Various types of modified nickel catalysts were prepared and their enantioface-differentiating (asymmetric) abilities were examined. It was found that the presence of aluminum or related metal compounds in the catalyst was unfavorable for the effective enantioface-differentiating catalyst. The modification with a solution containing tartaric acid and inorganic salt gave a high enantioface-differentiating ability to Raney nickel and reduced nickel catalyst. Among the modified nickel catalysts examined, the Raney nickel modified with tartaric acid and sodium bromide gave the best result with respect to the hydrogenation activity and enantioface-differentiating ability (optical yield 88%). NaBr adsorbed on Raney nickel was found to inhibit the nonenantioface-differentiating hydrogenation.
The enantioface-differentiating hydrogenation of methyl 3-oxoalkanoate (CH3(CH2)nCOCH2COOCH3, n=0, 6, 8, 10, 12) over the (R,R)-tartaric acid–NaBr–modified Raney nickel catalyst ((R,R)-TA–NaBr–MRNi) gave methyl (R)-3-hydroxyalkanoate (CH3(CH2)nCH(OH)CH2COOCH3, n=0, 6, 8, 10, 12) in an average optical yield of 85%. After the methyl ester had been converted to dicyclohexylammonium salt of 3-hydroxyalkanoic acid, the salt was recrystallized three times from acetonitrile and then treated with acid to give optically pure (R)-3-hydroxyalkanoic acid (CH3(CH2)nCH(OH)CH2COOH, n=0, 6, 8, 10, 12) in a reasonable yield. From the hydrogenation product with (S,S)-TA–NaBr–MRNi, optically pure (S)-3-hydroxyalkanoic acid was obtained by the same process as above.
Four stereoisomers of methyl 3-hydroxy-2-methylbutyrate (I), (+)-threo-I, (−)-threo-I, (+)-erythro-I, and (−)-rythro-I, were prepared, and their absolute configurations were determined. The configurations of (+)-threo-I, [α]D20+36.80° (c 5, methanol) and (+)-erythro-I, [α]D20+14.32° (c, 5, methanol) were assigned to (2S, 3S) and (2S, 3R) respectively.
In order to establish the stereochemistry of the sex attractant of various species of pine sawflies, (2S,3R,7R)-, (2S,3R,7S)- and (2S,3S,7S)-2-acetoxy- and 2-propionyloxy-3,7-dimethylpentadecane were synthesized. The alcohol moiety of each pheromone was prepared by the coupling of Grignard reagent of C12 block with tosylate of C5 block. The C12 blocks, (R)- and (S)-1-bromo-2-methylundecane, were prepared from (R)-(+)-pulegone. The C5 blocks, (2R,3S)- or (2S,3S)-2-methyl-3-tetrahydropyranyloxy-1-(tosyloxy)butane, were derived from (2S,3S)- or (2R,3S)-2-methyl-3-hydroxybutyric acid prepared by the enantio-differentiating hydrogenation of methyl 2-methyl-3-oxobutyrate over the asymmetrically modified nickle catalyst.
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