Cycloadditions to Pyrrolo[1,2-c]thiazoles and Pyrazolo[1,5-c]thiazoles

The pyrrolo[1,2-c]thiazole generated by dehydration of dimethyl 5-methyl-1H,3H-pyrrolo[1,2-c]thiazole-6,7-dicarboxylate 2-oxide acts as a thiocarbonyl ylide in its cycloaddition to electron deficient alkenes but as an azomethine ylide with electron deficient alkynes. The analogous pyrazolo[1,5-c]thiazole, generated similarly, acts as a thiocarbonyl ylide with both types of dipolarophile. This behaviour is partially explained by Frontier MO theory.