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.