The effect of electronic properties of substituents in the reactants on the [4+2]-cycloaddition of 3-vinyl-chromen-4-ones (dienes) and N-vinylpyrrolidines (dienophiles) has been studied. The conditions determining the formation of 4,4a-dihydroxanthones or benzophenones as the major products have been found. The aromatization of 4,4a-dihydroxanthones via pyran ring opening has been interpreted by quantum chemical calculations. A wide series of new 4,4a-dihydroxanthone and benzophenone derivatives have been isolated.

Methods for synthesizing new heterocyclic compounds based on 1,3,4-thiadiazoles were developed. It was shown that these compounds possessed pronounced antifungal activity and could be recommended for further investigation as potential antimicrobial agents with fungicidal activity.

Reactions of 2-alkylsulfanyl- and 2-alkoxy-4-hydroxy-6H-1,3-oxazin-6-ones with oxygen-centered nucleophiles were studied. 2-Alkoxy-4-hydroxy-6H-1,3-oxazin-6-ones reacted with water and alcohols to give the corresponding alkyl 3-amino-3-oxopropanoates as a result of opening of the oxazine ring at the C6-O bond, whereas their 2-alkylsulfanyl analogs turned out to be stable toward O-nucleophiles. The different reactivities of the title compounds were interpreted in terms of quantum-chemical calculations of their electronic structure.

Aromatic and heteroaromatic acid hydrazides reacted with oxalyl chloride in benzene or chloroform to give previously unknown 2-aryl(hetaryl)-4H-1,3,4-oxadiazine-5,6-diones whose structure was confirmed by the NMR, IR, and mass spectra. According to the spectral data and the results of quantumchemical calculations, the products exist mainly in the lactam form.

4-Hydroxy-6H-1,3oxazin-6-ones exhibit properties of weak OH acids. These compounds are readily methylated with diazomethane to give the corresponding 4-methoxy derivatives. According to the potentiometric titration data, the pKa values of 2-methoxy-and 2-methylsulfanyl-substituted 4-hydroxy-6H-1,3-oxazin-6-ones range from 7.45 to 8.42, depending on the substituent in position 5 of the heteroring. 4-Hydroxy-6H-1,3-oxazin-6-ones in biological media exist mainly in the neutral form.