Expected and unexpected reactions of 1,3-benzothiazine derivatives, II. Formation of isomeric 5,6-dihydro-1,5-benzothiazocines

A method for the synthesis of medium-sized 1,5-benzothiazocines by ring enlargement of 1,3-benzothiazines has been developed. When N-alkyl-1,3-benzothiazines were reacted with dimethyl acetylenedicarboxylate in acetonitrile at room temperature, regioisomeric 5,6-dihydro-2H-1,5-benzothiazocines and 5,6-dihydro-4H-1,5-benzothiazocines were obtained as novel ring systems. Dimethyl acetylenedicarboxylate could attack either the nitrogen or the sulfur atom of the thiazines. NMR spectroscopic and X-ray crystallographic investigations confirmed the structures of the products.