Poly- -leucine, enantioselective catalyst

Several examples are known of the enantioselective conversion of alkenes into epoxides with the use of polymer-supported oxidation catalysts. This can be traced to the pioneering work by Julia and Colonna in 1980. They demonstrated that highly enantioselective epoxidations of chalcones and related a, 3-unsaturated ketones can be achieved with the use of insoluble poly(a-amino acids) (116, Scheme 10.20) as catalysts [298-301]. The so-called Julia-Colonna epoxidation has been the object of several excellent reviews [302-306]. The terminal oxidant is H202 in aq. NaOH. With lipophilic amino acids as the components, such as (SJ-valine or (SJ-leucine, enantioselectivities as high as 96-97% ee were obtained. The enan-tioselectivity depends of several factors, including the side-chain of the amino acid, the nature of the end groups and the degree of polymerization. Thus, for instance,... 

In parallel, the three-phase reaction system was improved. It was found that the reaction was strongly enhanced when performed in the presence of a phase-transfer catalyst as achiral additive [35-40], Epoxidation of chalcone 9 with 10 mol% TBAB led to >99% conversion within 7 min and high enantioselectivity of 94% ee (Scheme 14.3) [35]. In the absence of TBAB the asymmetric poly-L-leucine-catalyzed epoxidation afforded only 2% conversion after 1.5 h. It was also reported that the amount of catalyst could be further reduced. 

Enantioselective heterogeneous catalytic hydrogenation using a chiral catalyst was pioneered by Aka-bori and Izumi, who prepared a palladium catalyst supported on silk fibroin. The oxime acetates of diethyl a-ketoglutarate or of ethyl phenylpyruvate were hydrogenated to form glutamic acid (7-15% ee) and phenylalanine (30% Similarly, a palladium-poly-L-leucine catalyst was used for the asym-... 

Another important asymmetric epoxidation of a conjugated systems is the reaction of alkenes with polyleucine, DBU and urea H2O2, giving an epoxy-carbonyl compound with good enantioselectivity. The hydroperoxide anion epoxidation of conjugated carbonyl compounds with a polyamino acid, such as poly-L-alanine or poly-L-leucine is known as the Julia—Colonna epoxidation Epoxidation of conjugated ketones to give nonracemic epoxy-ketones was done with aq. NaOCl and a Cinchona alkaloid derivative as catalyst. A triphasic phase-transfer catalysis protocol has also been developed. p-Peptides have been used as catalysts in this reaction. ... 

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