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

The synthesis of the nitro-sugar-containing trisaccharide found in the anti biotic cororubicin is described. This trisaccharide contains the nitro suga r L-decilonitrose which is attached to two digitoxose residues (a dideoxy s ugar) by beta-1,4 and alpha-1,4 linkages. The target trisaccharide synthesi zed was methyl 2,6-dideoxy-alpha-L-lyxo-hexopyranosyl-(1-->4)-2,3,6-trideox y-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 decilon itrose and as both the acceptor and donor in glycosyations, a beta-selectiv e glycosylation by means of a thioglycoside donor, and novel functional gro up transformations carried out on the assembled trisaccharide. These includ e oxidative cleavage of the O-benzylhydroxylamino group to nitro, carried o ut with dimethyldioxirane in one step. This transformation proved critical in completing the synthesis, which is the first reported of one of the deci lonitrose-containing trisaccharides found in antibiotics.