New 1,3,4-thiadiazolium-3-(unsubstituted) methanide 1,3-dipoles (azolium 1,3-dipoles): Useful synthons at -60°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°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 ¹H NMR spectra. An X-ray crystal structure is reported for methyl 1-{(Z)-1-[(Z)-2-methoxycarbonylvinylthiol-1-phenylmethylideneamino}-2-phenyl-1H- pyrrole-3-carboxylate 7a.