1,3-Allylic Strain as a Control Element in the Paternò−Büchi Reaction of Chiral Silyl Enol Ethers:  Synthesis of Diastereomerically Pure Oxetanes Containing Four Contiguous Stereogenic Centers

The facial diastereoselectivity in the Paternò−Büchi reaction of chiral silyl enol ethers and benzaldehyde was studied. The substituents (R^S, R^L) at the stereogenic carbon atom (−C^*HR^SR^L) attached to the β-position of the silyl enol ether were varied in order to evaluate the influence of steric bulk and electronic effects. The combined yields for the two diastereomeric 3-(silyloxy)oxetanes a and b range between 44% and 76%. In accordance with the 1,3-allylic strain model the facial diastereoselectivity (diastereomeric ratio (dr) = a/b) was best with large (R^L = t-Bu, SiMe₂Ph) and polar (R^L = OMe) substituents at the γ-position of the silyl enol ether (dr up to >95/5). Two regioselective ring opening reactions were applied to the product oxetanes 29a, 50, and 51. They furnished diastereomerically pure diols (52, 53) and triols (31) in excellent yields.