Ethyl 2-(12-Oxo-10,12-dihydroisoindolo[1,2-b] Quinazolin-10-yl) Acetate

Abstract

The title compound has been synthetized using a one-pot cascade process of ethyl (E)-3-(2-aminophenyl) acrylate and 2-formylbenzonitrile in the presence of Cs₂CO₃ as the catalyst. The synthetic route has been rationalized as a base-catalyzed tandem addition/cyclization/rearrangement initiated by the aniline molecule, followed by sequential aza-Michael addition/dehydrogenation. A theoretical investigation also provided a rationale for its fluorescence properties.

1. Introduction

Fused isoindoloisoquinolines and isoindoloquinolinones are the central cores of various natural alkaloids and synthetic structures with well-recognized biological and pharmacological activities (Figure 1). Several derivatives based on these structures have antibacterial, anti-fungal, anti-inflammatory, antihypertensive, anticonvulsants, sedatives, and antineoplastic properties. [1] It has also been reported that they can reduce oxidative, thermal, or light-induced degradation of organic polymers [2].

Given their significance in chemical, agro-chemical, and pharmaceutical industries, the attainment of these N-fused heterocycles is garnering attention from numerous researchers. As an example, following the discovery of batracylin [3,4] as a compound with potent in vivo and in vitro anticancer activity, the isoindoloquinazolinone nucleus has inspired the synthesis of many derivatives and conjugates (Scheme 1) [5]. Most of the reported structural modifications involved aromatic rings A and D [6,7], the replacement of the nitrogen atom 5 [8], and the functionalization of the amino group at position 8 [9,10]. Position 10 appendage of alkyl or aryl groups has also been reported [11,12].

To the best of our knowledge, there are no data in the literature indicating the installation of more elaborate functional groups at position 10. Nonetheless, this structural modification should be regarded as highly desirable for diversifying such an intriguing polyheterocyclic skeleton.

Here, we report the unexpected formation of isoindole [1,2-b] quinazolin-12-one derivative A with a unique one-pot cascade process using Cs₂CO₃ as the catalyst and without any external oxidant.

2. Results and Discussion

2.1. Synthesis

In the context of our ongoing research project on the synthesis of functionalized isoindolinones having heteroatoms in the third position [13,14,15,16] and their use in sequential reactions, we found that derivative A is directly obtained with a 24% isolated yield and 35% selectivity (with respect to the limiting reagent 1) when (E)-3-(2-aminophenyl) acrylate and 2-CN-benzaldehyde reacted under the conditions reported in Scheme 2. The product has been characterized by mass spectrometry, NMR (¹H, ¹³C, COSY, NOESY, HSQC and HMBC), IR, as well as UV-visible and fluorescence spectroscopies. The complete characterization is reported in the Supplementary Materials.

To rationalize the sequence of the cascade reactions leading to A, a series of control experiments were performed (Scheme 3). Interestingly, when the reaction was carried out with K₂CO₃ instead of Cs₂CO₃, the disappearance of the starting material 2 resulted in the formation of the solely isoindolinone derivative B in a nearly 60% yield and a slight recovering of unreacted aniline 1 (Scheme 3 eq. 1). Despite B being obtained in a moderate yield, no other products were detected in an appreciable amount. Under the assumption that B was a crucial intermediate during the formation of the final isoindoloquinazolinone A, we proceeded with our investigation by submitting isoindolinone B to the reaction conditions reported in Scheme 3 eq. 2. As shown, Cs₂CO₃ smoothly promoted the intramolecular aza-Michael reaction, producing a large majority of the tetracyclic compound C as a mixture 1/1 of diastereoisomers. Despite the use of aerobic conditions, the expected product A was only detected in traces at this stage. Conversely, conversion to A significantly increased when the aza-Michael annulation of B was conducted in the presence of starting aldehyde 2 (Scheme 3 eq. 3). Even if base-catalyzed dehydrogenation involving H in third position is reported [17], our results clearly indicate the involvement of 2 in the oxidative dehydrogenation process leading to the final target A. The aldehyde’s fate has not been thoroughly investigated, however, we still would like to point out that its use in larger quantities (two equivalents) did not produce a significant yield enhancement.

2.2. UV-Vis Absorption and Fluorescence Emission

In view of the increasing interest vs. the development of fluorescent small molecules [18,19], we herein report a preliminary photophysical characterization of isoindoloquinazolinone A by Uv-vis absorption and fluorescence spectroscopy in solution. The experimental spectra were also modelled through quantum-chemical calculations (without the inclusion of the solvent) in order to shed light on the character of the involved electronic transitions (see Computational Details).

The experimental and calculated UV absorption and emission spectra in dichloromethane are reported in Figure 2.

The experimental absorption spectrum shows a peak at 354 nm and a shoulder at 289 nm. Based on the calculated spectrum, which is satisfactorily consistent with the experimental one, we could assign the above peak as characterized by HOMO → LUMO and HOMO-1 → LUMO, respectively (see Figure 3), essentially corresponding to the π-π* transitions also involving an electronic density depletion of the exocyclic carbonyl. The fluorescence spectrum (Figure 2b) is characterized by an intense band at 439 nm. The calculated spectrum, still consistent with the experimental one, has revealed the π*-π nature of this transition dominated by LUMO → HOMO orbitals. Finally, it has to be noted that in both cases, i.e., absorption and fluorescence, the π*-π transitions are characterized by the lack of any relevant electronic density change. This suggests that the nature of the solvent has little effect on the spectral absorption and emission features that are currently under investigation in our laboratory.

3. Materials and Methods

3.1. General Information

The reactions were monitored via thin layer chromatography (TLC) using Merck Silica Gel 60 F254 plates and were visualized through fluorescence quenching at 254 nm. Column chromatographic purification of products was carried out using silica gel 60 (70–230 mesh, Merck, Milan, Italy). The NMR spectra were recorded on Bruker Avance 400 spectrometers (400 MHz, ¹H; 101 MHz, ¹³C). Spectra were referenced to residual CHCl₃ (7.26 ppm, ¹H; 77.00 ppm, ¹³C) or CH₂Cl₂ (5.32 ppm, ¹H; 54.0 ppm, ¹³C) when indicated. Yields are given for isolated products showing one spot on a TLC plate and no significant impurities detectable in the NMR spectrum. High-resolution mass spectra (HRMS) were acquired using a Xevo G2-XS QTof (Waters Corporation, Milford, CT, USA). The samples were ionized in positive ion mode using an electrospray (ESI) ionization source. The IR spectra were recorded on a Perkin-Elmer spectrometer (Spectrum Two FT-IR) equipped with a universal attenuated total reflectance accessory (UATR).

3.2. Materials

All chemicals and solvents were obtained from commercial sources and were used without further purification. Aniline 1 were prepared according to the reported procedures and gave spectra and analytical data as reported [20].

3.3. General Procedure for the Synthesis of Compound A

In a round-bottom flask, ethyl (E)-3-(2-aminophenyl) acrylate 1 (0.100 g, 0.523 mmol), 2-cyanobenzaldehyde 2 (0.096 g, 0.732 mmol), and Cs₂CO₃ (0.170g, 0.523 mmol) are dissolved in acetonitrile (3.5 mL). After 24 h (reaction monitored by TLC), acetonitrile was evaporated under reduced pressure. The residue was directly purified by flash chromatography over silica gel eluting with n-hexane AcOEt (hexane ethyl acetate from 4:1 to 3:2) to produce Ethyl 2-(12-oxo-10,12-dihydroisoindolo [1,2-b] quinazolin-10-yl) acetate (0.040 g, 24% Yield) as a white solid. ¹H-NMR (400 MHz, CD₂Cl₂) δ = 8.05 (dt, J = 7.5, 1.0 Hz, 1H); 7.91–7.84 (m, 1H); 7.72 (dtd, J = 21.4, 7.4, 1.2 Hz, 2H); 7.49–7.43 (m, 1H); 7.36 (ddd, J = 7.7, 4.9, 3.9 Hz, 1H); 7.32–7.22 (m, 2H); 5.74 (dd, J = 5.7, 3.9 Hz, 1H, CH); 3.91 (qd, J = 7.1, 2.1 Hz, 2H, CH₂); 3.20 (dd, J = 15.3, 5.7 Hz, 1H, CH); 2.83 (dd, J = 15.3, 3.9 Hz, 1H, CH); 0.99 (t, J = 7.1 Hz, 3H, CH₃). ¹³C-NMR (101 MHz, CD₂Cl₂) δ = 170.2; 167.4; 149.9; 141.4; 135.3; 133.8; 132.8; 131.1; 129.7; 128.3; 128.3; 127.3; 125.3; 123.8; 122.8; 61.3; 49.3; 41.2; 14.2. HR-MS (ESI) m/z calculated for C₁₉H₁₇N₂O₃ [M+H⁺] 321.1233, found 321.1239.

3.4. General Procedure for the Synthesis of Compound B

In a round-bottom flask, 2-cyanobenzaldehyde 2 (0.075 g, 0.572 mmol), ethyl (E)-3-(2-aminophenyl) acrylate 1 (0.131 g, 0.686 mmol), and K₂CO₃ (0.079 g, 0.572 mmol) are dissolved in acetonitrile (3.8 mL). After 24 h (reaction monitored by TLC), the reaction was diluted with CH₂Cl₂ (5 mL) and evaporated under reduced pressure. The residue was purified with flash chromatography over silica gel eluting with n-Hexane/AcOEt (Hexane/ethyl Acetate from 4:1 to 3:2) to produce ethyl 3-(2-((3-oxoisoindolin-1-yl) amino) phenyl) acrylate B (0.107 g, 58 % Yield) as a yellow solid. ¹H-NMR (400 MHz, CDCl₃) δ = 7.85 (s, 1H), 7.79 (d, J = 15.7 Hz, 1H, CH), 7.63 (d, J = 5.5 Hz, 2H), 7.56 (t, J = 7.2 Hz, 1H), 7.46 (d, J = 7.7 Hz, 1H), 7.33 (t, J = 7.7 Hz, 1H), 7.02 (s, 1H, NH), 6.93 (dt, J = 10.4, 5.8 Hz, 2H), 6.33 (d, J = 15.7 Hz, 1H, CH), 6.13 (d, J = 10.2 Hz, 1H, CH), 4.46 (d, J = 10.2 Hz, 1H, NH), 4.20 (q, J = 7.1 Hz, 2H, CH2), 1.29 (t, J = 7.1 Hz, 3H, CH3). ¹³C-NMR (101 MHz, CDCl₃) δ = 169.6; 166.9; 144.3; 144.0; 139.4; 132.7; 131.9; 131.5; 129.9; 128.6; 124.0; 123.6; 122.7; 120.5; 119.9; 113.9; 66.1; 60.6; 14.3. HR-MS (ESI) m/z calculated for C₁₉H₁₉N₂O₃ [M+H⁺] 323.1396, found 323.1403.

3.5. Computational Details

Quantum chemical calculations were carried out using the framework of Density Functional Theory (DFT) for the ground electronic state and Time-Dependent Density Functional Theory (TD-DFT) [21] for the vertical excited states using the B3LYP functional [22] and 6-311G* basis set. For the modelling of the absorption spectrum, the structure of isoindoloquinazolinone A was optimized in the ground state, and, subsequently, five vertical excited states were evaluated using this structure. Modelling of the emission spectrum was performed by optimizing the structure of isoindoloquinazolinone A in the first excited using TD-DFT. The spectra, both absorption and emission, were then modelled by calculating the relative intensities of the different peaks using the corresponding oscillator strengths and their shapes were estimated using a Gaussian function with an s selected in order to reproduce the experimental spectral width. All the calculations were carried out with the package Gaussian 09 [23]. Details of the optimized structures are reported in the Supplementary Materials.

4. Conclusions

In summary, a valuable highly fluorescent isoindolo[1,2-b] quinazolin-12-one derivative with potential applications in medicinal chemistry was synthesized for the first time through a novel one-pot five reactions process using solely Cs₂CO₃ as the catalyst. Despite its moderate selectivity, general atom economy and external oxidant-free conditions make this synthetic approach sustainable and environmentally friendly.