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EN
Asymmetric organometallic and organocatalytic processes in aqueous systems are currently of great interest. A few years ago, only a few practitioners studied the subject; now organic reactions in water have become one of the most exciting research areas. Nature has perfected the stereospecific aldol reaction by using aldolase enzymes. While virtually all the biochemical aldol reactions use unmodified donor and acceptor carbonyls and take place under catalytic control in an aqueous environment, the chemical domain of the aldol addition has mostly relied on prior transformation of carbonyl substrates, and the whole process traditionally is carried out in anhydrous solvents. The area of asymmetric aldol reactions in water has received much attention recently in light of the perception of both its green chemistry advantages and its analogy to eon-perfected enzyme catalysis. Only recently catalytic asymmetric reactions promoted by water-compatible Lewis acids with chiral ligands have been developed; most Lewis acids are not stable in water. Seminal work by List, Lerner, and Barbas on the intermolecular proline-catalyzed direct asymmetric aldol reaction opened a new platform for designing metal-free asymmetric catalysts, although their application was initially limited to organic solvents. Most recently, the challenge of developing efficient aqueous-phase organocatalytic processes has also been tackled. Recent progress in the area initiated constructive discussion on the role and practical merits of water as a solvent. This article describes recent developments in this area.
EN
The aldol reaction is one of the most important method for the stereoselective construction of polyketide natural products in both – living organisms and laboratory. The tremendous development in this field has led to development of many new variants of the aldol addition. There has been some success in the use of asymmetric catalysts, although they normally rely on a Mukaiyama-type process. This reaction required a conversion of a donor substrate into more reactive species such as enol silyl ether using not less than stoichiometric amounts of a silicon reagent and a base. From atom economic perspectives, such stoichiometric amounts of reagents should be excluded from the procedures. An exciting challenge in enhancement of the efficiency of the aldol reaction is to find a compound that will catalyze direct aldol addition without pre-formation of a nucleophile and to do so asymmetrically. Direct asymmetric aldol reaction, catalyzed by both metallic complexes and purely organic molecules now becomes one of the most desired tools in organic chemistry. After an initial period of validating methodology by using a wide range of important model reactions, the time has now been reached to address specific synthesis and solve pending problems of practical relevance. In this review we describe recently discovered, most important and most flexible catalysts for direct asymmetric aldol reaction and their application in total synthesis of target natural products and known compounds of biological and pharmaceutical relevance.
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