Lead-Mediated Highly Diastereoselective Allylation of Aldehydes with Cyclic Allylic Halides

Lead was found to efficiently mediate the allylation reactions of carbonyl compounds with cyclic allylic halides in the presence of stoichiometric amounts of lithium chloride and a catalytic amount of GaCl₃ (20 mol %), leading to the desired homoallylic alcohols in modest to high yields with excellent diastereocontrol (>99:1 syn/anti) and good functional group tolerance. In contrast, the use of either 2-pyridinecarboxaldehyde as the carbonyl substrate or (E)-cinnamyl bromide as the allylating agent produced the corresponding product with reversed diastereoselectivity (>99:1 anti/syn).