1,4,5-Thiadiazepines .2. Novel Ring Contraction Giving 1,4-Thiazine Derivatives

Cyclization of methyl (phenacylsulphonyl)acetate ( 3a ) and methyl 2-(phenacylsulphonyl)-propionate ( 3b ) with hydrazine gave 6-phenyl-2,3,4,7-tetrahydro-1,4,5-thiadiazepin-3-one 1,1-dioxide ( 4a ) and its 2-methyl derivative ( 4b ), respectively. The new thiadiazepines, unlike previously described derivatives, did not undergo desulphonylation when heated in various solvents. Brief reflux of 4a in acetic anhydride gave the O-acetyl derivative ( 6 ), while prolonged reflux resulted in extrusion of one nitrogen atom from the 7-membered ring. The mechanism of the ring contraction is discussed in terms of a bicyclic diaziridine intermediate ( 13 ).