Münchnone-Alkene Cycloadditions: Deviations from the FMO Theory. Theoretical Studies in the Search of the Transition State.

The dipolar cycloaddition reactions of 3-methyl-2-(4-nitrophenyl)-4-phenyl-1,3-oxazolium-5-olate (1) and chiral nitroalkenes derived from D-galacto- and D-manno-hept-1-enitols 2 and 3 were found to proceed in a regiospecific manner to afford acyclic pyrrole C-nucleosides (5 and 6) in satisfactory yields. This protocol constitutes a novel and efficient route to such substances. Remarkably, the regiochemistry of this mesoionic-based cycloadditive process is exactly opposite that anticipated from the FMO view of 1,3-dipolar cycloadditions. A preliminary semiempirical PM3 study also reveals the inconsistencies of semiempirical studies with experimental data by applying the FMO approach to münchnone cycloadditions. The structural characteristics of the reagents, products, and transition states have been determined, and this calculation also evaluates the influence of steric and electronic factors involved. Ab initio MO calculations using a model system consisting of 1,3-oxazolium-5-olate with 2-(hydroxymethyl)nitroethylene were also performed. The ab initio study justifies, for the first time, the experimental results of 1,3-dipolar cycloadditions with münchnones. The process occurs through a concerted, slightly asynchronous transition state.