The stereochemical outcome of electrophilic addition reactions on the 5,6-double bond in the spinosyns

The electrophilic addition of reagents to the 5,6-double bond in spinosyn A and spinosyn D systems occurred with high,pi -diastereofacial selectivity. Addition occurred preferentially from the beta face of the molecule with s electivities ranging from 5:1 to better than 30:1. Various NMR properties w ere investigated in order to distinguish the beta and alpha isomers with th e help of theoretical models of the products. These NMR properties include a C-13 gamma effect to C-11 and vicinal coupling between H-4 and H-5. To he lp rationalize the selectivity, computational studies on the transition sta tes for epoxidation were calculated using density functional theory. The re sults indicate that beta epoxidation is favored and that the geometries of the transition structures are consistent with torsional steering being the source of the selectivity.