Aza-C-disaccharides:  Synthesis of 6-Deoxygalactonojirimycin β-C(1→3) Linked with d-Altrofuranosides and d-Galactose

Cross-aldolization of 3-O-benzyl-N-(benzyloxycarbonyl)-2,6,7-trideoxy-2,6-imino-4,5-O-isopropylidene-β-d-glycero-l-mannoheptose ((−)-12, derived from d-glycero-d-gulo-heptono-1,4-lactone in eight steps) with (+)-(1R,4S,5S,6S)- and (−)-(1S,4R,5R,6R)-6-chloro-5-(phenylseleno)-7-oxabicyclo[2.2.1]heptan-2-one (obtained in one step from the “naked sugars” (−)- and (+)-7-oxabicyclo[2.2.1]hept-5-en-2-one) were highly stereoselective (lithium enolates, like modes), giving aldols (−)-14 and (+)-16, respectively. Stereoselective methods were developed for the conversion of (−)-14 into methyl 3-deoxy-3-C-[(1‘R)-2‘,6‘,7‘-trideoxy-2‘,6‘-imino-β-d-glycero-l-manno-heptitol-1‘-yl]-α- and -β-d-altrofuranoside ((+)-1α,β). The aza-C-disaccharide 1α prefers a anti conformation (bonds C(2‘)−C(3‘) and C(1‘)−C(3) are antiperiplanar) for the β-d-galactoside moiety (³J_H,H coupling constants, NOEs). Aldol (+)-16 was converted stereoselectively into 3-deoxy-3-C-[(1‘S)-2‘,6‘,7‘-trideoxy-2‘,6‘-iminio-β-d-glycero-l-manno-heptitol-1‘-yl]-β-d-galactose trifluoroacetate.