Epoxide From alkene, enantioselective

The ready availability of achiral and racemic epoxides from simple alkene precursors renders epoxide ring-opening an appealing approach to asymmetric synthesis. The inherent strain-induced reactivity of epoxides can be enhanced by coordination of the epoxide oxygen to a Lewis acid, thereby creating the possibility for chiral Lewis acids to catalyze enantioselective ring opening events. 

Shi and co-workers reported in 1996 the enantioselective epoxidation of unfunctionalized alkenes mediated by the fructose-derived ketone 10 (Figure 10.10) [46]. Corresponding oxiranes were obtained with excellent enantioselectivities (Equation 10.23). Yang and co-workers also reported the enantioselective epoxidation of alkenes, employing a cyclic ketone derived from binaphthyldicarboxylic add [47]. 

Asymmetric epoxidationJacobsen s salen-based catalyst 1, derived from (R,R)-or (S,S)-diphcnyl-l,2-diaminoethane (16,157) can effect asymmetric epoxidation of alkenes with NaOCI, but the enantioselectivity is generally only moderate ( 70% ee) in the case of ci.v-alkencs. Subsequently, this group has examined salen-based catalysts... 

Thus, a large range of alkenes were epoxidised with success (Scheme 7.3). The stereospecificity of the reaction was shown by the exclusive formation of ds-epoxides from (Z)-alkenes. In contrast, ( )-alkenes were poorly active, giving low yields and enantiomeric excesses. Simple aliphatic alkenes were also good substrates, which is of primary importance, since few methods are available for these substrates. Moreover, the reaction worked well with [Z) enol esters, giving the corresponding epoxides with high enantioselectivity (up to >99% enantiomeric excess). ... 

By contrast, chloroperoxidase-catalyzed epoxidation of alkenes proceeds with excellent enantioselectivites [1333, 1334]. For styrene oxide it was demonstrated that all the oxygen in the product is derived from hydrogen peroxide, which implies a true oxygen-transfer reaction (path 3, Scheme 2.174) [1335]. As depicted in Scheme 2.178, unfunctionalized c/s-alkenes [1336] and 1,1-disubstituted olehns [1337, 1338] were epoxidized with excellent selectivities. On the other hand, aliphatic terminal and irans-l,2-disubstituted alkenes were epoxidized in low yields and moderate enantioselectivities [1339]. 

From a biosynthetic point of view, CPO is very useful for enantioselective epoxidations of alkenes and vinylaro-matic compounds (Scheme 36.28) as well as for the stereoselective oxidation of heteroatoms. ... 

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