MUNCHNONE-ALKENE CYCLOADDITIONS - DEVIATIONS FROM THE FMO THEORY - THEORETICAL-STUDIES IN THE SEARCH OF THE TRANSITION-STATE

The dipolar cycloaddition reactions of l-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 regios pecific manner to afford acyclic pyrrole C-nucleosides (5 and 6) in sa tisfactory yields. This protocol constitutes a novel and efficient rou te to such substances. Remarkably, the regiochemistry of this mesoioni c-based cycloadditive process is exactly opposite that anticipated fro m the FMO view of 1,3-dipolar cycloadditions. A preliminary semiempiri cal PM3 study also reveals the inconsistencies of semiempirical studie s with experimental data by applying the FMO approach to munchnone cyc loadditions. 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-oxazoli um-5-olate with 2-(hydroxymethyl)nitroethylene were also performed. Th e ab initio study justifies, for the first time, the experimental resu lts of 1,8-dipolar cycloadditions with munchnones. The process occurs through a concerted, slightly asynchronous transition state.