The stereochemistry of the reaction of allylsilanes with osmium tetroxide and of the epoxidation and methylenation of allylsilanes

Abstract

The allylsilanes E-7 react with osmium tetroxide, with m-chloroperbenzoic acid and with the Yamamoto methylenating reagent to give the diols 8 and 9, the epoxides 10 and11, and the cyclopropanes 12 and13, respectively. The reactions are more selective for the formation of the even-numbered diastereoisomers when the substituent on the stereogenic centre is an isopropyl or phenyl group than when it is a methyl group. The corresponding Z-allylsilanes, Z-7, are even more selective in this sense, giving more of the diol 14 than of 15, and only the epoxides 16 and the cyclopropanes 18. The ground-state conformation, as measured by the coupling constants in 24 and 25, correlates with the diastereoselectivity, as does a simple calculation of the relative energies of the two most appropriate conformations of the starting materials.