Asymmetric Aldol Reaction of Allenoates: Regulation for the Selective Formation of Isomeric Allenyl or Alkynyl Aldol Adduct

A highly stereoselective synthesis of 3-butynyl-threo-aldol adducts is achieved from the reaction of allyl allenoate with a chiral bromoborane in the presence of iPr₂NEt, followed by addition of BF₃·OEt₂ as an additive to scavenge excess base and then aldehydes, whereas isomeric allenyl aldol adducts are formed in the absence of a Lewis acid additive from methyl allenoate.