Chalcone phase-transfer-catalyzed epoxidation

Quite recently, one of the most efficient phase-transfer-catalyzed epoxidation methods for chalcone-type enones was developed by the Park-Jew group [11], A series of meta-dimeric cinchona PTCs with modified phenyl linkers were prepared. Among this series, the 2-fluoro substituted catalyst 5, exhibited unprecedented activity and enantioselectivity for the epoxidation of various trans-chalcones in the... 

Murphy and coworkers prepared similar guanidinium salts 54 from commercially available ethyl (J )-3-hydroxybutyrate and (S)-(—)-mahc acid [86]. The application of the catalysts 54 in the phase-transfer-catalyzed epoxidation of the chalcones delivered high enantioselectivities. Recently, Tan and coworkers developed a series of chiral pentanidium salts 55, which could be successfully applied to the asymmetric phase-transfer-catalyzed conjugate addition [87] as well as a-hydroxylation reactions [88]. 

Phase-Transfer-Catalyzed Asymmetric Epoxidation of E-Chalcone [58] (p. 415)... 

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. 

Asymmetric epoxidation of chalcone, in basic conditions, by hydrogen peroxide may be catalyzed by poly-(S)-alanine [84], giving ee s of up to 97%. The system maybe considered as a case of phase-transfer catalysis. The scope of this reaction is however not very broad. 

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