Immobilized poly-L-leucine

To immobilized poly-L-leucine (7.0 g) was added THF (50 mL), urea hydrogen peroxide (2.07 g, 22 mmol) and DBU (4.11 mL, 27.5 mmol). This mixture was stirred for 3-5 min, after which the enone (4.01 g, 18.4 mmol) in THF (10 mL) was added. After a further 3 h, additional urea hydrogen peroxide (1.06 g, 11.3 mmol) and DBU (2.5 mL, 16.1 mmol) were added. After 28 h, the reaction was filtered to remove the poly-L-leucine. The filtrate was added to saturated aqueous ammonium chloride solution and extracted with ethyl acetate. The combined organic layers were dried (MgS04) and concentrated in vacuo. The acid-sensitive... 

Roberts has shown that the asymmetric epoxidation of chalcone can be catalysed by polyamino acid derivatives under non-aqueous conditions [13]. This improved reaction involves the use of a urea-hydrogen peroxide complex in THF, in the presence of an organic base (DBU) and immobilized poly-(L)-leucine. Under these conditions, the reaction of chalcone derivatives and related substrates provided the corresponding epoxides in 70-99% yield and 83-95% ee within 30 min. Several substrates with enolisable enones have also been epoxidized successfully [14]. 

The biomimetic protocol was invented by Julia and Colonna, and involves the use of polyamino acids (such as poly-(L)-leucine) as the catalysts for peroxide oxidation of chalcones, styryl alkyl ketones and conjugated alkenones. The substrate range is broad, especially when using immobilized catalysts and an organic solvent containing the substrate, urea-hydrogen peroxide and an organic base (Scheme 22)[101]. 

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. 

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. 

This procedure utilizes immobilized (polystyrene) poly-L-leucine, which is more easily separated from the products. (For experimental details see Chapter 14.13.9). 

Poly-L-leucine and a polymeric BENOL-zinc complex have been evaluated for the epoxidation of enones. Poly-L-leucine immobilized on silica is also effective. 

To solution of chalcone (1 eq.) in anhydrous tetrahydrofuran was added urea-hydrogen peroxide (1.2 eq.), diazabicycloundecene (1.2 eq.), and poly-L-leucine (12 mol % optionally immobilized on silica, i.e., one part catalyst, three parts silica). The mixture was stirred at room temperature until the reaction was complete (TLC) ( 7 days). The catalyst was removed by rapid filtration and washed with EtOAc. The combined organic fractions were evaporated in vacuo to yield crude epoxide. It was found that 45% of chalcone had been oxidized within the first 30 min, and the overall epoxyketone has 91% e.e. 

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