. [Google Scholar] [CrossRef] [PubMed]
  25. Joshi, H.C.; Antonov, L. Excited-State Intramolecular Proton Transfer: A Short Introductory Review. Molecules 2021, 26, 1475. [Google Scholar] [CrossRef]
  26. Ameer-Beg, S.; Ormson, S.M.; Brown, R.G.; Matousek, P.; Towrie, M.; Nibbering, E.T.J.; Foggi, P.; Neuwahl, F.V.R. Ultrafast Measurements of Excited State Intramolecular Proton Transfer (ESIPT) in Room Temperature Solutions of 3-Hydroxyflavone and Derivatives. J. Phys. Chem. A 2001, 105, 3709–3718. [Google Scholar] [CrossRef]
  27. Sarkar, M.; Ray, J.G.; Sengupta, P.K. Effect of reverse micelles on the intramolecular excited state proton transfer (ESPT) and dual luminescence behaviour of 3-hydroxyflavone. Spectrochim. Acta A Mol. Biomol. 1996, 52, 275–278. [Google Scholar] [CrossRef]
  28. Zhao, X.; Li, X.; Liang, S.; Dong, X.; Zhang, Z. 3-Hydroxyflavone derivatives: Promising scaffolds for fluorescent imaging in cells. RSC Adv. 2021, 11, 28851. [Google Scholar] [CrossRef]
  29. Butun, B.; Topcu, G.; Ozturk, T. Recent Advances on 3-Hydroxyflavone Derivatives: Structures and Properties. Mini Rev. Med. Chem. 2018, 18, 98–103. [Google Scholar] [CrossRef]
  30. Russo, M.; Orel, V.; Takko, P.; Šranková, M.; Muchová, L.; Vítek, L.; Klán, P. Structure–Photoreactivity Relationship of 3-Hydroxyflavone-Based CO-Releasing Molecules. J. Org. Chem. 2022, 87, 4750–4763. [Google Scholar] [CrossRef]
  31. Jiang, G.; Jin, Y.; Li, M.; Wang, H.; Xiong, M.; Zeng, W.; Yuan, H.; Liu, C.; Ren, Z.; Liu, C. Faster and More Specific: Excited-State Intramolecular Proton Transfer-Based Dyes for High-Fidelity Dynamic Imaging of Lipid Droplets within Cells and Tissues. Anal. Chem. 2020, 92, 10342–10349. [Google Scholar] [CrossRef] [PubMed]
  32. Kamariza, M.; Keyser, S.G.L.; Utz, A.; Knapp, B.D.; Ealand, C.; Ahn, G.; Cambier, C.J.; Chen, T.; Kana, B.; Huang, K.C.; et al. Toward Point-of-Care Detection of Mycobacterium tuberculosis: A Brighter Solvatochromic Probe Detects Mycobacteria within Minutes. JACS Au 2021, 1, 1368–1379. [Google Scholar] [CrossRef] [PubMed]
  33. Bernini, R.; Crisante, F.; Ginnasi, M.C. A Convenient and Safe O-Methylation of Flavonoids with Dimethyl Carbonate (DMC). Molecules 2011, 16, 1418–1425. [Google Scholar] [CrossRef] [PubMed]
  34. Koirala, N.; Thuan, N.H.; Ghimire, G.P.; Thang, D.V.; Sohng, J.K. Methylation of flavonoids: Chemical structures, bioactivities, progress and perspectives for biotechnological production. Enzym. Microb. 2016, 86, 103–116. [Google Scholar] [CrossRef] [PubMed]
  35. Liu, Y.; Fernie, A.R.; Tohge, T. Diversification of Chemical Structures of Methoxylated Flavonoids and Genes Encoding Flavonoid-O-Methyltransferases. Plants 2022, 11, 564. [Google Scholar] [CrossRef] [PubMed]
  36. Ohtani, H.; Ikegawa, T.; Honda, Y.; Kohyama, N.; Morimoto, S.; Shoyama, Y.; Juichi, M.; Naito, M.; Tsuruo, T.; Sawada, T. Effects of various methoxyflavones on vincristine uptake and multidrug resistance to vincristine in P-gp-overexpressing K562/ADM cells. Pharm. Res. 2007, 24, 1936–1943. [Google Scholar] [CrossRef]
  37. Juvale, K.; Stefan, K.; Wiese, M. Synthesis and biological evaluation of flavones and benzoflavones as inhibitors of BCRP/ABCG. Eur. J. Med. Chem. 2013, 67, 115–126. [Google Scholar] [CrossRef]
  38. Khan, D.; Parveen, I.; Shaily, S.S. Design, Synthesis and Characterization of Aurone Based α,β-unsaturated Carbonyl-Amino Ligands and their Application in Microwave Assisted Suzuki, Heck and Buchwald Reactions. Asian J. Org. Chem. 2022, 11, e202100638. [Google Scholar] [CrossRef]
  39. Khan, D.; Parveen, I. Chroman-4-one-Based Amino Bidentate Ligand: An Efficient Ligand for Suzuki-Miyaura and Mizoroki-Heck Coupling Reactions in Aqueous Medium. Eur. J. Org. Chem. 2021, 35, 4946–4957. [Google Scholar] [CrossRef]
  40. Joo, Y.H.; Kim, J.K.; Kang, S.-H.; Noh, M.-S.; Ha, J.Y.; Choi, J.C.; Lim, K.M.; Lee, C.H.; Chung, S. 2,3-Diarylbenzopyran derivatives as a novel class of selective cyclooxygenase-2 inhibitors. Bioorg. Med. Chem. Lett. 2003, 13, 413–417. [Google Scholar] [CrossRef]
  41. Prasanna, S.; Manivannan, E.; Chaturvedi, S.C. Quantitative structure–activity relationship analysis of a series of 2,3-diaryl benzopyran analogues as novel selective cyclooxygenase-2 inhibitors. Bioorg. Med. Chem. Lett. 2004, 14, 4005–4011. [Google Scholar] [CrossRef]
  42. O’Brien, D.F.; Gates, J.W., Jr. Some Reactions of 3-Hydroxy-1-phenylpyrazole. J. Org. Chem. 1966, 31, 1538–1542. [Google Scholar] [CrossRef]
  43. Milišiūnaitė, V.; Arbačiauskienė, E.; Řezníčková, E.; Jorda, R.; Malínková, V.; Žukauskaitė, A.; Holzer, W.; Šačkus, A.; Kryštof, V. Synthesis and anti-mitotic activity of 2,4- or 2,6-disubstituted- and 2,4,6-trisubstituted-2H-pyrazolo[4,3-c]pyridines. Eur. J. Med. Chem. 2018, 150, 908–919. [Google Scholar] [CrossRef] [PubMed]
  44. Urbonavičius, A.; Fortunato, G.; Ambrazaitytė, E.; Plytninkienė, E.; Bieliauskas, A.; Milišiūnaitė, V.; Luisi, R.; Arbačiauskienė, E.; Krikštolaitytė, S.; Šačkus, A. Synthesis and Characterization of Novel Heterocyclic Chalcones from 1-Phenyl-1H-pyrazol-3-ol. Molecules 2022, 27, 3752. [Google Scholar] [CrossRef] [PubMed]
  45. Shen, X.; Zhou, Q.; Xiong, W.; Pu, W.; Zhang, W.; Zhang, G.; Wang, C. Synthesis of 5-subsituted flavonols via the Algar-Flynn-Oyamada (AFO) reaction: The mechanistic implication. Tetrahedron 2017, 73, 4822–4829. [Google Scholar] [CrossRef]
  46. Bhattacharyya, S.; Hatua, K. Computational insight of the mechanism of Algar–Flynn–Oyamada (AFO) reaction. RSC Adv. 2014, 4, 18702–18709. [Google Scholar] [CrossRef]
  47. Ferreira, D.; Brandt, E.V.; Volsteedt, F.D.R.; Roux, D.G. Parameters regulating the α- and β-cyclization of chalcones. J. Chem. Soc. Perkin Trans 1975, 1, 1437–1446. [Google Scholar] [CrossRef]
  48. Pati, S.K.; Marks, T.J.; Ratner, M.A. Conformationally Tuned Large Two-Photon Absorption Cross Sections in Simple Molecular Chromophores. J. Am. Chem. Soc. 2001, 123, 7287–7291. [Google Scholar] [CrossRef]
  49. Jutand, A.; Mosleh, A. Rate and Mechanism of Oxidative Addition of Aryl Triflates to Zerovalent Palladium Complexes. Evidence for the Formation of Cationic (.sigma.-Aryl)palladium Complexes. Organometallics 1995, 14, 1810–1817. [Google Scholar] [CrossRef]
  50. Kumar, A.; Rao, M.L.N. Pot-economic synthesis of diarylpyrazoles and pyrimidines involving Pd-catalyzed cross-coupling of 3-trifloxychromone and triarylbismuth. J. Chem. Sci. 2018, 130, 165. [Google Scholar] [CrossRef]
  51. Dahlén, K.; Wallén, E.A.A.; Grøtli, M.; Luthman, K. Synthesis of 2,3,6,8-Tetrasubstituted Chromone Scaffolds. J. Org. Chem. 2006, 71, 6863–6871. [Google Scholar] [CrossRef] [PubMed]
  52. Akrawi, D.A.; Patonay, T.; Kónya, K.; Langer, P. Chemoselective Suzuki–Miyaura Cross-Coupling Reactions of 6-Bromo-3-(trifluoromethylsulfonyloxy)flavone. Synlett 2013, 24, 860–864. [Google Scholar] [CrossRef]
  53. Nuzillard, J.-M. Use of carbon-13 NMR to identify known natural products by querying a nuclear magnetic resonance database—An assessment. Magn Reson Chem 2023, 1–7. [Google Scholar] [CrossRef] [PubMed]
  54. Dolbier, W.R. Guide to Fluorine NMR for Organic Chemists; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2016. [Google Scholar]
  55. Arbačiauskienė, E.; Martynaitis, V.; Krikštolaitytė, S.; Holzer, W.; Šačkus, A. Synthesis of 3-substituted 1-phenyl-1H-pyrazole-4-carbaldehydes and the corresponding ethanones by Pd-catalysed cross-coupling reactions. ARKIVOC 2011, 11, 1–21. [Google Scholar] [CrossRef]
  56. Solum, M.S.; Altmann, K.L.; Strohmeier, M.; Berges, D.A.; Zhang, Y.; Facelli, J.C.; Pugmire, R.J.; Grant, D.M. 15N Chemical Shift Principal Values in Nitrogen Heterocycles. J. Am. Chem. Soc. 1997, 119, 9804–9809. [Google Scholar] [CrossRef]
  57. Williamson, R.T.; Buevich, A.V.; Martin, G.E.; Parella, T. LR-HSQMBC: A Sensitive NMR Technique To Probe Very Long-Range Heteronuclear Coupling Pathways. J. Org. Chem. 2014, 79, 3887–3894. [Google Scholar] [CrossRef]
  58. Barczyński, P.; Szafran, M.; Ratajczak-Sitarz, M.; Nowaczyk, Ł.; Dega-Szafran, Z.; Katrusiak, A. Structure of 2,3-dicarboxy-1-methylpyridinium chloride studied by X-ray diffraction, DFT calculation, NMR, FTIR and Raman spectra. J. Mol. Struct. 2012, 1018, 21–27. [Google Scholar] [CrossRef]
  59. Iwatsuki, S.; Kanamitsu, Y.; Ohara, H.; Kawahata, M.; Danjo, H.; Ishihara, K. Crystal Structure of a Methanesulfonate Salt of 4-(N-Methyl)pyridinium Boronic Acid. X-ray Struct. Anal. Online 2012, 28, 63–64. [Google Scholar] [CrossRef]
  60. Macdonald, A.L.; James Trotter, J. Crystal and molecular structure of o-benzoquinone. J. Chem. Soc. Perkin Trans. 1973, 2, 476–480. [Google Scholar] [CrossRef]
  61. Allinger, N.L.; Chen, K.-H.; Lii, J.-H.; Durkin, K.A. Alcohols, ethers, carbohydrates, and related compounds. I. The MM4 force field for simple compounds. J. Comput. Chem. 2003, 24, 1447–1472. [Google Scholar]
  62. Li, P.; Su, W.; Lei, X.; Xiao, Q.; Huang, S. Synthesis, characterization and anticancer activity of a series of curcuminoids and their half-sandwich ruthenium(II) complexes. Appl. Organomet. Chem. 2017, 31, e3685. [Google Scholar] [CrossRef]
  63. Milišiūnaitė, V.; Arbačiauskienė, E.; Bieliauskas, A.; Vilkauskaitė, G.; Šačkus, A.; Holzer, W. Synthesis of pyrazolo[4′,3′:3,4]pyrido[1,2-a]benzimidazoles and related new ring systems by tandem cyclisation of vic-alkynylpyrazole-4-carbaldehydes with (het)aryl-1,2-diamines and investigation of their optical properties. Tetrahedron 2015, 71, 3385–3395. [Google Scholar] [CrossRef]
  64. Arbačiauskienė, E.; Krikštolaitytė, S.; Mitrulevičienė, A.; Bieliauskas, A.; Martynaitis, V.; Bechmann, M.; Roller, A.; Šačkus, A.; Holzer, W. On the Tautomerism of N-Substituted Pyrazolones: 1,2-Dihydro-3H-pyrazol-3-ones versus 1H-Pyrazol-3-ols. Molecules 2018, 23, 129. [Google Scholar] [CrossRef]
  65. Titi, A.; Messali, M.; Alqurashy, B.A.; Touzani, R.; Shiga, T.; Oshio, H.; Fettouhi, M.; Rajabi, M.; Almalki, F.A.; Hadda, T.B. Synthesis, characterization, X-ray crystal study and bioctivities of pyrazole derivatives: Identification of antitumor, antifungal and antibacterial pharmacophore sites. J. Mol. Struct. 2020, 1205, 127625. [Google Scholar] [CrossRef]
  66. Sharma, S.; Brahmachari, G.; Kant, R.; Gupta, V.K. One-pot green synthesis of biologically relevant novel spiro[indolin-2-one-3,4′-pyrano[2,3-c]pyrazoles] and studies on their spectral and X-ray crystallographic behaviors. Acta Crystallogr. B Struct. Sci. Cryst. Eng. Mater. 2016, 72, 335–343. [Google Scholar] [CrossRef] [PubMed]
  67. Shynkar, V.V.; Mély, Y.; Duportail, G.; Piémont, E.; Klymchenko, A.S.; Demchenko, A.P. Picosecond Time-Resolved Fluorescence Studies Are Consistent with Reversible Excited-State Intramolecular Proton Transfer in 4′-(Dialkylamino)-3-hydroxyflavones. J. Phys. Chem. A 2003, 107, 9522–9529. [Google Scholar] [CrossRef]
  68. Spadafora, M.; Postupalenko, V.; Shvadchak, V.; Klymchenko, A.; Mély, Y.; Burger, A.; Benhida, R. Efficient synthesis of ratiometric fluorescent nucleosides featuring 3-hydroxychromone nucleobases. Tetrahedron 2009, 65, 7809–7816. [Google Scholar] [CrossRef]
  69. Klymchenko, A.S.; Ozturk, T.; Pivovarenko, V.G.; Demchenko, A.P. A 3-hydroxychromone with dramatically improved fluorescence properties. Tetrahedron Lett. 2001, 42, 7967–7970. [Google Scholar] [CrossRef]
  70. Klymchenko, A.S.; Kenfack, C.; Duportail, G.; Mély, Y. Effects of polar protic solvents on dual emissions of 3-hydroxychromones. J. Chem. Sci. 2007, 119, 83–89. [Google Scholar] [CrossRef]
  71. Voicescu, M.; Ionescu, S.; Gatea, F. Effect of pH on the fluorescence characteristics of some flavones probes. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2014, 123, 303–308. [Google Scholar] [CrossRef]
  72. Klymchenko, A.S.; Pivovarenko, V.G.; Ozturk, T.; Demchenko, A.P. Modulation of the solvent-dependent dual emission in 3-hydroxychromones by substituents. New J. Chem. 2003, 27, 1336–1343. [Google Scholar] [CrossRef]
  73. Klymchenko, A.S.; Demchenko, A.P. Multiparametric probing of intermolecular interactions with fluorescent dye exhibiting excited state intramolecular proton transfer. Phys. Chem. Chem. Phys. 2003, 5, 461–468. [Google Scholar] [CrossRef]
  74. Zhao, X.; Liu, Y.; Zhou, L.; Li, Y.; Chen, M. Time-dependent density functional theory study on excited state intramolecular proton transfer of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one. J. Lumin. 2010, 130, 1431–1436. [Google Scholar] [CrossRef]
  75. Chen, L.; Fu, P.-Y.; Wang, H.-P.; Pan, M. Excited-State Intramolecular Proton Transfer (ESIPT) for Optical Sensing in Solid State. Adv. Optical Mater. 2021, 9, 2170097. [Google Scholar] [CrossRef]
  76. Ormson, S.M.; Brown, R.G.; Voller, F.; Rettig, W. Switching between charge- and proton-transfer emission in the excited state of a substituted 3-hydroxyflavone. J. Photochem. Photobiol. A Chem. 1994, 81, 65–72. [Google Scholar] [CrossRef]
  77. Lebeau, B.; Innocenzi, P. Hybrid materials for optics and photonics. Chem. Soc. Rev. 2011, 40, 886–906. [Google Scholar] [CrossRef]
  78. Mohammad-Pour, G.S.; de Coene, Y.; Wiratmo, M.; Maan, A.; Clays, K.; Masunov, A.E.; Crawford, K.E. Modular synthesis of zwitterionic, xanthene bridged, low twist angle chromophores with high hyperpolarizability. Mater. Adv. 2022, 3, 7520–7530. [Google Scholar] [CrossRef]
  79. Andreu, R.; Carrasquer, L.; Santiago Franco, C.; Garín, J.; Orduna, J.; de Baroja, N.M.; Alicante, R.; Villacampa, B.; Allain, M. 4H-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear Optical Chromophores. J. Org. Chem. 2009, 74, 6647–6657. [Google Scholar] [CrossRef]
  80. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. A Complete Structure Solution, Refinement and Analysis Program. J.Appl. Cryst. 2009, 42, 339–341. [Google Scholar] [CrossRef]
  81. Sheldrick, G.M. SHELXT-Integrated Space-Group and Crystal-Structure Determination. Acta Cryst. 2015, A71, 3–8. [Google Scholar] [CrossRef]
  82. Bourhis, L.J.; Dolomanov, O.V.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. The anatomy of a comprehensive constrained, restrained refinement program for the modern computing environment – Olex2 dissected. Acta Cryst. 2015, A71, 59–75. [Google Scholar]
Figure 1. Structural formulas of selected 3-hydroxyflavones and their relative compounds.
Figure 1. Structural formulas of selected 3-hydroxyflavones and their relative compounds.
Molecules 28 06599 g001
Scheme 1. Reagents and conditions: (i) appropriate carbaldehyde, NaOH, EtOH, 55 °C, 3–5 h, in accordance with ref. [44]; (ii) NaOH, EtOH, H2O2, −25 °C, 2 h, then rt, 16 h.
Scheme 1. Reagents and conditions: (i) appropriate carbaldehyde, NaOH, EtOH, 55 °C, 3–5 h, in accordance with ref. [44]; (ii) NaOH, EtOH, H2O2, −25 °C, 2 h, then rt, 16 h.
Molecules 28 06599 sch001
Figure 2. Mechanisms proposed for the transformation reaction of compound 2a to compound 3a.
Figure 2. Mechanisms proposed for the transformation reaction of compound 2a to compound 3a.
Molecules 28 06599 g002
Scheme 2. O-Methylation of compound 3a.
Scheme 2. O-Methylation of compound 3a.
Molecules 28 06599 sch002
Scheme 3. N-Methylation of compound 3h.
Scheme 3. N-Methylation of compound 3h.
Molecules 28 06599 sch003
Scheme 4. Reagents and conditions: (i) TEA, Tf2O, DCM, 0–24 °C, 16 h; (ii) (hetero)aryl boronic acid, K3PO4, KBr, Pd(PPh3)4, dioxane, 90 °C, 16 h (for 8ae); (iii) tert-butyl acrylate, TEA, Pd(PPh3)2Cl2, DMF (dry), 100 °C (for 8f), 72 h; (iv) phenylacetylene, TEA, CuI, Pd(PPh3)2Cl2, DMF (dry), 65 °C, 1 h (for 8g).
Scheme 4. Reagents and conditions: (i) TEA, Tf2O, DCM, 0–24 °C, 16 h; (ii) (hetero)aryl boronic acid, K3PO4, KBr, Pd(PPh3)4, dioxane, 90 °C, 16 h (for 8ae); (iii) tert-butyl acrylate, TEA, Pd(PPh3)2Cl2, DMF (dry), 100 °C (for 8f), 72 h; (iv) phenylacetylene, TEA, CuI, Pd(PPh3)2Cl2, DMF (dry), 65 °C, 1 h (for 8g).
Molecules 28 06599 sch004
Figure 3. Relevant 1H-13C HMBC, 1H-13C LR-HSQMBC, 1H-13C H2BC, 1H-15N HMBC, 1H-1H NOESY, and 1,1-ADEQUATE correlations, as well as 1H NMR (italics), 13C NMR, and 15N NMR (bold) chemical shifts of compounds 3a (DMSO-d6), 4 (DMSO-d6), and 8f (CDCl3).
Figure 3. Relevant 1H-13C HMBC, 1H-13C LR-HSQMBC, 1H-13C H2BC, 1H-15N HMBC, 1H-1H NOESY, and 1,1-ADEQUATE correlations, as well as 1H NMR (italics), 13C NMR, and 15N NMR (bold) chemical shifts of compounds 3a (DMSO-d6), 4 (DMSO-d6), and 8f (CDCl3).
Molecules 28 06599 g003
Scheme 5. 15N NMR (bold) chemical shifts of compounds 3h (DMSO-d6), 5 (DMSO-d6), 9 (TFA-d), and 10 (TFA-d).
Scheme 5. 15N NMR (bold) chemical shifts of compounds 3h (DMSO-d6), 5 (DMSO-d6), 9 (TFA-d), and 10 (TFA-d).
Molecules 28 06599 sch005
Figure 5. ORTEP view of compound 5: (a) asymmetric unit; (b) crystal cell and hydrogen bonds.
Figure 5. ORTEP view of compound 5: (a) asymmetric unit; (b) crystal cell and hydrogen bonds.
Molecules 28 06599 g005
Figure 6. (a) UV–vis absorption spectra of compounds 3ah in MeOH; (b) fluorescence emission spectra (λex = 380 nm) of compounds 3ah in MeOH.
Figure 6. (a) UV–vis absorption spectra of compounds 3ah in MeOH; (b) fluorescence emission spectra (λex = 380 nm) of compounds 3ah in MeOH.
Molecules 28 06599 g006
Figure 7. Depiction of the ESIPT process in 3a.
Figure 7. Depiction of the ESIPT process in 3a.
Molecules 28 06599 g007
Figure 8. (a) UV–vis absorption spectra of compounds 3ah in aprotic solvents; (b) fluorescence emission spectra (λex = 380 nm) of compounds 3ah in aprotic solvents.
Figure 8. (a) UV–vis absorption spectra of compounds 3ah in aprotic solvents; (b) fluorescence emission spectra (λex = 380 nm) of compounds 3ah in aprotic solvents.
Molecules 28 06599 g008
Figure 9. UV–vis electronic absorption spectra of compounds 5 and 6 in THF.
Figure 9. UV–vis electronic absorption spectra of compounds 5 and 6 in THF.
Molecules 28 06599 g009
Table 1. Selected bond lengths [Å] for compound 5.
Table 1. Selected bond lengths [Å] for compound 5.
Atom Atom Length/Å Atom Atom Length/Å
N1N21.3839(12)C4C51.5140(14)
N1C7A1.3233(14)C4O141.2265(14)
N2C31.3458(14)C5C61.4047(15)
N2C81.4287(13)C5O151.2723(13)
C3C3A1.3842(14)C6O71.4135(12)
C3AC41.4411(14)C6C161.4308(14)
C3AC7A1.4017(14)O7C7A1.3387(12)
Table 2. Selected bond angles [°] for compound 5.
Table 2. Selected bond angles [°] for compound 5.
Atom Atom Atom Angle/° Atom Atom Atom Angle/°
C7AN1N2102.23(8)O14C4C5120.99(10)
C3N2N1113.26(8)C6C5C4119.08(9)
C8N2N1119.35(8)O15C5C4116.91(9)
C8N2C3127.39(9)O15C5C6124.01(10)
C3AC3N2106.46(9)O7C6C5123.66(9)
C4C3AC3134.37(10)C16C6C5125.60(10)
C7AC3AC3104.07(9)C16C6O7110.74(9)
C7AC3AC4121.56(9)C7AO7C6116.70(8)
C5C4C3A113.86(9)C3AC7AN1113.97(9)
O14C4C3A125.15(10)O7C7AN1120.98(9)
Table 3. Absorption (λabs of the absorption maxima and ε), fluorescence emission (λN*em, λT*em, ratio IN*/IT* and quantum yield Φf) parameters, and Stokes shifts for 3ah in MeOH (λex = 380 nm); sh = shoulder.
Table 3. Absorption (λabs of the absorption maxima and ε), fluorescence emission (λN*em, λT*em, ratio IN*/IT* and quantum yield Φf) parameters, and Stokes shifts for 3ah in MeOH (λex = 380 nm); sh = shoulder.
EntryComp.λabs (nm)ε × 103 (dm3 mol−1 cm−1)λN*em (nm)λT*em (nm)IN*/IT*Stokes Shift (cm−1)Φf (%)
13a337sh
311
70.89
78.45
4825820.1028927
12491
59.3
23b341sh
317
112.50
119.51
4285860.0095961
12261
42.7
33c355
320sh
240
93.74
66.27
54.91
4465820.1615747
10987
13.4
43d361
311
261
43.82
34.60
23.65
4795800.4066824
10459
52.7
53e353
321sh
310sh
293
245sh
70.76
60.39
57.42
56.73
55.59
4355910.0435340
11408
76.1
63f365
317sh
266
80.34
59.96
41.23
4385820.0464566
10215
55.8
73g360
317
260
138.88
116.75
53.16
4355750.0544789
10386
42.6
83h355sh
329
49.99
61.54
4936110.0317885
11802
13.1
Table 4. Absorption (λabs absorption maxima and ε), fluorescence emission (λN*em, λT*em, ratio IN*/IT*, and quantum yield Φf) parameters, and Stokes shifts for 3ah in aprotic solvents (a THF, b DMF, and c toluene) (*λex = 380 nm); sh = shoulder.
Table 4. Absorption (λabs absorption maxima and ε), fluorescence emission (λN*em, λT*em, ratio IN*/IT*, and quantum yield Φf) parameters, and Stokes shifts for 3ah in aprotic solvents (a THF, b DMF, and c toluene) (*λex = 380 nm); sh = shoulder.
EntryComp.λabs (nm)ε × 103 (dm3 mol−1 cm−1)λN*em (nm)λT*em (nm)IN*/IT*Stokes Shift (cm−1)Φf (%)
13aa339sh
315
58.67
69.15
4665880.0148039
12492
59.2
23ba339
317
240
49.24
53.74
22.77
4285900.0196134
6134
75.5
33ca353
318sh
260
69.71
52.84
30.30
4285910.0054964
11408
19.2
43da362
311
268
58.61
44.19
23.99
4295940.0064314
10789
39.6
53ea352
334sh
295
283
63.18
59.61
46.02
45.73
4165980.0124371
11687
50.6
63fa357
327sh
266
35.90
28.75
15.87
4235930.02343,701
11148
41.2
73ga352
317
262
82.58
75.99
32.57
4215840.0204656
11286
55.0
83ha351sh
335sh
319
40.75
57.39
64.62
4426100.2215866
12097
30.1
93ab338sh
315
64.22
75.82
4285890.0096221
12608
45.5
103ac357sh
338
322
43.31
57.48
62.27
4305810.0044755
10800
67.7
Table 5. Absorption (λabs absorption maxima and ε) of compounds 5 and 6 in THF.
Table 5. Absorption (λabs absorption maxima and ε) of compounds 5 and 6 in THF.
EntryComp.λabs (nm)ε × 103 (dm3 mol−1 cm−1)
155280.55
4990.48
3460.39
2990.42
2610.30
265280.54
4990.49
3480.52
2970.49
2610.37
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Urbonavičius, A.; Krikštolaitytė, S.; Bieliauskas, A.; Martynaitis, V.; Solovjova, J.; Žukauskaitė, A.; Arbačiauskienė, E.; Šačkus, A. Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues. Molecules 2023, 28, 6599. https://doi.org/10.3390/molecules28186599

AMA Style

Urbonavičius A, Krikštolaitytė S, Bieliauskas A, Martynaitis V, Solovjova J, Žukauskaitė A, Arbačiauskienė E, Šačkus A. Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues. Molecules. 2023; 28(18):6599. https://doi.org/10.3390/molecules28186599

Chicago/Turabian Style

Urbonavičius, Arminas, Sonata Krikštolaitytė, Aurimas Bieliauskas, Vytas Martynaitis, Joana Solovjova, Asta Žukauskaitė, Eglė Arbačiauskienė, and Algirdas Šačkus. 2023. "Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues" Molecules 28, no. 18: 6599. https://doi.org/10.3390/molecules28186599

Article Metrics

Back to TopTop