Crossed aldol condensation stereoselective

The stereochemical course of several Co2(CO)6-mediated reactions has been studied. For example, although alkynyl aldehydes undergo crossed aldol condensation with trimethylsilyl enol ethers with little stereoselectivity, their hexacarbonyldicobalt derivatives react with moderate to excellent syn diastereoselectivity.96 101 The mechanism behind this selectivity has not been fully elucidated and is complicated by the lluxional nature of the intermediate cations. This stereoselective reaction has been successfully applied to the synthesis of /3-lactam antibiotics.100... 

Stereodefined 9-BBN enolboranes which contain a directing chiral auxiliary undergo highly selective crossed aldol condensations as do other dialkylboryl systems (eq 40). The conjugate addition of B-Br-9-BBN also produces enolboranes which condense with aldehydes to produce, after the elimination of the elements of B-HO-9-BBN, a-bromomethyl enones stereoselectively (eq41). ... 

Bivalent tin enolates may also be used in cross-aldol condensations between two ketones. In reactions of tin enolates with aliphatic ketones, little stereoselectivity is observed, but with aromatic ketones (such as acetophenone), the major diastereoisomer formed is the threo-xsomtt [equation (51)]. The reaction... 

Optically active -silyl acetic acid and siLylmethylamines have been made to determine chiral amines and acids respectively, ol-silyl esters and PhCHO used in cross aldoL condensation with moderate stereoselectivity, while... 

The cross-aldol reaction is actively studied with the aim of improved control of the stereoselectivity.58 The use of silyl enol ethers for a condensation with aldehydes constitutes important progress, but catalysis by Lewis acids can be unsatisfactory for acid labile substrates, and the predominant anti stereoselectivity is not always optimal. An attempt was made to solve this problem by running the reaction sonochemically in the presence of alumina, without any solvent.59 products are absent, and the anti-isomer forms predominantly (Eq. 16). 

The aldol reaction has long been recognized as one of the most useful synthetic tools. Under classical aldol reaction conditions, in vhich basic media are usually employed, dimers, polymers, self-condensation products, or a,j5-unsaturated carbonyl compounds are invariably formed as byproducts. The lithium enolate-mediated aldol reaction is regarded as one useful synthetic means of solving these problems. Besides the vell-studied aldol reaction based on lithium enolates, very versatile regio- and stereoselective carbon-carbon bond forming aldol-type reactions have been established in our laboratory by use of boron enolates (1971), silicon enolates-Le vis acids (1973), and tin(II) enolates (1982). Here we describe the first t vo topics, boron and silicon enolate-mediated crossed aldol reactions, in sequence. 

In other studies, analysis of the products of reaction between formaldehyde and guanosine at moderate pH shows a new adduct—formed by condensing two molecules of each reactant—which has implications for the mechanism of DNA cross-linking by formaldehyde,17 while the kinetics of the mutarotation of N-(/ -chlorophcnyl)-//-D-glucopyranosylamine have been measured in methanolic benzoate buffers.18 For a stereoselective aldol reaction of a ketene acetal, see the next section. 

Importantly, with a-silyloxy acetaldehyde, the syn aldol is the major dimer (threose derivative). Thus, applying Mukaiyama condensations with 27 (see O Scheme 23), hexoses such as idose, gulose, and galactose can be prepared. A highly stereoselective protocol for the cross coupling of aldehydes and ketones with a-thioacetal aldehydes has been developed... 

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