Recent advances in asymmetric Wittig-type reactions

Reactions in which no new sp(3) stereocenters are formed can nevertheless be used for asymmetric synthesis if substrates are employed which contain either a stereogenic unit or another prostereogenic unit apart from the reacting functional group. In this way, asymmetric versions of such powerful reactions as the Wittig reaction and its variants can be developed. Recently, the interest in this class of asymmetric reactions has grown, and several useful new reagents and methods have been reported. In this review, a short discussion of different substrate types is followed by an overview of the results obtained in various asymmetric Wittig-type reactions. Despite the profound importance of the Wittig reaction and its variants in synthetic chemistry, there still remain mechanistic details which have not been completely elucidated. In some cases, however, structure determinations on intermediates isolated from asymmetric alkenylation reactions have given mechanistic insights. These observations are discussed, and an attempt is also given to rationalize results obtained in reactions involving reagents in which 8-phenylmenthol has been used as a chiral auxiliary. Finally, the possibilities for developing catalytic asymmetic Wittig-type reactions are briefly discussed.