Synthesis of α-D-(1→3) and α-D-(1→4)-C-linked galactosides of D-mannose derivatives. Conformation of α-C-galactosides

Reductive radical alpha-D-galactosidation of 5-exo-(benzeneselenyl)-6-endo-chloro-3-methylidene-7- -oxabicyclo[2.2.1]heptan-2-one with acetobromo-D-galactose, followed by ketone reduction led to (+)-(1R,2S, 3R,4S,5S,6S)-5-exo-(benzeneselenyl)-6-endo-chloro-3-endo-[(2',3',4',6'-t etra-O-acetyl-alpha-D-galactopyranosyl)me- thyl]-7-oxabicyclo[2.2.1]heptan-2-endo-ol which was converted into (+)-methyl 3-deoxy-3-[(alpha-D-galactopyranosyl)methyl]-alpha-D-mannopyranoside (1), (+)-3-deoxy-3-[alpha-D-galactopyranosyl)methyl]-D-mannitol (2) and 4-deoxy-4-[(2',3',4',6'-tetra-O-acetyl-alpha-D-galactopyranosyl)methyl]- 2,3,6-tri-O-acetyl-D-mannono-1,5-lactone (3). H-1-MNR data of the alpha-C-galactosides confirmed that their preferred conformations involve antiperiplanar arrangements for the C-linked substrates and bond sigma C(1'),C(2') of the alpha-(1-->3)-C-galactoside unit. The alpha-(1-->3)-C-galactoside of methyl mannopyranoside 1 adopts a conformation similar to that proposed for methyl 3-O-(alpha-D-galactopyranosyl)-alpha-D-mannopyranoside.