New 1,3,4-thiadiazolium-3-(unsubstituted) methanide 1,3-dipoles (azolium 1,3-dipoles): useful synthons at -60 degrees C: a new ring interconversion route to 1,2,3-trisubstituted and 1,2,3,4-tetrasubstituted 1-{[1-(vinylthio)-1-phenylmethylidene]amino}pyrroles

1,3,4-Thiadiazolium-3-(unsubstituted) methanides have been trapped with substituted alkyne dipolarophiles at -60 degrees C giving high yields of 2,3-di- and 2,3,4-tri-substituted 1-([1-(vinylthio)-1-phenylmethylidene]amino)pyrroles from a cycloaddition rearrangement sequence. The conformation of these molecules results in an unusual proton shielding effect in the H-1 NMR spectra. An X-ray crystal structure is reported for methyl 1-((Z)-1-[(Z)-2-methoxycarbonylvinylthio]-1-phenylmethylideneamino)-2-phenyl-1H-pyrrole-3-carboxylate 7a.