[1,2]-Wittig rearrangement: Stereochemical features and synthetic utilities

The stereochemical features and synthetic potentials are described of the classic [1,2]-Wittig rearrangement which is now recognized to proceed via the radical cleavage-recombination mechanism. First the [1,2]-Wittig rearrangement of enantio-defined alpha-benzyloxypropyllithium and its (R)-alpha-methylbenzyloxy analogs is shown to proceed predominantly with inversion of configuration at the Li-bearing terminus and retention of configuration at the migrating carbon. Second, the [1,2]-Wittig rearrangement of O-glycosides of both acetal and ketal types, including those derived from D-ribose and D-glucose, is demonstrated to proceed with complete retention of either the alpha- or beta-anomeric configuration, along with efficient stereocontrol over the newly created chiral center on the side chain to afford the novel classes of stereo-defined C-glycosides in good yields.