Cycloaddition Chemistry of Anhydro-4-hydroxy-1,3-thiazolium Hydroxides (Thioisomünchnones) for the Synthesis of Heterocycles

A series of α,α-disubstituted thioisomünchnones were prepared by treating acyclic or cyclic thioamides with bromoacetyl chloride in the presence of triethylamine. The resulting mesoionic dipole was found to undergo bimolecular 1,3-dipolar cycloaddition with several different dipolarophiles. When a hydrogen atom is present in the α-position of the thioamide, the initially formed N-acyl iminium ion undergoes proton loss to produce a S,N-ketene acetal at a faster rate than thioisomünchnone formation. The cycloaddition behavior of several substituted thioisomünchnones which possess a tethered alkene were also examined. The stereochemical outcome of the intramolecular reaction is the consequence of an endo cycloaddition of the neighboring π-bond across the transient thioisomünchnone dipole. Molecular mechanics calculations reveal that the endo diastereomers are significantly lower in energy than the corresponding exo isomers. Attempts to cycloadd the thioisomünchnone dipole across a tethered indolyl ring were unsuccessful. Instead, products derived by nucleophilic cyclization of the indolyl ring onto the thio N-acyl iminium ion were formed in good yield.