Addition of achiral allylstannanes to achiral aldehydes in the presence of Yamamoto's chiral (acyloxy)borane catalyst (III) and trifluoroacetic anhydride affords homoallylic alcohols with high diastereo- and enantioselectivity. For example, crotonoaldehyde (1c) and (E)-tributyl(2-methyl-2-pentenyl)stannane (2) gave (3S,4S,5E)-3-ethyl-2-methyl-1,5-heptadien-4-ol (3c) in 85% yield and 95% enantiomeric excess.