Polymeric asymmetric reducing agents: preparation and reducing performance of chitosan/dihydronicotinamide conjugates having L- and D-phenylalanine spacer arms

Conjugates composed of chitin and the dihydronicotinamide group having L- and D-phenylalanine spacer arms have been prepared and evaluated as polymer-supported asymmetric reducing agents. Though the coupling reaction of N-nicotinoyl-L- or D-phenylalanine with chitosan resulted in poor substitution, the reaction with trityl-chitosan was efficient in solution to attain high substitution degrees. The derivatives were quaternized and reduced to generate the dihydronicontinamide structure. Reduction of ethyl benzoylformate with the resulting conjugates having L-phenylalanine and D-phenylalanine spacer arms produced (-)-excess and (+)-excess ethyl mandelate, respectively, indicating that the chiral selectivity can be controlled by the molecular structure of the spacer arm to synthesize a target enantiomer. The polymeric reducing agents could be regenerated after reaction to allow repeated use and the reducing performance proved to remain in the same level in four consecutive runs. (C) 2002 Elsevier Science Ltd. All rights reserved.