The reaction of unstabilised 1,3,4-oxadiazolium-N-methanide 1,3-dipoles with alkenes: N-substituted Delta-2-pyrrolines: contrast with the 1,3,4-thiadiazolium analogue

Treatment of transient 2,5-diaryl-1,3,4-oxadiazolium-3-methanides, unstabilized 1,3-dipoles, at 60 degreesC with acrylonitrile, methyl acrylate, dimethyl maleate, dimethyl fumarate, chloroacrylonitrile, dimethyl- and diethyl acetylenedicarboxylates, methyl propiolate and butyne-3- one gave in all cases a 1-N-benzamido-substituted pyrrole or pyrroline product. This contrasts with the corresponding 1,3,4-thiadiazolium-methanide 1,3-dipole where the cycloadducts were stable. The reactions involved cycloadditions followed by ring-opening through 1,2- or 1,4-conjugate elimination processes. An X-ray crystal structure is reported on 1-{N-benzoyl-N-[Z-1,2- dimethoxycarbonylvinyl] amino}-2-phenyl-3,4-dimethoxycarbonyl pyrrole, 17.