Synthesis of a ‘direct-linked’ C-disaccharide from a pyranulose glycoside

Syntheses of five ‘direct linked’ C-disaccharides 8a–e were reported. The (Et3SiH/BF3·Et2O) reduction of pyranulose glycoside 1 yielded (6S)- and (6R)-6-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)pyran-3(2H,6H)-one (2a and 2b) in a ratio of ca. 2:1 and in 88% combined yield. The absolute stereochemistry of each was determined from its CD spectrum. The reduction of 2a with NaBH4 in methanol afforded two allylic alcohols 6a and 6b in 14 and 73% yield, respectively. The reduction of 2b with NaBH4 afforded 6c and 6d in 30 and 56% yield, respectively. Cis hydroxylation of the double bond in compounds 6a–d with osmium tetroxide gave 7a–e. The stereoisomers 7a–e were separated and their configuration was established by 1H NMR spectroscopy. Debenzoylation of compounds 7a–e with aqueous sodium carbonate produced deprotected C-disaccharides 8a–e.