Johnson-type acetals derived from dimethyl tartrate give, after opening with Me2BBr and cupratedisplacement, secondary alcohols with high diastereoselectivity (>30:1). The mechanism proposed for theinduction of diastereoselectivity is downstream from the ring fission. It implies a direct participation of theLewis acid as a source of nucleophile and the stereospecific transformation of the resulting bromo acetalthrough an invertive and temperature-dependent process. The acetals are prepared by reaction of thedesired aldehyde with dimethyl tartrate. Removal of the auxiliary is accomplished through SmI2 reductionor by an addition−elimination protocol using methoxide.
