Cross-aldolization of 3-O-benzyl-N-(benzyloxycarbonyl)-2,6,7-trideoxy-2,6-imino-4,5-O-isopropy lidene-beta-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 -imino-beta-D-glycero-L-manno-heptitol-1'-yl]-alpha and -beta-D-altro-furanoside ((+)-1 alpha,beta). The aza-C-disaccharide 1 alpha prefers a anti conformation (bonds C(2')-C(3') and C(1')-C(3) are antiperiplanar) for the beta-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 D-glycero-L-manno-heptitol-1'-yl]-beta-D-galactose trifluoroacetate.