Highly stereoselective synthesis of vicinal diols by stannous chloride-mediated addition of hydroxyallylic stannanes to aldehydes

A new protocol for the synthesis of vicinal diols was accomplished by the reaction of unprotected α-hydroxymethylmetals, as hydroxymethyl anion equivalents, with aldehydes. The treatment of hydroxyallylic stannanes, which were prepared from α,β-unsaturated aldehydes and Bu3SnLi in situ, with various aldehydes gave but-3-en-1,2-diols in the presence of SnCl2. The stereochemistry of the diol and olefin moieties demonstrated syn- and E-selectivities, respectively. We propose the following reaction mechanism; transmetalation of a hydroxyallylic stannane with SnCl2 gives a rearranged allylic tin(II) species that undergoes aldehyde addition via a cyclic transition state. The strict interaction between the unprotected hydroxy moiety and the tin(II) center accounts for the selectivity.