Asymmetric [2,3]-Wittig rearrangement as a general tool for asymmetric synthesis
Recent advances in the asymmetric (2,3)-Wittig rearrangement as a general tool for asymmetric synthesis of homoallylic alcohols are described. First, the synthetic utilities of the asymmetric transmission type and the two asymmetric induction types are demonstrated, including applications in asymmetric syntheses of natural products and steroid side chains. Second, some examples are presented of the enantioselective versions involving a chiral ligand-bound boron enolate and organolithium as the migrating terminus. Finally, the mechanistic grounds of the asymmetric versions are discussed on the basis of the stereochemical analyses of the asymmetric version involving an enantiomerically-defined carbanion terminus. The (2,3)-Wittig rearrangement is a special class of (2,3)-sigmatropic rearrangement which involves an a- oxy carbanion as the migrating terminus to afford various types of homoallylic alcohols. As can be seen in the general formula (eq. l), this type of carbanion rearrangement possesses synthetically valuable features, including (a) the regiospecific carbon-carbon bond formation with allylic transposition of the oxygen function, (b) the stereoselective formation of a new olefinic bond, and (c) the stereoselective creation of vicinal chiral centers. Thus, the (2,3)-Wittig rearrangement currently enjoys widespread application in many facets of organic synthesis, particularly in the context of acyclic stereocontrol and natural product synthesis (ref. 1). The most significant feature is its ability of efficient diastereocontrol over the newly-created chiral centers through the proper choice of the combination of G group and substrate geometry. Some of the highly diastereoselective variants thus developed are shown below which provide a higher than 95% of either threo or erythro diastereoselectivity (refs. 1, 2).