Polyamino acids, chalcone epoxidations

The asymmetric epoxidation of several chalcones (39) and other electron-poor olefins in a triphase system (water/organjc solvent/chiral polyamino acid) afford optically active oxirans with optical yields of up to 96%. The influence of the molecular structure of the catalysts and substrates, the solvent, and the temperature on the stereochemistry was investigated by a group of chemists from Italy and Spain 77). 

Polyamino acid 8 catalyzed the epoxidation of chalcones in the presence of alkaline hydrogen peroxide. The Julia-Colonna reaction furnished the corresponding oxiranes with excellent enantioselectivities (Equation 10.21) [43]. 

The epoxidation ofa, (3-unsaturated ketones catalysed by polyamino acids is known as the Julia-Colonna epoxidation.This three-phase procedure utilises aqueous hydrogen peroxide as oxidant along with a water immiscible solvent and solid poly-L-leucine as a catalyst and is mainly effective in the epoxidation of chalcone (4.79) and derivatives. ... 

In addition to the great advances in asymmetric epoxidation techniques using electrophilic oxidants, there have been significant developments toward the catalytic nucleophific epoxidation of electron-deficient olefins. In 1980, Julia and co-workers reported on the first highly enantioselec-tive epoxidation (in >90% ee) of chalcones catalyzed by polyamino acids (Scheme 35.22). Typically, the reaction was carried out under mild conditions in a triphasic system (toluene, water, and polymer catalyst) using hydrogen peroxide as an oxidant. A relatively large quantity of the polymer was required for the reaction (typically 0.4 g of polymer to 0.5 g of substrate), but the polymer could be recovered and reused after the application. 

SCHEME 35.22. The first highly enantioselective epoxidation of chalcones catalyzed by polyamino acids. 

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]. 

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