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 dihydronicotinamide 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.