Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 317

of 2,2-disubstituted vinyl silanes can undergo desilylation to obtain 1,1-
disubstituted terminal epoxides [64].
Two extreme epoxidation transition states - spiro and planar - have been pro-
posed for the epoxidation with dioxiranes (Figure 15.4). Both experimental and
computational studies suggest that the spiro transition state is energetically favored
due to a stabilizing secondary orbital interaction between the nonbonding orbital
of the dioxirane oxygen and the

* orbital of the olefin [3, 4, 17, 46, 47, 65-72].
Studies have also shown that spiro transition state A is favored for the epoxidations
of trans - and trisubstituted olefins with ketone 1 based on the obtained epoxide
configurations (Figure 15.5). The stereochemical outcome of the epoxidation
reveals that planar transition state B is competing and that the competition is
dependent upon the steric and electronic nature of the olefin substituents [46, 47].
Generally speaking, conjugating aromatic rings, alkenes, and alkynes to the react-
ing double bond enhances the aforementioned secondary orbital interaction, thus
favoring spiro A over planar B, which leads to an increase in the enantioselectivity
of the resulting epoxide. A small R 1 group and/or a large R 3 group is also beneficial
for the enantioselectivity by sterically favoring spiro A and/or disfavoring planar
B. The transition state model is validated by the results of kinetic resolution of

π

Figure 15.4 Spiro and planar transition states for the dioxirane epoxidation of olefins.

Figure 15.5

Competing spiro and planar transition states for the epoxidation with ketone 1.