Azines and azoles: CXXIX. Synthesis,structure, and some reactions of 2-aryl-4-sulfanyl-6H-1,3-thiazin-6-ones

Reactions of 2-aryl-4-chloro-6H-1,3-thiazin-6-ones with sodium sulfide in aqueous alcohol at 18–20°C led to the formation of a readily separable mixture of 2-aryl-4-sulfanyl-6H-1,3-thiazin-6-one sodium salts (yield >70%) and bis(2-aryl-6-oxo-6H-1,3-thiazin-4-yl) sulfides (<10%). The latter can also be obtained in more than 50% yield by treatment of 2-aryl-4-sulfanyl-6H-1,3-thiazin-6-one sodium salts with 2-aryl-4-chloro-6H-1,3-thiazin-6-ones. Methylation of 2-aryl-4-sulfanyl-6H-1,3-thiazin-6-ones afforded the corresponding methylsulfanyl derivatives (yield >90%) regardless of the alkylating agent, solvent, temperature, reactant concentration, and their ratio. 2-Aryl-4-sulfanyl-6H-1,3-thiazin-6-ones in the crystalline state and in solutions in polar and nonpolar protic and aprotic solvents exist preferentially as 4-sulfanyl-6-oxo tautomers, and they undergo almost complete ionization in neutral aqueous, alcoholic, and aqueous-alcoholic media (pK _a = 4.3). Reactions of 4-sulfanyl-2-phenyl-6H-1,3-thiazin-6-one with ammonia, amines, and difunctional N-centered nucleophiles involve cleavage of the C⁶-S bond in the thiazine ring and subsequent recyclization of linear intermediates to pyrimidines and diazole derivatives. The structure of the isolated compounds was confiirmed by ¹H and ¹³C NMR, IR, and UV spectra.