A comparison of steady-state NOE coefficients measured for vinyl-substituted isopropyl ethers with conformations generated by a molecular mechanics program allowed a characterization of the most favorable ground state conformations. With high confidence it was possible to assign the lowest energy conformation to one which is characterized by a diastereo-zeroplane consisting of the s-trans vinyl group, the stereogenic center and the methyl group (Fig. 3). A stereochemical model of the transition state for [2+2]cycloaddition of chlorosulfonyl isocyanate and vinyl ethers is proposed, based on the lowest energy conformation derived from NOE coefficients, which agrees well with the experimental facts and provides a sound explanation of the direction of asymmetric induction.
