Synthesis and Conformational Study of Chiral Oxepines: The Baylis_Hillman Reaction and RCM Approach with Sugar ldehyde

摘要

he Baylis_Hillman reaction of 3-O-allyl-a-D-xylo-pentodialdo-1,4-furanose 3 afforded a diaster_eomeric mixture of D-gluco- and L-ido-configured a-methylene-_-hydroxy esters 4a and 4b, respectively, in a ratio of 2:3. Reduction of the ester functionality in 4a/4b gave alcohols 5a/5b. The diene thus formed in 5a/5b was subjected to ring-closing metathesis (Grubbs's second-generation catalyst) to afford oxa-bicyclic ring system 6a/6b in high yield. Further manipulation of the acetonide functionality in 6a and 6b afforded new polyhydroxylated oxepines 1a/2a and lb/2b, respectively. The ~1H NMR of oxepines la and lb in D_20 showed doubling of signals indicating their existence in two different rotamers/conformers. This fact was substantiated by calculating energetics of 1 and 2 conformers using the density functional theory and correlating the calculated ~1H NMR chemical shift pattern with that of the experimental spectra.