Reaction of 4H-3.1-benzothiazine-4-lhiones 1 with S-methylisothiosemicarbazide hydroiodide yields bamino-1.3.4-thiadiazoles 2 insteud of the espected 1.2,4-triazolo[3,2-c]quinazolines. Structures of compounds 2 have been established by mcans of '>C-N M R analysis and X-ray crystallography. 3-Arnino-4(3/f )-quinazolinethiones 4 react with ketcne dithioacetals 5 to give the new mesoionic t,3,~thiadiazolo[3.2-c]quinazolinylium-2-methanides 6 which undergo ring opening by the action or hydrochloric acid to givc z-methylene-1,3.4-thiadiamles 7. Compounds 2 are convcrted into the corresponding oxygen analog 9 by treatment with hydrochloric acid.The reaction of primary amines with 4H-3,1-benzothiazine-4-thiones to give 4(3H)-quinazolinethiones is well-documented. In this context, we have studied the reaction with several bifunctionalized nitrogen nucleophiles giving fused quinazoline derivatives'). On the other hand, we have reported the preparation of mesoionic 1,2,4-triazolo[3,2-c]-quinazolines from 3-amino-4(3H)-quinazolinethiones and aromatic isothiocyanates" or diaryl~arbodiimides~). We report here attempts to synthesize 2-substituted S-aryl-l,3,4-thiadiazoles by two approaches: a) from 4H-3,l-benzothiazine-4-thiones and S-methylisothiosemicarbazide to give 2-amino-l,3,4-thiadiazoles and b) via mesoionic 1,3,4-thiadiazolo[3,2-c]quinazolines to give 2-methylene-l,3,4-thiadiazoles.In general, the most common procedure for the synthesis of 5-substituted 2-amino-l,3,4-thiadiazoles is the acylation of thiosemicarbazide or derivatives followed by dehydration under acidic conditions4). However, no general useful procedure for the preparation of 2-methylene-l,3,4-thiadiazoles has hitherto been reported, it has only been briefly mentioned 5,6) the preparation of mesoionic heptafulvenes derived from the 1,3,4-thiadiazole ring system.Our approach to the preparation of 5-aryl-2-amino-1,3,4-thiadiazoles is based on the ring opening of 4H-3,l-benzothiazine-Cthiones by the action of thiosemicarbazide derivatives. Thus, the reactions between 1 and S-methylisothiosemicarbazide hydroiodide7) in ethanolic solutions in the