Highly Enantioselective Alk‐2‐enylation of Aldehydes through an Allyl‐Transfer Reaction

Enantiopure homoallylic alcohols such as 1 (conveniently prepared from alk-2-enyl metal reagents with (−)- or (+)-menthone) serve as alk-2-enyl donors to aldehydes. The allyl-transfer reaction, which proceeds via a chairlike six-membered-ring transition state, gives the corresponding α adduct of the homoallylic alcohols, 2, in good yields with >99 % ee. pTSA=p-toluenesulfonic acid.