Synthesis of (1 -> 3)-C and homo(1 -> 3)-C-linked iminodisaccharides starting from levoglucosenone and isolevoglucosenone

The reaction of 2,5-(benzyloxycarbonyl)imino-2,5-dideoxy-3,4-O-isopropylidene-L-ribose ((-)-15) with levoglucosenone (5) in the presence of Et2AlI gave a 3,4-dideoxy-D-glycero-hex-3-enopyranos-2-ulose derivative ((-)-16) that was converted into the (1-->3)-C-linked imino-disaccharide: methyl 3,4-dideoxy-3-[(1'S)-2',5'-dideoxy-2',5'-imino-D-ribitol-1'-C-yl)-alpha- D-lyxo-hexo-pyranoside ((+)-22). The addition of benzyl alcohol to isolevoglucosenone (3), followed by cross-aldol condensation with 3,6-[tert-butoxycarbonyl]imino-2,3,6-trideoxy-4,5-O-isopropylidene-L-ara bino-hexose ((+)-30) generated, after water elimination, a single enone ((+)-31) that was converted into a homo-(1-->3)-C-linked imino-disaccharide: 3-deoxy-3-(1',2',3',6'-tetradeoxy-3',6'-imino-L-arabino-hexitol-1'-C-yl) -beta-D-galactofuranose (44).