Stereoselective 1,2-additions of alpha-alkoxymethyllithiums to aldehydes.

[reaction: see text]. A chiral derivative of tributylstannylmethanol, readily prepared from L-valine, undergoes Sn-Li exchange to provide an alpha-alkoxyorganolithium that adds to aldehydes with up to 91:9 dr. The diastereoselectivity depends on the solvent and alkyllithium used for transmetalation. Treatment of adducts with acid allowed recovery of the chiral auxiliary and diol with complete stereochemical integrity.