Studies on heterocyclic compounds. Part XVI. Crystal and molecular structures of three products of 1,4-dipolar cycloaddition of dimethyl acetylenedicarboxylate to benzothiazole

Abstract

Benzothiazole reacts with dimethyl acetylenedicarboxylate in methanol to give tetramethyl 1H-pyrido[2,1-b]-benzothiazole-1,2,3,4-tetracarboxylate (6) and dimethyl 4-formyl-2,3-dihydro-1,4-benzothiazine-2,3-dicarboxylate (7). A similar reaction of benzothiazole with dimethyl acetylenedicarboxylate in dimethylformamide affords tetramethyl 4aH-pyrido[2,1-b]benzothiazole-2,3,4,4a-tetracarboxylate (8). The structures of these compounds were confirmed by the ¹H and ¹³C n.m.r. and mass spectra and X-ray analyses.