Addition of lithiated 5-hydroxymethyl-1,3-dithiane to benzaldehyde: HMPA-controlled trans stereoselectivity.
Addition of 5-substituted dithianyl anions to carbonyl compounds normally produces trans adducts. The presence of a nucleophilic hydroxymethyl group in position 5 dramatically decreases the trans stereoselectivity of the reaction in THF. The trans/cis ratio shows a bell curve dependence on HMPA, fitted to a quantitative model involving a series of equilibrated ion pairs, of which an intermediate contact ion pair possessing three (effective) HMPA molecules yields the trans adduct with much higher stereoselectivity. [structure: see text]
Li
Zaiguo
Z
Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208-2436, USA.
Kurchan
Alexei N
AN
Kutateladze
Andrei G
AG