The vinylalumination of α-substituted aldehydes gave anti- and syn-adducts with moderate diastereoselectivity. The diastereomeric ratio was inverted by the addition of lithium or sodium perchlorates. Thus, both anti- and syn-adducts were isolated and transformed into the biologically active conduritols, pericosine B and C, respectively. Formal synthesis of pericosine A was achieved with the anti-adduct. The rationales for the different diastereoselectivity are discussed.