AbstractThe reaction of 6‐chloro‐2‐[1‐methyl‐2‐(Mmemylthiocarbamoyl)hydrazino]quinoxaline 4‐oxide 5 with acetic anhydride or trifluoroacetic anhydride resulted in dehydrative cyclization to give 2‐(N‐acetyl)‐memylamino‐8‐chloro‐4‐methyl‐4H‐1,3,4‐thiadiazino[5,6‐b]quinoxaline 6 or 8‐chloro‐2‐(N‐trifluoroacetyl)methylamino‐4‐methyl‐4H‐1,3,4‐thiadiazino[5,6‐b]quinoxaline 9, respectively. The oxidation of compound 6 or 9 with 2‐fold molar amount of m‐chloroperbenzoic acid afforded the 4H‐1,3,4‐thiadiazino‐[5,6‐b]quinoxaline 1,1‐dioxide 8 or 13, respectively. The acetyl group of compound 6 was hardly hydrolyzed, but the trifluoroacetyl group of compound 9 was easily hydrolyzed to change into 8‐chloro‐4‐methyl‐2‐memylamino‐4H‐1,3,4‐thiadiazino[5,6‐b]quinoxaline 10. The acylation of compound 10 with acetic anhydride, trifluoroacetic anhydride, phenyl isocyanate, and chloroacetyl chloride furnished the 2‐(N‐acetyl)methylamino 6, 2‐(N‐trifluoroacetyl)methylamino 9, 2‐(1‐methyl‐3‐phenylureido) 11, and 2‐(N‐chloroacetyl)methylamino 12 derivatives, respectively.