Glycosylation of branched amino and nitro sugars. 2. Synthesis of the cororubicin trisaccharide

The synthesis of the nitro-sugar-containing trisaccharide found in the antibiotic cororubicin is described. This trisaccharide contains the nitro sugar L-decilonitrose which is attached to two digitoxose residues (a dideoxy sugar) by beta-1,4 and alpha-1,4 linkages. The target trisaccharide synthesized was methyl 2,6-dideoxy-alpha-L-lyxo-hexopyranosyl-(1-->4)-2,3,6-trideoxy-3-C-methyl-3-nitro-beta-L-ribo-hexopyranosyl-( 1-->4)-2,6-dideoxy-alpha-L-lyxo-hexopyranoside The principle features of the synthesis are the use of an O-benzylhydroxylamino sugar as the precursor to the nitro sugar decilonitrose and as both the acceptor and donor in glycosyations, a beta-selective glycosylation by means of a thioglycoside donor, and novel functional group transformations carried out on the assembled trisaccharide. These include oxidative cleavage of the O-benzylhydroxylamino group to nitro, carried out with dimethyldioxirane in one step. This transformation proved critical in completing the synthesis, which is the first reported of one of the decilonitrose-containing trisaccharides found in antibiotics.