General procedure: 2-amino-6-methylbenzoic acid (1 eq.) and an excess of formamide (6 eq.) were added to a three-necked flask. The reaction mixture was heated at 140 °C for 4-6 h. The reaction progress was monitored by thin layer chromatography. Upon completion of the reaction, ice water was added to the mixture. If the product precipitated, the solid was collected by filtration, washed with water, subsequently dissolved in ethyl acetate, dried over anhydrous MgSO4 and concentrated to give pure 5-methylquinazolin-4(1H)-one. If the product did not precipitate, the reaction mixture was extracted with ethyl acetate and the organic phase was dried and concentrated with anhydrous MgSO4 to give the target product. For the synthesis of amino derivatives 10, 16 and 22: Substituted nitroquinazolin-4(3H)-one derivatives (0.3 g, 1.56 mmol), 6 mL of ethyl acetate and SnCl2-2H2O (2.12 g, 9.42 mmol) were added in a reaction flask and reacted at reflux for 8 hours. After cooling to room temperature, the reaction was quenched with saturated sodium bicarbonate solution and extracted with ethyl acetate (3 x 50 mL). The organic layers were combined, dried and concentrated with anhydrous MgSO4 to give the target amino derivatives. For the synthesis of compounds 26-30, 31a and 32: Substituted o-aminobenzoic acid (1 g) was dissolved in excess acetic anhydride (10 mL) and stirred at room temperature for 4-7 h. The progress of the reaction was monitored by thin layer chromatography. After removal of the solvent under vacuum, the residue was stirred with aqueous ammonia solution for 7 hours. After completion of the reaction, it was extracted with ethyl acetate (3 x 10 mL), the organic layers were combined, dried and concentrated with anhydrous MgSO4 to obtain the target compound. Synthesis of compound 31 was obtained by reduction of 2-methyl-8-nitroquinazolin-4(3H)-one intermediate (31a) with reference to the method reported in Scheme 1.
