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 azometh
ine ylide with electron deficient alkynes. The analogous pyrazolo[1,5-c]thi
azole, generated similarly, acts as a thiocarbonyl ylide with both types of
dipolarophile. This behaviour is partially explained by Frontier MO theory
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