New route to alpha-adducts of homoallylic alcohols by an acid-catalyzed stereospecific allyl-transfer reaction from gamma-adducts

Allylation of aldehydes by an allyl-transfer reaction from the gamma-adducts of homoallylic alcohols has been successfully carried out to give the corresponding alpha-adducts regiospecifically. The reaction proceeds via a hemiacetal (11), derived from an aldehyde and the homoallylic alcohol, followed by a six-membered cyclic transition state (2-oxonia[3.3]-sigmatropic rearrangement) in the presence of a Lewis acid. Moreover, the gamma-adducts are restructured into the corresponding alpha-adducts via a similar transition state by an acid catalyst, in which chirality in both anti- and syn-gamma-adducts is stereospecifically transferred to the corresponding E- and Z-alpha-adducts, respectively, with > 98% ee.