Polymer-supported chalcones

The successfully generated chalcones could be cleaved by treatment with trifluoro-acetic acid or used for the subsequent synthesis of pyrimidines [45], Condensation of the polymer-bound chalcones with benzamidine hydrochloride under microwave irradiation for 30 min furnished the corresponding pyrimidines in good yields after TFA-induced cleavage. This new robust support/linker system for SPOT synthesis has been demonstrated to be compatible with a range of organic reactions and highly applicable for microwave conditions. 

The Wang group also reported the asymmetric epoxidation of chalcone derivatives with their polymer-supported dimeric PTC 61 using fert-butyl hydroperoxide as an oxidant (Scheme 4.18) [23]. 

Polymer-supported (diacetoxyiodo)benzene (PSDIB) can promote an oxidative rearrangement of 2 -benzyloxy-chalcones to form the intermediate acetal 748, which can be isolated or directly treated with aqueous base to afford... 

The use of numerous polymer-supported optically active phase transfer catalysts was further extended by Kelly and Sherrington11351 in a range of phase transfer reactions including a variety of displacement reactions, such as sodium borohydride reductions of prochiral ketones, epoxidation of chalcone, addition of nitromethane to chalcone and the addition of thiophenol to cyclohexanone. Except in the chalcone epoxidation, all the examined resin catalysts proved to be very effective. However, with none of the chiral catalyst system examined was any significant ee achieved. The absence of chiral induction is a matter of debate, in particular over the possible reversibility of a step and the minimal interaction within an ion pair capable of acting as chiral entities in the transition state and/or the possible degradation of catalysts and leaching. 

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,... 

Scheme 10.20 Chiral polymer-supported oxidation catalysts and the use of one of them in the enantioselective epoxidations of chalcones.

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