Reaction of 2‐dimethylaminomethylene‐1,3‐diones with Dinucleophiles. IV. Synthesis of 5,7‐dihydrothiopyrano[3,4‐ c ]pyrazol‐4(1 H )‐ones, 5 H ‐Thiopyrano[4,3‐ d ]isoxazol‐4(7 H )‐one and 6 H ‐Thiopyrano[3,4‐ d ]pyrimidin‐5(8 H )‐ones

The reaction of 2 H ‐thiopyran‐3,5(4 H ,6 H )‐dione with N,N ‐dimethylformamide dimethyl acetal gave in good yield 4‐dimethylaminomethylene‐2 H ‐thiopyran‐3,5(4 H ,6 H )dione (II), which afforded 1‐substituted 5,7‐dihydrothiopyrano[3,4‐ c ]pyrazol‐4(1 H )‐ones with aliphatic and aromatic hydrazines, 5 H ‐thiopyrano[4,3‐ d ]isoxazol‐4(7 H )‐one (IV) with hydroxylamine hydrochloride and 2‐substituted 6 H ‐thiopyrano[3,4‐ d ]pyrimidin‐5(8 H )‐ones with amidines and guanidines, generally in satisfactory yields. 4‐( t ‐Butylhydrazonoformyl)‐2 H ‐thiopyran‐3,5(4 H ,6 H )‐dione was isolated as an intermediate in the reaction of II with t ‐butylhydrazine, whereas formamidine gave with II 4‐iminoformyl‐2 H ‐thiopyran‐3,5(4 H ,6 H )‐dione as the sole product. The isoxazole IV isomerized easily with sodium methoxide to 3,4,5,6‐tetrahydro‐5,5‐dihydroxy‐3‐oxo‐2 H ‐thiopyrano‐4‐carbonitrile.