Chiral acyl anion and enolonium ion equivalents. Asymmetric synthesis of α-methoxy-aldehydes

Abstract

The reaction of the lithium derivative of (+)-(S)-p-tolyl p-tolylthiomethyl sulphoxide [(+)-(S)-(1)] with benzaldehyde and phenylacetaldehyde in tetrahydrofuran at –78 °C gives, after methylation of the hydroxy-group, reduction of the sulphoxide, and hydrolysis of the resulting dithioacetal, the corresponding α-methoxy-aldehydes, (–)-(R)-2-methoxy-2-phenylacetaldehyde and (+)-(R)-2-methoxy-3-phenylpropionaldehyde, enantiomeric excess 70 and 46%, respectively. The reaction of (+)-(S)-(1) with benzaldehyde in tetrahydrofuran at room temperature in the presence of Triton B gives the new optically pure reagent (E)-(+)-(S)-1-p-tolysulphinyl-1-p-tolylthio-2-phenylethylene [(+)-(S)-(2)], which is a chiral enolonium equivalent. The addition of sodium methoxide to (+)-(S)-(2) in methanol gives, after reduction of the sulphoxide and hydrolysis of the resulting dithioacetal, (+)-(S)-2-methoxy-2-phenylacetaldehyde, enantiomeric excess 60%. The unfavourable equilibrium in addition of methoxide to (+)-(S)-(2) means that the latter is not very useful as a chiral enolonium equilvalent in reactions of this type.