Synthesis of the C-disaccharide alpha-C(1 -> 3)-L-fucopyranoside of N-acetylgalactosamine

Radical C-glycosidation of racemic 5-exo-benzeneselenyl-6-endo-chloro-3-methylidene-7-oxabicyclo[2.2.1]hept an-2-one ((+/-)-2) with alpha -acetobromofucose (3) provided a mixture of alpha -C-fucosides that were reduced with NaBH4 to give two diastereomeric alcohols that were separated readily. One of them ((-)-6) was converted into (-)-methyl 2-acetamido-4-O-acetyl-2,3-dideoxy-3-C(3',4',5'-tri-O-acetyl-2',6'-anhyd ro- 1',7'-dideoxy-alpha -L-glycero-D-galacto-heptitol-1'-C-yl)-alpha -D-galactopyranuronate ((-)-11) and then into (-)-methyl 2-acetamido-2,3-dideoxy-3-C-(2',6'-anhydro- 1',7'-dideoxy-alpha -L-glycero-D-galacto-heptitol- 1'-C-yl)-beta -D-galactopyranoside a new alpha -C(1 -->3)-L-fucopyranoside of N-acetylgalactosamine. Its H-1 NMR data shows that this C-disaccharide (alpha -L-Fucp-(1 -->3)CH2-beta -D-GaINAc-OMe) adopts a major conformation in solution similar to that expected for the corresponding O-linked disaccharide, i.e., with antiperiplanar sigma (C-3',C-2') and sigma (C-1',C-3) bonds.