English

• 1.   Introduction

  2-Thioxoquinazolin-4(1H)-ones are important heterocyclic compounds that widely present in natural products as well as medicinal, and pharmacological compounds [1]. In addition, several thioxoquinazolin analogues have been developed as antitumor, antibiotic, antidefibrillator and antipyretic agent (Fig. 1). Furthermore, they display a broad range of applications for diabetes [2], cancer [3], and selective plant grow regulators [4, 5]. Given the importance of these nitrogen heterocyclic compounds, the development of mild, high yielding and clean synthesis of these important compounds is daunting challenge and has been extensively investigated in literatures [6-12]. The classical methods for the synthesis of quinazolinedione ring system are the reaction of anthranilic acid and their derivatives with isothiocyanates or their equivalents [13-27]. However the difficult handling of unstable isothiocyanate as well as preparation of costly and toxic reagents, such as thiophosgene limits its practical applications. On the other hand, the dithiocarbamates are safe and highly efficient equivalent to isothiocyanate and were found to show broad application owing to their easy preparation in large scale from simple and readily available starting material [28-34]. In this context, our interest in green synthesis of dithiocarbamate derivatives [35, 36], we aimed to develop an efficient and facile approach to synthesize a series of thioxoquinazolin-4(1H)-ones from dithiocarbamate derivatives under the mild and safe conditions.

  图 1

  

  图 1  Selected examples of quinazolin analogues with pharmacological activities.

  Figure 1.  Selected examples of quinazolin analogues with pharmacological activities.

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  2.   Experimental

  2.1   Materials and instrumentation

  All chemicals and solvents were commercially available. All products were confirmed by ¹H NMR, FT-IR spectroscopy and mass spectrometry. ¹H NMR spectra were recorded on 500 or 300 MHz ¹H NMR, ¹³C NMR 125.7 and 75 MHz NMR spectrometer using DMSO-d₆ as a solvent and chemical shifts have been expressed in (ppm) downfield from TMS. Water and ethanol were distilled before used. Melting points were recorded on Buchi 535 melting point apparatus and are uncorrected. FT-IR spectra were determined on a Bruker Vector-22 infrared spectrometer using KBr disks.

  2.2   General procedure for the synthesis of 2-thioxo-2, 3-dihydroquinazolin-4(1H)-one

  To a well stirred solution of anthranilic acid 1 (0.5 mmol) and triethylamine (1.0 mmol) in ethanol (1 mL) was added dithiocar-bamate 2 (0.5 mmol), and the reaction mixture was heated at 60 ℃ until the completion of the reaction (monitored by TLC). Once the reaction mixture has cooled to room temperature, resulted in the precipitation of a white solid, which was filtered, washed with water (10 mL) and recrystallized from ethanol or ethyl acetate to furnish pure 2-thioxoquinazolin-4(1H)-one derivatives without tedious work-up.

  3-Benzyl-2-thioxo-2, 3-dihydroquinazolin-4(1H)-one (3a): ¹H NMR (500 MHz, DMSO-d₆): δ 5.24 (s, 2H, CH₂), 7.19-7.27 (m, 7H, Ar-H), 7.74-781 (m, 2H), 12.01 (brs, 1H, NH); ¹³C NMR (125 MHz, DMSO-d₆): δ 50.4, 116.3, 116.6, 124.9, 126.1, 127.8, 129.3, 140.8, 127.9, 135.8, 140.6, 159.7, 175.2.

  3-Butyl-2-thioxo-2, 3-dihydroquinazolin-4(1H)-one (3b): ¹H NMR (500 MHz, DMSO-d₆): δ 0.94 (t, 3H, J=6.8 Hz), 1.31-1.42 (m, 2H), 1.63-1.72 (m, 2H), 4.40 (t, 2H, J=6.8 Hz), 7.18-7.20 (m, 2H) 7.74 (t, 1H, J=6.8 Hz), 7.96 (d, 1H, J=6.8 Hz), 12.90 (brs, 1H, NH); IR (neat, cm^-1): 3250, 3144, 2955, 2935, 1652, 1626, 1538, 1490, 1340, 1272, 1184, 1128, 990, 798, 758, 690.

  3-Phenyl-2-thioxo-2, 3-dihydroquinazolin-4(1H)-one (3c): ¹H NMR (500 MHz, DMSO-d₆): δ 7.25-7.28 (m, 2H), 7.35 (t, 1H, J=7.7 Hz), 7.40-7.96 (m, 6H), 13.03 (brs, 1H, NH); ¹³C NMR (125 MHz, DMSO-d₆): δ 115.6, 116.1, 124.3, 127.4, 128.0, 128.8, 128.9, 135.5, 139.3, 139.6, 159.7, 176.0.

  3-(4-Butylphenyl)-2-thioxo-2, 3-dihydroquinazolin-4(1H)-one (3d): ¹H NMR (500 MHz, DMSO-d₆): δ 0.89 (t, 3H, J=6.9 Hz), 1.32-1.34 (m, 2H), 1.57-158 (m, 2H), 2.46 (t, 2H, J=6.7 Hz); 7.14-7.24 (m, 8H), 7.30-7.91 (m, 1H), 12.90 (brs, 1H, NH).

  3-Cyclopropyl-2-tioxo-2, 3-dihydroquinazolin-4(1H)-one (3e): ¹H NMR (500 MHz, DMSO-d₆): δ 1.15-1.17 (m, 4H), 2.82-2.83 (m, 1H), 7.33-7.36 (m, 2H), 7.72-7.92 (m, 2H), 12.76 (brs, 1H, NH).

  3-Phenylpropyl-2-thioxoquinazolin-4(1H)-one (3f): ¹H NMR (500 MHz, DMSO-d₆): δ 2.65-2.66 (m, 2H), 3.32 (t, 2H, J=7.1 Hz), 4.44 (t, 2H, J=6.9 Hz), 7.27-7.31 (m, 5H), 7.33-7.37 (m, 2H), 7.73-7.94 (m, 2H), 12.90 (brs, 1H, NH).

  3-[(Furan-2-yl) methyl]-2, 3-dihydro-2-thioxoquinazolin-4(1H)-one (3g): ¹H NMR (500 MHz, DMSO-d₆): δ 5.65 (s, 2H), 6.34-6.37 (m 2H), 7.34-7.39 (m, 2H), 7.53-7.97 (m, 3H, 12.08 (brs, 1H, NH).

  3-(3-Mthoxyphenyl)-2, 3-dihydro-2-thioxoquinazolin-4(1H)-one (3h): ¹H NMR (500 MHz, DMSO-d₆): δ 3.74 (m, 3H), 6.88-7.04 (m, 3H), 7.20-7.35 (m, 3H), 7.43-7.94 (m, 2H), 13.01 (brs, 1H, NH). ¹³C NMR (125 MHz, DMSO-d₆): δ 56.4, 116.2, 116.9, 124.8, 126.5, 127.4, 128.9, 130.4, 133.5, 136.1, 136.9, 138.7, 139.9, 154.1, 16.8, 177.6.

  2-Thioxo-3-(p-tolyl)-2, 3-dihydro-2-thioxoquinazolin-4(1H)-one (3i): ¹H NMR (500 MHz, DMSO-d₆): δ 2.37 (s, 3H), 7.10-7.25 (m, 2H), 7.26-7.38 (m, 3H), 7.45-7.95 (m, 3H), 12.06 (brs, 1H, NH). ¹³C NMR (125 MHz, DMSO-d₆): δ 18.1, 114.8, 115.9, 128.0, 128.7, 129.1, 129.7, 130.9, 132.4, 135.5, 138.4, 139.38, 160.2, 175.8.

  3.   Results and discussion

  Initially, we have investigated the condensation reaction of anthranilic acid 1 and dithiocarbamate 2 [36] as a model reaction in water (Table 1). The results indicate that when the condensation reaction was carried out at room temperature and 100 ℃, it gave lower yields of product even after prolonged reaction time (Table 1, entries 1-2). On the other hand, when the model reaction was carried out in the presence of the triethylamine at reflux condition, the desired product was obtained in moderate yields (58%). A detailed inspection of the reaction showed that other base such as NaOH, KOH, and Na₂CO₃ did not improve the yield of the reaction. Moreover, different organic solvents were further studied. As shown in Table 1, the nonpolar solvents such as CH₂Cl₂ and toluene gave only moderate yields of the products (58% and 55%, respectively), the polar solvents (CH₃CN and DMF) gave much better yields than that of water. However, ethanol can give an excellent isolated yield (95%) for this reaction (Table 1, entry 10) and appeared to be the best media for this reaction in term of yields and product separation. Under optimum conditions, the reaction time was also studied (Table 1, entries 15-18). About 8 h of reaction was sufficient for the completion of reaction. After the completion of the reaction, the reaction mixture was solidified and could be monitored visually. So, the reaction mixture was allowed to cool to room temperature, and the product was isolated by filtration after water addition.

  表 1

  

  表 1  Optimization of reaction condition for the formation of 2-thioxoquinazolin-4(1H)-one.^a

  Table 1.  Optimization of reaction condition for the formation of 2-thioxoquinazolin-4(1H)-one.^a

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  Encouraged by the initial success, we applied the optimal condition to the synthesis of a series of substituted 2-thioxoquinazolin-4(1H)-one and the results were found in Table 2. Various aliphatic and aromatic dithiocarbamates 2 were synthesized from the corresponding amines, carbon disulfide and Michael acceptor and were evaluated for the condensation reaction with anthranilic acid 1. Various dithiocarbamates prepared from different primary aliphatic amines such as benzylamine, butyl amine, 1-phenylethylamine, 2-phenylethylamine, 3-phenylpropylamine and cyclopropylamine reacted with 2-aminobenzoic acid, smoothly under optimized condition to give the corresponding 2-thioxoquinazolin-4(1H)-one in good to excellent yield.

  表 2

  

  表 2  The preparation of 2-thioxo-2, 3-dihydroquinazolin-4(1H)-one derivatives from dithiocarbamates.^a

  Table 2.  The preparation of 2-thioxo-2, 3-dihydroquinazolin-4(1H)-one derivatives from dithiocarbamates.^a

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  Furthermore, dithiocarbamates derived from primary aryl amines such as aniline, 4-methoxyaniline, 4-butylaniline and 4-chloroaniline were also found to react with 2-aminobenzoic acid to afford the desired products in good yield (Table 2). In a similar manner, we also studied the reaction of dithiocarbamates derived from secondary amines, such as pyrrolidine and diethylamine under the same reaction conditions. However, this condensation reaction between 2-aminobenzoic acid and dithiocarbamates did not take place and only starting materials were recovered. As an extension of this work, we carried out the reaction in 2-aminobenzamide and isatoic anhydride with dithiocarbamates using identical strategy. As outlined in Table 2, these gave the corresponding 2-thioxoquinazolin-4(1H)-one 3 in good yield.

  Based on our observations, a possible mechanism is depicted in Scheme 1. Treatment of dithiocarbamate with an excess of trimethylamine in refluxing ethanol resulted in isothiocyanate intermediate A, and then a second addition between A and anthranilic acid derivatives (2-aminobenzoic acid, 2-aminobenzamide and methyl anthranilate that in situ generated from the reaction of isatoic anhydride with ethanol in the presence of Et₃N) occurred, leading to thiourea intermediate B, which underwent intramolecular cyclization and subsequent deamination to furnish the dihydroquinazolin-4(1H)-one products [37-40].

  Scheme 1

  

  图 Scheme 1  Suggested mechanism for the dihydroquinazolin-4(1H)-one synthesis from dithiocarbamate-anthranilic acid derivatives.

  Scheme 1.  Suggested mechanism for the dihydroquinazolin-4(1H)-one synthesis from dithiocarbamate-anthranilic acid derivatives.

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  4.   Conclusion

  In summary, we have demonstrated a conceptually practical synthesis of extremely useful 2-thioxo-2, 3-dihydroquinazolin-4(1H)-one derivatives from readily available dithiocarbamates and 2-aminobenzoic acid in good to excellent yields. This approach used a wide variety of dithiocarbamates prepared from commercially available primary amine substrates in large scale. The products can be isolated by a simple filtration of the reaction mixture without chromatography.

  Acknowledgment

  Financial support of this work by the Chemistry and Chemical Engineering Research Center of Iran is gratefully appreciated.

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• 

  Figure 1  Selected examples of quinazolin analogues with pharmacological activities.

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  Scheme 1  Suggested mechanism for the dihydroquinazolin-4(1H)-one synthesis from dithiocarbamate-anthranilic acid derivatives.

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  Table 1.  Optimization of reaction condition for the formation of 2-thioxoquinazolin-4(1H)-one.^a

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  Table 2.  The preparation of 2-thioxo-2, 3-dihydroquinazolin-4(1H)-one derivatives from dithiocarbamates.^a

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•