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 in the 21(st) century. This is partly in response to the requirement for pharmaceuticals 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 attention. A variety of simple, complex and polymeric ligands have been developed for use in asymmetric transformations in which Lewis acids or transition metals play a key role. Traditional methods for the synthesis of enantiomeric series of products require chiral starting materials with opposite configurations. 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 enantiomeric series of products during chiral Lewis acid catalysis can be achieved 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 examined in detail to access enantiomeric products are: Control of architecture of the ligand-Lewis acid-substrate complex (geometry 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 catalysis. It is a compilation of results from various labs organized according to bond formations.