Activated silk optics

Catalytic asymmetric hydrogenation is a relatively developed process compared to other asymmetric processes practised today. Efforts in this direction have already been made. The first report in this respect is the use of Pd on natural silk for hydrogenating oximes and oxazolones with optical yields of about 36%. Izumi and Sachtler have shown that a Ni catalyst modified with (i ,.R)-tartaric acid can be used for the hydrogenation of methylacetoacetate to methyl-3-hydroxybutyrate. The group of Orito in Japan (1979) and Blaser and co-workers at Ciba-Geigy (1988) have reported the use of a cinchona alkaloid modified Pt/AlaO.i catalyst for the enantioselective hydrogenation of a-keto-esters such as methylpyruvate and ethylpyruvate to optically active (/f)-methylacetate and (7 )-ethylacetate. 

During the early phase of an investigation there is usually very little information available and therefore, the goal is to find a "lead" which then can be developed and improved further to give e.g. a synthetically useful system. The approach most often used is what one could call "screening with a concept experiments are set up in order to test an intuitive idea or a more or less well defined hypothesis. A good illustration for this approach is described by Izumi [4] "we expected simply that an optically active product should be produced from the influence of the optically active environment, like baking a waffle. We used silk fibroin as a waffle iron ". 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

As early as 1904, Willstatter attempted to separate optical isomers on the optically active natural polymers wool and silk [10]. About 35 years later, the first partial chromatographic resolution of the enantiomers ofp-phenylene-bis-imino-cam-phor on lactose was achieved by Henderson and Rule [11], and a few years later by Lecoq for the enantiomers of ephedrine [12], and by Prelog and Wieland for the enantiomers ofTroeger s base [13]. 

A number of asymmetric reductions have been reported using asymmetric hydrogenation catalysts. Akabori et al. (1956) reduced (XII) on a palladium on silk catalyst and obtained, after hydrolysis, optically active phenylalanine. Hartung (Beamer et al., 1960) has discussed such asymmetric reductions in terms of asymmetric active sites on the catalyst surface. An ordinary catalyst is envisaged as having equal numbers of enantiomorphic sites, some of which, however, may be linked so that only one at a time may function. To prepare a catalyst having a disparity... 

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