Control of stereochemistry during aldol addition reactions has attracted considerable interest over the years as the aldol reaction is one of the most fundamental tools for the construction of new carbon–carbon bonds. Several strategies have been implemented whereby eventually any single possible stereoisomeric aldol product can be accessed by choosing the appropriate procedure. With earlier methods, stoichiometric quantities of chiral reagents were required for efficient asymmetric induction, with the auxiliary most often attached covalently to the substrate carbonyl. Lewis acid catalyzed addition reactions of silyl enolates to aldehydes (Mukaiyama reaction) later opened the way for catalytic asymmetric induction. In the last few years, both chiral metal complexes and small chiral organic molecules have been found to catalyse the direct aldol addition of unmodified ketones to aldehydes with relatively high chemical and stereochemical efficiency. These techniques along with the more recent developments in the area are discussed in this tutorial review.

Chiral, metal-free Brønsted bases have been demonstrated capable of catalyzing several types of C–C and C–X bond-forming reactions with high chemical and stereochemical efficiency, thereby complementing other major organocatalytic asymmetric approaches such as enamine catalysis. This critical review describes the most fundamental developments in the area, which have been documented in the last couple of years, along with major mechanistic implications. Discussion includes the limitations of the present protocols, identifies the most salient gaps, and gives a prospect of future improvements and applications (122 references).