Aufbau von oligosacchariden mit glycosylfluoriden unter lewissäure-katalyse

Abstract

Glycosylation of 1,2:3,4-di- O-isopropylidene-α- d-galactopyranose ( 6), as well as its 6-trimethylsilyl ether 7 with 2,3,4,6-tetra- O-acetyl-β- d-glucopyranosyl fluoride ( 5) was achieved stereospecifically in a mild and fast manner in the presence of Lewis acids like, e.g., titanium tetrafluoride, to give the β-(1→6)-linked disaccharide derivative 1. By use of 2,3,4,6-tetra- O-benzyl-β- d-glucopyranosyl fluoride ( 8) or its α anomer 10 and titanium tetrafluoride in acetonitrile with 6 or 7, a fast reaction proceeds preponderantly to yield 1,2:3,4-di- O-isopropylidene 6- O-(2,3,4,6-tetra- O-benzyl-β- d-glucopyranosyl)-α- d-galactopyranose ( 2). In ether, however, mainly the α-(1→6) anomer was formed. These model systems were used to elucidate the limiting conditions for this procedure, and mechanistic conceptions are discussed. By glycosylation at O-4 of 1,6:2,3-dianhydro-β- d-mannopyranose ( 11) with the perbenzylated α-fluoride 10 both the α- and the β- d-(1→4) disaccharide derivatives 12 and 14 were obtained, but 5 gave exclusively the β- d-(1→4) compound 16. Opening of the anhydro rings of 12 led to the synthesis of N-acetyl-maltosamine ( 22). 1,6-Anhydro-2-azido-4- O-benzyl-2-deoxy-β- d-glucopyranose was glycosylated with methyl (2,3,4-tri- O-acetyl-β- d-galactopyranosyl fluoride)uronate under titanium tetrafluoride catalysis to give the β- d-(1→3)-linked disaccharide 16, subsequently transformed into 29.