1,3-allylic strain as a control element in the Paterno-Buchi reaction of chiral silyl enol ethers: Synthesis of diastereomerically pure oxetanes containing four contiguous stereogenic centers

The facial diastereoselectivity in the Patemo-Buchi reaction of chiral silyl enol ethers and benzaldehyde was studied. The substituents (R(S), R(L)) at the stereogenic carbon atom (-C*HR(S)R(L)) attached to the beta-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) = alb) was best with large(R(L) = t-Bu, SiMe(2)Ph) and polar (R(L) = OMe) substituents at the gamma-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.