jo981675b_si_001.pdf (1.34 MB)
Download file

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

Download (1.34 MB)
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.

History