Abstract
Alkaline treatment of 5-(o-acetamidophenylthio)-4-chloro-2-phenyl-3 (2H)-pyridazinone (II) resulted in the exclusive formation of 3-phenyl-10H-benzo (b) pyridazino-[4, 5-e][1, 4] thiazine-4-(3H)-one (III) via Smiles rearrangement. Isomeric 2-phenyl-10Hbenzo [b] pyridazino [4, 5-e][1, 4] thiazine-1 (2H)-one (IV) was obtained preferentially upon treatment of deacetyl derivative (I) with acid together with II. The structures of III and IV were established both by comparison of their chemical properties and by an unequivocal synthesis of IV. A novel ring contraction of IV to spiro (2-phenyl-4 (3H)-pyrazolone-3, 2'-benzothiazoline)(XIV) was observed.
