Cyclization/[3+2]-Cycloaddition Reaction of N′-(2-Alkynylbenzylidene) Hydrazides

Significance

Reported is the reaction of N′-(2-alkynylbenzylidene)hydrazides with dimethyl acetylenedicarboxylate either under AgOTf catalysis or in the presence of halides. Several Lewis acids were screened for the former, but silver triflate afforded the best results yielding isoquinoline-based azomethine ylides A in mediocre to excellent yields. In trying to mediate the reaction with an electrophile, the authors were successful in obtaining a similar product B utilizing bromine, but stumbled upon the transformation to the fused pyrazolo-1,2-dihydroisoquinolines (C) when iodine was applied. The yields were generally good, starting materials are easily formed from the condensation of 2-alkynylbenzaldehydes with hydrazine, but the substrate scope in both cases was poorly studied. A mechanism was suggested but without support.