キトサン,D-(+)-グルコサミン誘導体を配位子とする金属錯体存在下でのジエチル亜鉛の芳香族アルデヒド類への触媒的不斉付加反応

概要

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Chitin is an ubiquitous natural biopolimer consisted of D-glucosamine with β-(1,4) bonds but hardly used for chemical transformation because of its extremely low solubility. On the other hand, chitosan is easily obtained from chitin by deacetylation and soluble in acidic media. It is also optically active and expected to be used as a chiral source, if it could be soluble in organic solvent. Chitosan and its monomer, D-(+)-glucosamine, were modified in order to become soluble in organic solvent and examined as a chiral ligand for asymmetric addition of diethylzinc to aldehydes. First, chemically modified chitosans were synthesized from chitosan, and used as a ligand. Although the reaction of benzaldehyde with Et_2Zn in the presence of chitin and chitosan gave the racemic 1-phenyl-1-propanol, N-phthaloyl-6-O-tritylchitosan as a ligand accelerated the reaction to produce (R)-1-phenyl-1-propanol with up to 61% ee. Next, D-(+)-glucosamine derivatives were synthesized and used as a ligand. Benzyl-2-acetylamino-4,6-O-anisylidene-2-deoxy-α-D-glucopyranoside and Ti complex accelerated the reaction and was found to be the most efficient catalyst for the asymmetric addition to the various aldehydes with diethylzinc, giving the (R)-1-phenyl-1-propanol in high yield and with high enantiomeric excess up to 94%.