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 (+)-(1 R,4 S,5 S,6 S )- and (−)-(1 S,4 R,5 R,6 R )-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 ((+)- 1 α,β). The aza- C -disaccharide 1 α prefers a anti conformation (bonds C(2‘)−C(3‘) and C(1‘)−C(3) are antiperiplanar) for the β- d -galactoside moiety ( 3 J 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.