Compounds 2a and 3a were obtained as orange and bright yellow solids, respectively. Each showed strong absorption at 1766 cm-'and a single vinylic hydrogen at 6 6.62 and 6.29, respectively. The specific stereochemical assignment is based on NOE studies. Thus, irradiation of the vinylic peak of 2 (6 6.62) gave an NOE response only in the ortho hydrogens of the phenyl group. In contrast, irradiation of the 6 6.29 peak in 3 gave an ...