The addition of dimethyl and diethyl acetylendicarboxylate (DMAD) and (DEAD
) to 6-methyl-1,2,4-triazine-5-(4H) thione-3-(2H)-one afforded 2-methoxycar
bonyl and 2-ethoxycarbonyl-9-methyl-1,3-thiazino [2,3-d]1,2,4-triazine 4,6-
dione (2a) respectively.