Poly-D-leucine

Bentley et al.m recently improved upon Julia s epoxidation reaction. By using urea-hydrogen peroxide complex as the oxidant, l,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the base and the Itsuno s immobilized poly-D-leucine (Figure 4.2) as the catalyst, the epoxidation of a, (3-unsaturated ketones was carried out in tetrahydrofuran solution. This process greatly reduces the time required when compared to the original reaction using the triphasic conditions. 

The white solid was placed under high vacuum overnight to give immobilized poly-D-leucine (4.4 g) as a white powder. 

In a lOmL round-bottomed flask equipped with a magnetic stirrer bar were placed tetrahydrofuran (0.8 mL) and immobilized poly-D-leucine (100 mg). (7 )-Benzylidene acetophenone (50 mg), l,8-diazabicyclo[5.4.0] undec-7-ene (90 mg), and urea-hydrogen peroxide (27 mg) were added to the mixture. The thick white reaction mixture was stirred vigorously for 30 minutes. 

After completion, the mixture was poured into ethyl acetate and the catalyst was removed by filtration using two filter papers in a Buchner funnel. The poly-D-leucine residue was washed with ethyl acetate (2 x 10 mL), with water (2 x 10 mL) and with brine (10 mL). 

The epoxidation of enones with poly-D-leucine is complementary to other strategies. Enders et u/.[8] introduced a new method for the asymmetric epoxidation of a-enones using diethylzinc, oxygen and (1R, lR)-or (IS1, 2S)-N-methylpseudoephedrine as chiral auxiliary. 

Methods A Immobilised poly-(L)-leucine, urea hydrogen peroxide, DBU, THE B Immobilised poly-(D)-leucine, urea hydrogen peroxide, DBU, THE... 

In a similar fashion to those results obtained for the oxidation process, on switching from poly-(L)-leucine to poly-(D)-leucine the opposite configuration of the polyether was observed (absolute configuration of products unknown). Unexpected, however, was the observation that poly-(L)-phenylalanine furnished the opposite enantiomer to that observed employing poly-(L)-leucine. Thus, it has been shown that addition of nucleophiles other than peroxide anion can be catalysed by polyamino acids with significant stereocontrol [22]. 

The improved Julia-Colonna epoxidation conditions have been successfully employed for poly-(D)-leucine. 

Thus, epoxidation of the diene 6 using poly-(D)-leucine affords the epoxide ent-12 in 90% yield and 95% e.e. (It is worth noting at this point that the per-... 

Both, poly-L-leucine and poly-D-leucine systems are readily available for research use. World patents are pending on commercial apphcations [52]. The catalysts are stable to air, fight, heat, and moisture and they are active at low cat-... 

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