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Communication

A Green Synthesis of 3-Selanyl-Isoflavones via Lipase Mediated Selenylation/Cyclization of Enaminones

1
Key Laboratory of Molecular Enzymology and Engineering of Ministry of Education, School of Life Sciences, Jilin University, Changchun 130023, China
2
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130023, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(7), 413; https://doi.org/10.3390/catal14070413
Submission received: 29 April 2024 / Revised: 18 June 2024 / Accepted: 20 June 2024 / Published: 28 June 2024

Abstract

:
Herein, a green biocatalytic approach using lipase as a catalyst has been developed for the synthesis of 3-selanyl-isoflavones through the selenylation/cyclization of 2-hydroxyphenyl enaminones and diphenyl di-selenide under mild conditions. The environmentally friendly method reached high yields of 87–95% in a short time at 30 °C, with 17 examples of 3-selanyl-isoflavones successfully prepared. Furthermore, we have investigated the possible mechanisms underlying this reaction.

Graphical Abstract

1. Introduction

Flavonoids, although sporadically found in nature, represent one of the most prevalent heterocycles in the pharmaceutical and agrochemical industries [1,2,3]. Flavonoid derivatives exhibit anti-inflammatory, anti-Alzheimer’s, anti-oxidant, and anti-tumor properties [4,5]. The incorporation of selenium moieties into the molecular structures of flavonoids holds immense potential in generating compounds with enhanced biological activity. Consequently, various strategies have been established for constructing organo-selenium compounds [6,7]. Owing to these unique properties, organic researchers have devoted their efforts to integrating selenium elements into flavonoid scaffolds, aiming to establish an efficient strategy for constructing selenium-containing flavonoid derivatives with improved pharmacological features.
As a highly promising compound across multiple fields, substantial research efforts have been devoted to developing various synthesis methods for 3-selanyl-isoflavones. In 2018, Zhu et al. introduced a synthesis method involving the interaction of flavone analogs and various aromatic iodoarene, utilizing KSeCN as a cheap selenium source to obtain ArSe-substituted flavones in good yields (Scheme 1a) [8]. In the same year, Ding’s group reported on the synthesis of 3-selanyl-isoflavones using NIS (N-iodosuccinimide) as a catalyst, along with flavone derivatives and diphenyl di-selenide as key reactants (Scheme 1b) [9]. In 2021, Liu et al. demonstrated a visible-light-promoted synthesis of 3-selanyl-4H-chromen-4-one compounds via the reaction between enaminones and di-selenides (Scheme 1c) [10]. Following that, Xu and colleagues devised a highly effective approach for synthesizing ArSe-substituted chromone derivatives in 2022. This method involved combining selenium powder and aromatic iodides in a one-pot reaction under simple conditions (Scheme 1d) [11]. Subsequently in 2023, Xia’s group also utilized enaminones and di-selenides to establish a synthesis of 3-selenylated chromones, employing select-fluor as a gentle oxidant (Scheme 1e) [12]. Furthermore, Doerner and his team successfully synthesized 3-selanyl-isoflavones in the same year, via a one-pot reaction using the eco-friendly reagent trichloro-iso-cyanuric acid and ethanol as a solvent under mild conditions, with 22 examples in high yields (Scheme 1f) [13]. Despite these advancements, the majority of these approaches rely on costly and environmentally harmful reagents, like metal catalysts, and are conducted under harsh conditions, which contradict the principles of green chemistry. Consequently, there is an urgent demand for the creation of new, mild reaction conditions and highly efficient methods for the synthesis of 3-selanyl-isoflavones [14].
Over the past decades, the biocatalytic oxidation method has emerged as a mature and practical strategy with wide applicability [15]. Compared to conventional methods, the biocatalytic system offers reduced energy consumption and can be conducted under mild reaction conditions. Under this background, urea hydrogen peroxide (UHP), as a biological green oxidant, has captured the interest of numerous researchers, due to its stability at room temperature, affordability, and user-friendly nature compared with traditional oxidants, such as H2O2 [16]. Research indicates that UHP can be utilized in various oxidation reactions in anhydrous organic solvents when combined with carboxylic anhydrides [17], which has also prompted some researchers to delve into its potential influence in related selenization reactions [18,19].
Simultaneously, numerous enzymes, such as hemoproteins [20,21,22,23,24], monooxygenases [25,26,27,28,29], laccases [30,31], and lipases [32,33], can catalyze oxidation reactions. Among these enzymes, lipases have been extensively studied due to their broad substrate compatibility, high stability under extreme conditions, excellent enantio-selectivity, and environmentally friendly characteristics. Our research group, along with other researchers, has reported on lipase-catalyzed oxidation reactions, including epoxidation [34], Dakin reaction [35], and oxidation of amines [36], using various solvents such as water, organic solvents, and ionic liquids [34,37,38]. These studies have clearly demonstrated the feasibility and promising potential of lipase-catalyzed oxidation reactions. The work presented here is part of our ongoing project on biocatalytic oxidation reactions and represents the first report on the lipase-mediated oxidative synthesis of 3-selanyl-isoflavones.

2. Results and Discussion

In this study, ethyl acetate served as the reaction medium and substrate for the in-situ generation of peroxyacetic acid, with urea-hydrogen peroxide (UHP) as the oxidant. Lipase-mediated oxidation utilized o-hydroxyphenyl enaminone 1a and diphenyl di-selenide 2 as the model substrates for synthesizing 3-hydroxy chromone 3a. As illustrated in Table 1, PPL, MML and CSL exhibited extremely poor catalytic activity with yields below 5% (entries 1–3), while Cal-B provided a moderate yield of 3a (67%) in this oxidation (entry 4). However, inactivated Cal-B, whether denatured by heating or PMSF, was unable to facilitate this oxidation (entries 5–6). On the other hand, a significantly high yield (90%) was achieved when Novozym 435, a commercial immobilized form of Cal-B, was utilized in this reaction (entry 7). It was interesting that there was a decline in yield when UHP was replaced with H2O2 (entry 8). At the same time, our control experiments on Novozym 435 demonstrated that the enzyme and UHP are essential components for the reaction, as the absence of either will prevent the reaction from proceeding (entries 9–11). Furthermore, experiments using mixed solvents were conducted, but the yield was significantly lower compared to that in pure ethyl acetate (entries 12–13). Thus, based on these results, Novozym 435 is determined to be the most effective biocatalyst when used in conjunction with UHP in ethyl acetate.
Temperature plays a crucial role in determining the progress of lipase-catalyzed reactions. As shown in Figure 1, the reaction temperature varied from 20 °C to 80 °C to assess its impact. The results showed that enzyme activity continuously increased with the rise of temperature, reaching 90% yield at 30 °C. However, a minimal growth of yield could be observed, though the temperature had already grown up to 60 °C, followed by an obvious plunge over 60 °C. This can be attributed to the fact that higher temperatures enhance the thermal movement of substrate molecules, leading to more collisions and ultimately speeding up the enzyme-catalyzed reaction rate. Nonetheless, excessively high temperatures can also lead to enzyme inactivation, making the yield start to slump. Therefore, the optimal temperature was determined to be 30 °C, as the yield was almost no longer growing (only by about 1%) with increasing temperature.
Novozym 435 is a commonly used immobilized lipase known for its excellent reusability in various practical applications. In this work, the reusability of Novozym 435 was assessed under optimized reaction conditions. After each batch, the filtered Novozym 435 was washed with ethyl acetate and reused in the same reaction. As shown in Figure 2, the yield of 3a remained relatively high (86%) after six cycles, but the yield witnessed a drop afterwards, possibly due to enzyme leakage from the carrier during washing, leading to enzyme loss in the subsequent cycle. Enzyme activity may also be decreased during the biocatalytic process, which is another reason for the decrease of yield.
Under the optimized conditions, the enzymatic method was systematically evaluated using 2-hydroxyphenyl enaminones (1) and diphenyl di-selenide (2), as outlined in Scheme 2. The reactions proceeded smoothly and were minimally affected by electronic or steric factors, resulting in the formation of products 3a–3q with yields ranging from 88% to 95%. The presence of either electron-withdrawing or electron-donating substituents on the phenyl group of 1 did not significantly impact the product yields. These results definitively demonstrate that compound 1, bearing diverse substituents at the 2-, 3-, 4-, and 5-positions of the aromatic rings, effectively participated in the reactions.
Based on the findings of these results and previous references [18,39], we proposed a potential mechanism for the lipase-catalyzed reaction. As shown in Scheme 3, ethyl acetate is transformed in situ into peracetic acid by UHP and lipase, serving as a strong oxidant to convert 2 into organo-seleninic acid I. Then, I can be correspondingly transformed into peroxy-seleninic acid II with further oxidation, which can generate organo-selenenic acid III subsequently. After that, selenic acid III is protonated and converted into highly reactive substance IV, which can promptly attack the C=C bond of 1a, producing selenium ion V. Following this, selenium ion V undergoes nucleophilic substitution and intramolecular cyclization to form VI, ultimately generating the target product 3a through the elimination of dimethylamine.

3. Materials and Methods

3.1. Materials

Cal-B (C. antarctica lipase B), MML (Lipase from Aspergillus niger), CSL (Lipase from Candida sp. 99–125), PPL (Porcine pancreatic lipase). Novozym 435 (protein content: 10%) was purchased from Novo Nordisk Co., Ltd. (Beijing, China). The quantity (per gram of carrier) of protein bound to the carrier in the commercial immobilized lipases were 100 mg for Novozym 435 when determined using a conventional Kjeldahl method. In addition, 2-Hydroxyphenyl enaminones were prepared in the laboratory. Diphenyl di-selenide, urea hydrogen peroxide (UHP) and all other chemical reagents were bought from Bide Pharmatech. Ltd. and Energy-Chemical Ltd. (Shanghai, China). All commercially available reagents and solvents were used as received without additional purification.
Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a 400 MHz spectrometer. Chemical shifts for protons are reported in parts per million downfield from tetra-methyl-silane (TMS) and are referenced to the residual protium in the NMR solvent (CHCl3 = δ 7.26 ppm). 1H NMR data are presented as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad), coupling constant in Hertz (Hz), integration. Mass spectra were recorded on the Bruker MicrOTOF Q II and an Orbitrap FusionTM TribridTM mass spectrometer (Thermo Scientific, San Jose, CA, USA) coupled with HESI ion source. A 100 MHz spectrometer was used to report 13C NMR spectra, while a 76 MHz spectrometer and 376 MHz spectrometer was used to record 77Se NMR and 19F NMR spectra data separately. Data for 13C NMR, 77Se NMR and 19F NMR were reported in terms of chemical shifts (δ ppm). High resolution mass spectra (HRMS) were obtained by use of an Orbitrap Fusion Tribrid mass spectrometer (Thermo Scientific, San Jose, CA, USA) coupled with HESI ion source.

3.2. General Procedure for Lipase-Catalyzed Synthesis of 3-Selanyl-Isoflavones (3)

Novozym 435 (5 mg) was added into a mixture of 2-hydroxyphenyl enaminones (1, 0.2 mmol), diphenyl di-selenide (2, 0.1 mmol) and UHP (0.5 mmol) in ethyl acetate (1 mL) at 30 °C for 2 h.
Following the completion of the reaction, which was monitored by TLC, the mixture was filtered and the residue was washed with ethyl acetate. Subsequently, the mixture underwent separation by TLC with ethyl acetate/petroleum ether as the mobile phase and was dried in a vacuum to obtain the pure solid product (3). All isolated products were well characterized through 1H NMR spectroscopy, and the results were found to be essentially in agreement with previously published literature [9,12,13,40], where the 13C NMR data also could be found. As for the compounds that were not reported before, we further performed analyses on these compounds utilizing 13C NMR, 77Se NMR, and 19F NMR spectroscopy techniques. Moreover, we tested the data for HRMS of all the compounds. The details can be found in our Supplementary Materials (SM).

4. Conclusions

In summary, we have developed a highly efficient synthesis of 3-selanyl-isoflavones through lipase-mediated selenylation/cyclization of 2-hydroxyphenyl enaminones and diphenyl di-selenide in high yields (87–95%). Compared with chemical catalysis, our method is more environmentally friendly, using lipase as a green catalyst, and the reaction can occur under mild conditions (ethyl acetate as a solvent, lipase as a green catalyst, 2 h at 30 °C). Overall, the research conducted on the development of 3-selanyl-isoflavones could lead to the exploration of novel possibilities in enzymatic organic synthesis.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14070413/s1, Figure S1: Experimental Section; Figure S2: Data of Products; Figure S3: 1H NMR, 13C NMR, 19F NMR and 77Se NMR Spectra of Products.

Author Contributions

Investigation, methodology, visualization, writing—original draft, and formal analysis, W.K.; methodology, Y.P.; visualization, W.S.; formal analysis, W.K., C.W., Y.P., W.S. and X.C.; supervision, conceptualization, funding acquisition, and writing review and editing, Z.W. and L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Development Program of Jilin Province (No. 20230101135JC).

Data Availability Statement

Data presented in this study are available in the Supplementary Materials. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. Previous works on the synthesis of 3-selanyl-isoflavones. (a) Zhu’s work; (b) Ding’s work; (c) Liu’s work; (d) Xu’s work; (e) Xia’s work; (f) Doerner’s work.
Scheme 1. Previous works on the synthesis of 3-selanyl-isoflavones. (a) Zhu’s work; (b) Ding’s work; (c) Liu’s work; (d) Xu’s work; (e) Xia’s work; (f) Doerner’s work.
Catalysts 14 00413 sch001
Figure 1. Effect of different temperature on the synthesis of 3-selanyl-isoflavones. Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 2 h.
Figure 1. Effect of different temperature on the synthesis of 3-selanyl-isoflavones. Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 2 h.
Catalysts 14 00413 g001
Figure 2. The reusability of Novozym 435 on the synthesis of 3-selanyl-isoflavones. Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 30 °C, 2 h.
Figure 2. The reusability of Novozym 435 on the synthesis of 3-selanyl-isoflavones. Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 30 °C, 2 h.
Catalysts 14 00413 g002
Scheme 2. Scope of selenylation/cyclization reaction with enaminones and di-selenide via lipase. Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 30 °C, 2 h.
Scheme 2. Scope of selenylation/cyclization reaction with enaminones and di-selenide via lipase. Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), Novozym 435 (5 mg), UHP (0.5 mmol), ethyl acetate (1 mL), 30 °C, 2 h.
Catalysts 14 00413 sch002
Scheme 3. Proposed mechanism for lipase synthesis of 3-selanyl-isoflavones.
Scheme 3. Proposed mechanism for lipase synthesis of 3-selanyl-isoflavones.
Catalysts 14 00413 sch003
Table 1. The effect of enzyme origin and oxidant on the synthesis of 3-selanyl-isoflavones.
Table 1. The effect of enzyme origin and oxidant on the synthesis of 3-selanyl-isoflavones.
Catalysts 14 00413 i001
EntryEnzyme 1OxidantYield (%)
1PPLUHP<5
2MMLUHP<5
3CSLUHP<5
4Cal-BUHP67
5Cal-B 2UHP13
6Cal-B 3UHP9
7Novozym 435UHP90
8Novozym 435H2O271
9Novozym 435None<5
10NoneUHP<5
11NoneNone<5
12Novozym 435 4UHP<5
13Novozym 435 5UHP7
Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), enzyme (5 mg), oxidant (0.5 mmol), ethyl acetate (1 mL), 30 °C, 2 h; 1 PPL (Porcine pancreatic lipase); Cal-B (C. antarctica lipase B); Novozym 435 (a commercial immobilized lipase B from C. antarctica) MML (Mucor miehei lipase); CSL (Candida sp. lipase). 2 Cal-B was obtained by heating to 100 °C for 12 h; 3 Cal-B was inactivated by phenyl-methane-sulfonyl fluoride (PMSF, an inversible inhibitor); 4 Mixed solvents (water/ethyl acetate = 10:1, 1 mL) instead of ethyl acetate; 5 Mixed solvents (methanol/ethyl acetate = 10:1, 1 mL) instead of ethyl acetate.
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MDPI and ACS Style

Kan, W.; Piao, Y.; Song, W.; Chen, X.; Wang, C.; Wang, Z.; Wang, L. A Green Synthesis of 3-Selanyl-Isoflavones via Lipase Mediated Selenylation/Cyclization of Enaminones. Catalysts 2024, 14, 413. https://doi.org/10.3390/catal14070413

AMA Style

Kan W, Piao Y, Song W, Chen X, Wang C, Wang Z, Wang L. A Green Synthesis of 3-Selanyl-Isoflavones via Lipase Mediated Selenylation/Cyclization of Enaminones. Catalysts. 2024; 14(7):413. https://doi.org/10.3390/catal14070413

Chicago/Turabian Style

Kan, Wenbo, Yuming Piao, Wenning Song, Xiaoxuan Chen, Chunyu Wang, Zhi Wang, and Lei Wang. 2024. "A Green Synthesis of 3-Selanyl-Isoflavones via Lipase Mediated Selenylation/Cyclization of Enaminones" Catalysts 14, no. 7: 413. https://doi.org/10.3390/catal14070413

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