The unusual thermal transformations of the dimethyl 2,2,3,3-tetramethyl-7a-R-1,2,3,7a-tetrahydroimidazo[1,2-b]isoxazole-6,7-dicarboxylate derivatives

Dimethyl 2,2,3,3-tetramethyl-7a-R-1,2,3,7a-tetrahydroimidazo[1,2-b]-isoxazole-6,7-dicarboxylate derivatives were shown to undergo thermal rearrangement along two competing routes giving rise to dimethyl 2-(2-R-4,5-dihydro-1H-3λ-5-imidazol-3-ylidene)-3-oxosuccinate derivatives and (or) dimethyl-2-oxo-3-(1-R-imidazolidin-2-ylidene)succinates. The latter undergoes intramolecular cyclization producing, in case of N-unsubstituted derivatives, methyl 5,6-dioxo-1-R-2,3,5,6-tetrahydro-1H-pyrrolo[1,2-a]imidazol-7-carboxylates.