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.