Reversal of stereochemistry in enantioselective transformations. Can they be planned or are they just accidental?

Development of new methodology for the preparation of enantiomerically pure compounds (EPCs) is at the forefront in the realm of synthetic chemistry i n the 21(st) century. This is partly in response to the requirement for pha rmaceuticals to be single entities. The preparation of both enantiomers of a target molecule is a challenging endeavor. Of a plethora of methodologies available for the preparation of EPCs, catalytic methods using metal salts as Lewis acids in conjunction with chiral ligands have received the most a ttention. A variety of simple, complex and polymeric ligands have been deve loped for use in asymmetric transformations in which Lewis acids or transit ion metals play a key role. Traditional methods for the synthesis of enanti omeric series of products require chiral starting materials with opposite c onfigurations. However, some chiral sources, for instance sparteine, sugars , amino acids, etc., only one enantiomer is readily accessible. Therefore, the development of general strategies to produce enantiomeric products from a single chiral starting material is important. In principle, access to en antiomeric series of products during chiral Lewis acid catalysis can be ach ieved by careful manipulation of the important components of the reaction: Lewis acid, ligand, and the substrate. Additional factors such as additives and reaction conditions also play a role. Strategies which have been exami ned in detail to access enantiomeric products are: Control of architecture of the ligand-Lewis acid-substrate complex (geometr y control) Structural modifications of the ligand Modification of the substrate using different achiral templates The use of additives This review will focus on recent progress of such examples in asymmetric ca talysis. It is a compilation of results from various labs organized accordi ng to bond formations.