Trans-Disubstituted alkenes asymmetric epoxidation

The Sharpless asymmetric dihydroxylation works best for trans disubstituted alkenes, while the Jacobsen epoxidation works best for cis disubstituted alkenes. Even In this small area, there is a need for better and more general methods. Organic chemistry has a long way to go. 

Enolisable aldehydes such as 101 or 103 do not give quite such good yields but the ees are still good and the diastereoselectivity in favour of the trans epoxides 102 and 104 is excellent. The secret of this method is the simple preparation of the reagent 96. In the next chapters you will see that superior catalytic methods are available for asymmetric epoxidation of allylic alcohols and of m-alkenes but they are less good for the trans disubstituted alkenes that would give 97, 102, or 104. You will also see catalytic versions of sulfur ylid epoxidation. 

We recently reported our results on the asymmetric epoxidation of trans-disubstituted and trisubstituted alkenes, using Oxone as oxidant, catalyzed by readily available arabinose-derived 4-uloses containing tunable steric blockers that control the enantioselectivity of the epoxidation.Ulose (3), containing a 2, 3 -diisobutyl acetal unit, was the most efficient catalyst and displayed good enantioselectivity. 

The diaryl prolinol/TBHP system has been found to be suitable for the asymmetric epoxidation of a variety of poorly investigated trans-disubstituted or trisubstituted electron-poor alkenes (Scheme 7.5). ... 

This catalytic system provides high enantioselectivities for a range of epoxides (Figure 19.2), including those derived from trisubstituted and traus-1,2-disubstituted alkenes, with complete stereospecificity (retention of the alkene geometry in the epoxide product) [17]. The reaction has been shown to be chem-oselective for the alkene of enynes [18], provided monoepoxides upon reaction with conjugated trans-dienes [19] and afforded up to 93% ee for the asymmetric epoxidation of fluoro-olefins [20]. However, decreased enantioselectivity was observed for both cis- and terminal alkenes. The catalytic system has also been applied to the resolution and desymmetrization of cyclic trisubstituted alkenes [21]. 

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