Preparation of Novel Chiral Ligands from Natural Sugars and Their Application to Catalytic Asymmetric Synthesis

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We developed the highly enantioselective hydrogenation of dehydroamino acids or imines using novel water-soluble chiral rhodium and iridium catalysts bearing hydrophilic phosphine-phosphinite or diphosphinite ligand derived from α,α-trehalose.(1) Preparation of Water-soluble Chiral Rhodium Catalyst having Phosphine-phosphinite LigandMesylation of 2,3:4,6-di-ο-isopropylidene-α-D-glucopyranosyl-(1,1)-4,6-ο-isopropylidene-α-D-gluco-pyranoside (1) derived from α,α-trehalose with MsCl gave 2,3-di-ο-mesyl disaccharide 2. The treatment of 2 with NaOEt gave 2,3-anhydro disaccharide 3. The ring-opening of an epoxide moiety in 3 with KPPh_2 afforded 3-hydroxy-2-diphenylphosphino derivative 4. The reaction of 4 with Ph_2PCl gave 2,3:4,6-di-ο-isopropylidene-α-D-glcopyranosyl-(1,1)-4,6-ο-isopropylidener-2-(diphenyphosphi-no)-2-deoxy-3 -0-(diphenyl- phosphino)-α-D-altropyranoside (5). Finally, the reaction of 5 with [Rh(acac)(cod)] followed by treatment with aqueous HBF_4 afforded a cationic rhodium … More complex 6 bearing six free hydroxyl groups.(2) Preparation of Water-soluble Chiral Iridium Catalyst having Diphosphinite LigandReaction of 1 with Ph_2PCl gave 2,3:4,6-di-ο-isopropylidene-α-D-glcopyranosyl-(1,1)-4,6-ο-isopropylidene-2,3-di-ο-(phenyphosphino)-α-D-altropyranoside (7). The reaction of 7 with [Ir(acac)(cod)] followed by treatment with aqueous HBF_4 afforded a cationic iridium complex 8 bearing six free hydroxyl groups.(3) Asymmetric Hydrogenation of Dehydroamino Acids and their Esters with 6 in an Aqueous MediumThe title complex 6 was found to be an effective catalyst for enantioselective hydrogenation of dehydroamino acids and their esters leading to (R)-α-amino acid derivatives in water or an aqueous/organic biphasic medium. In the biphasic system, the catalyst is immobilized in the aqueous phase. Therefore, the aqueopus phase involving catalyst can be recovered by simple phase separation and reused for further operation in hydrogenation to give almost same level of enantiomeric excess of a hydrogenated product.(4) Asymmetric Hydrogenation of Imines with 8The title complex 8 can act as a catalyst for enantioselective hydrogenation of several imines to give enantiomericaly enriched secondery amines. Less