Catalyst‐controlled enantioselective nitroaldol (or Henry) reactions are presented. These reactions represent powerful C–C bond‐forming tools and the resulting nitro alcohol (nitroaldol) products can be transformed into a number of nitrogen‐ and oxygen‐containing derivatives (nitroalkenes, amino alcohols, amino acids etc.). In addition to substrate‐controlled nitroaldol reactions (not covered), catalytic systems that provide good stereoselectivity have been developed in recent years. Through the use of preformed silyl nitronates as activated substrates, catalytic systems involving metallic Lewis acids or tetraalkylammonium salts have been deployed in fluoride‐mediated nitroaldol reactions. In a further advance, catalytic systems capable of promoting direct nitroaldol reactions between unmodified nitroalkanes and aldehydes (ketones) with high yields and stereoselectivities have been developed. To this end, ambifunctional systems based on La, Zn, Cu, Co and Mg metal species are described, together with some organocatalysts. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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.

[2 + 2] Cycloaddition reactions between ketenes, bearing amino‐, oxy‐, or halo‐ groups, and imines are recognized as being amongst the most important and direct routes to β‐lactams. Alkyl‐substituted ketenes also furnished the corresponding β‐lactams upon reaction with activated imines (iminoesters). In general, ketenes are generated from the appropriate acid chloride and a tertiary amine. The major or sole product of the cycloaddition is usually the cis‐β‐lactam, although a few exceptions showing trans selectivity are known. In this way β‐lactams with a widely varying substitution pattern at the C‐3 and C‐4 positions of the ring are constructed stereoselectively. The diastereoselection of the cycloaddition process can be controlled with variable success from chiral groups attached to either the ketene or the imine component, or alternatively to both. This method, in turn, has proved to be valuable for the synthesis of precursors of important β‐lactam antibiotics, and new successful applications can be expected in the near future.

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).

Organocatalysts based on diarylprolinol ethers mediate a broad range of reactions involving enamine or iminium‐ion activation or a combination of both. The reactions are characterized by efficient steric control of the approach of the respective electrophile/nucleophile to the enamine/iminium‐ion counterpart, thus complementing reactions involving hydrogen‐bond donors as directing key elements.