Although the reaction of heterosubstitutedallyliccarbanions (2) with aldehydes generally produces a mixture of the γ- and α-adducts (4 and 3), syn-3 and anti-3 can be prepared either exclusively or predominantly by the proper choice of the organometallic compound added.
Regio- and stereo-selective coupling between trimethylsilyl allyl carbanions and aldehydes is achieved in the presence of an additive, ‘M’; use of R2BCl or EtAlCl2 as the additive gives the threo isomer (3) predominantly, while use of Bu3SnCl–BF3 affords the erythro isomer exclusively.