Diastereoselective additions of chiral vinylzinc reagents to α-chiral aldehydes

Additions of vinylic zinc bromide reagents to alpha-chiral aldehydes (R-1 = CH2OTBS, R-2 = Me; R-1 = Me, R-2 = OTBS) in the presence of lithiated (+) or (-) N-methylephedrine proceed with predominant reagent control to afford anti or syn adducts stereoselectively, except when the aldehydes possess an alkoxy substituent at the alpha or beta-positions (R-1 = Me, R-1 = OBn; R-1 = CH2OBn, R-2 = Me), in which case chelation-controlled adducts predominate.