In the doctoral thesis 2,3-disubstituted (1 – 130) and 3-substituted quinazolin-4(3H)-one derivatives (130 – 136) are synthesized in deep eutectic solvents to investigate their application in the synthesis of heterocycles. Syntheses of quinazolin-4(3H)-one derivatives were performed conventionally, by microwave-assisted and ultrasound-assisted synthesis and mechanochemical reactions as well. Deep eutectic solvents for the synthesis of new compounds were prepared by mixing of choline chloride (ChCl) with various amides, carboxylic acids, carbohydrates and alcohols in a specific molar ratio. The optimization of the reaction conditions for each class of compounds was carried out on the selected model reaction by choosing the most favourable deep eutectic solvents, volume, temperature and reaction time. The influence of the amount of balls in mechanochemical synthesis was also optimized. 2-mercapto-3-substituted quinazolin-4(3H)-one derivatives (1 – 50) were synthesized in ChCl: urea deep eutectic solvent by reaction of anthranilic acids and aliphatic and aromatic isothiocyanates conventionally with yields 19 – 76%, by microwave-assisted synthesis (10 – 49%), by ultrasound-assisted synthesis (17 – 64%) and by mechanochemical synthesis (48 – 88%). 2-methyl-3-substituted quinazolin-4(3H)-one derivatives (51 – 87) were synthesized by the reaction of 2-methylbenzoxazin-4-one and aromatic amines and coumarinyl hydrazides and pyridine-2,6-dicarbohydrazide in ChCl: urea conventionally with yields 17 – 77%, by microwave-assisted synthesis (10 – 47%), by ultrasound-assisted synthesis (15 – 56%) and by mechanochemical synthesis (45 – 87%). Quinazolin-4(3H)-one Schiff bases (88 – 130) were synthesized in ChCl: malonic acid from 3-amino-2-methylquinazolin-4(3H)-ones and aromatic aldehydes conventionally with yields 63 – 97%, by microwave-assisted synthesis (39 – 68%), by ultrasound-assisted synthesis (40 – 80%) and by mechanochemical synthesis with high yields 79 – 97%. 3-substituted quinazolin-4(3H)-one derivatives (130 – 136) were detected in traces obtained by a three-component reaction of anthranilic acids, trimethyl orthoformate and aromatic amines. The best yields of the reactions in the synthesis of 2,3-disubstituted quinazolin-4(3H)-one derivatives (1 – 130) were obtained by mechanochemical synthesis, which is characterized by the shortest reaction time and the purity of the isolated products. Lower yields were obtained with conventional synthesis, ultrasound-assisted synthesis, and the lowest yields were achieved by microwave-assisted synthesis. With the use of deep eutectic solvents in the synthesis, the harmful impact of organic solvents and catalysts on the environment is reduced, the reaction time is shorter, the isolation and purification of products are simple, and the recycling of solvents is enabled. The structures of all synthesized compounds were confirmed by 1H- and 13C-NMR spectroscopy and mass spectrometry.