Origin of diastereoselectivity in the tandem oxy-Cope/Claisen/ene reaction: experimental and theoretical studies of the ring inversion mechanism.

We report herein a detailed investigation into the reaction mechanism of the oxy-Cope/Claisen/ene reaction. A series of chiral substrates was prepared, subjected to the tandem sequence, and the enantiomeric excess of the final products was evaluated. The observed conservation of enantiomeric excess was taken as evidence that the ring inversion of the intermediary enol ether does not occur. DFT calculations were used to map out the potential energy surface for the reaction and evaluate the relative energies of the ring inversions relative to those of the Claisen and ene reactions. Transition state energies thus obtained were found to support the presence of a high-energy transition state for the ring inversion of B to D provided R(1) not equal H. In addition, the calculations lent further support to the hypothesis that the selectivity of the transannular ene reaction is under Curtin-Hammett control.