Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 315

Scheme 15.4 Catalytic cycle of epoxidation mediated by ketone 1.

and/or 9. The epoxidation was found to be much more efficient at higher pH,
possibly by suppressing the competing Baeyer-Villiger oxidation and favoring the
formation of intermediate 6 and dioxirane 7 [54, 55]. Under these reaction condi-
tions, ketone 1 appears to be reactive enough that the autodecomposition of Oxone
at high pH is presumably overridden.
Ketone 1 provides high levels of enantioselectivity for the epoxidation of a wide
range of 1,2- trans - and trisubstituted olefins (Figure 15.1) [46, 47, 56]. Conjugated
dienes and enynes [57-59] (Figure 15.2), enol esters [60-63], and 2,2-disubstituted
vinyl silanes [64] are also suitable substrates (Figure 15.3). Optically active
α -acyloxy ketones can be obtained by stereoselective rearrange-

ment of the chiral enol ester epoxides. The choice of Lewis acid determines
whether the rearrangement would lead to retention or inversion in configuration
(an example is shown in Scheme 15.5) [61, 62]. Additionally, the chiral epoxides

-hydroxyl and α

Scheme 15.5

Rearrangement of chiral enol ester epoxides.