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-adduct s of homoallylic alcohols has been successfully carried out to give the cor responding alpha-adducts regiospecifically. The reaction proceeds via a hem iacetal (11), derived from an aldehyde and the homoallylic alcohol, followe d by a six-membered cyclic transition state (2-oxonia[3.3]-sigmatropic rear range ment) in the presence of a Lewis acid. Moreover, the gamma-adducts ar t: restructured into the corresponding alpha-adducts via a similar transiti on state by an acid catalyst, in which chirality in both azti- and syn-gamm a-adducts is sterio-specifically transferred to the corresponding E- and Z- alpha-adducts, respectively, with > 98 % ee.