Azines and azoles: CXXV. New regioselective synthesis of 1-amino-6-methyluracils

Readily accessible 5-acetyl-4-hydroxy-3,6-dihydro-2H-1,3-thiazine-2,6-dione reacts with substituted hydrazines and carboxylic acid hydrazides under mild conditions to give the corresponding hydrazones. Under severe conditions (heating in boiling dimethylformamide) the reaction is accompanied by extrusion of COS with formation of substituted 1-amino-6-methyluracils. Reactions of 5-acetyl-4-hydroxy-3,6-dihydro-2H-1,3-thiazine-2,6-dione with monosubstituted alkyl-and arylhydrazines take different pathways, depending on the conditions. Heating of equimolar mixtures of the reactants in ethanol or propan-1-ol leads to the formation of 2-substituted 5-methyl-3-oxo-2,3-dihydro-1H-pyrazole-4-carboxamides rather than 1-amino-6-methyluracil derivatives.