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Samarium Ion-Promoted Cross-Aldol Reactions and Tandem Aldol/Evans−Tishchenko Reactions

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journal contribution
posted on 15.01.1999, 00:00 by Ling Lu, Hung-Yu Chang, Jim-Min Fang
Cross-aldol reactions of carbonyl compounds were achieved by the catalysis of SmI2 or SmI3, together with molecular sieves, at ambient temperature. 1,3-Dichloroacetone and 1-chloroacetone can be used as acceptor substrates in the cross-aldol reactions with donor substrates such as acetone, cyclopentanone, and cyclohexanone. The cross-aldol reactions with (R)-glyceraldehyde acetonide gave optically pure compounds 2532, the stereochemistry of which was in agreement with a chairlike chelate transition state of dipolar mode. SmI2−molecular sieves or SmI3−molecular sieves also functioned as effective Lewis acids to catalyze tandem aldol/Evans−Tishchenko reactions. The aldol/Evans−Tishchenko reactions of methyl ketones with aldehydes occurred at 0 °C to give α,γ-anti diol monoesters 53a59a. When the reactions were conducted at room temperature, a certain degree of transesterification took place. The aldol/Evans−Tishchenko reactions of ethyl or benzyl ketones with aldehydes yielded α,β-anti−α,γ-anti diol monoesters 60a65a. However, the aldol/Evans−Tishchenko reactions of cyclic ketones with benzaldehyde occurred with a different stereoselectivity to give α,β-syn−α,γ-anti diol monoesters 66a76a. The structures of products were determined by chemical and spectroscopic methods including an X-ray diffraction analysis of 72a derived from the reaction of 4-tert-butylcyclohexanone and benzaldehyde. A reaction mechanism involving dissociation−recombination of aldols followed by intramolecular stereoselective hydride shift is proposed, based on some experimental evidence, to explain the dichotomous stereoselectivity using acyclic or cyclic ketones as the reaction substrates.