Construction of 6,10-syn- and -anti-2,5-Dioxabicyclo[2.2.1]heptane Skeletons via Oxonium Ion Formation/Fragmentation: Prediction of Structure of (E)-Ocellenyne by NMR Calculation.

A highly efficient and stereoselective route to potential synthetic intermediates for ocellenyne and related C acetogenin natural products with 6,10-syn- and 6,10-anti-2,5-dioxabicyclo[2.2.1]heptane core structures has been developed by means of an iterative biogenesis-inspired oxonium ion formation/fragmentation sequence. In accordance with chemical transformations, the most likely stereostructure for (E)-ocellenyne on the basis of GIAO C NMR calculations possesses a 6,10-anti-2,5-dioxabicyclo[2.2.1]heptane core, as predicted from a plausible biosynthetic pathway, instead of the spectroscopically proposed 6,10-syn-2,5-dioxabicyclo[2.2.1]heptane skeleton.