Stereoselective synthesis of vicinal diols by the stannous chloride-mediated reaction of unprotected hydroxyallylic stannane with carbonyl compounds

The highly stereoselective synthesis of vicinal diols was accomplished by the reaction of hydroxymethyl anion equivalents with carbonyl compounds. The reaction of hydroxyallylic stannanes with various aldehydes gave but-3-en-1,2-diols in the presence of SnCl₂. The diol moiety showed syn-diastereoselectivity. A mechanism for this reaction was proposed: transmetalation of the hydroxyallylic stannane with SnCl₂ gives an active rearranged allylic tin(II) species, which adds to carbonyl compounds through a cyclic transition state. The use of either α-alkoxy or hydroxy carbonyl compounds instead of aldehydes also gave the corresponding vicinal diols with high selectivity. In this case, either the alkoxy or the hydroxy group effectively coordinates to the tin(II) center to form a transition state that leads to high selectivity. The reaction of hydroxyallylic stannane with α-alkoxy aldehyde afforded the product through a cyclic transition state according to the Felkin-Anh model. This system was applied to sugar synthesis because conventional treatment of protected (R)-glyceraldehyde led to d-arabinose. The configurations of the major products were determined by means of X-ray structural analysis.