Studies of the synthesis of rubranitrose and crystal structure of methyl 2,3,6-trideoxy-3-C-methyl-3-nitro-alpha-D-ribo-hexopyranoside

The branched-chain nitro sugar methyl 2,3,6-trideoxy-3-C-methyl-3-nitro-alpha-D-ribo-hexopyranoside 4 was investigated as a precursor to D-rubranitrose, a nitro sugar found in the antibiotic rubradirin. X-ray cyrstallographic analysis of 4 shows that the pyranose ring adopts the C-4(1) conformation with the methoxy group at C-l and the nitro group at C-3 in a 1,3-diaxial relationship. There is an intermolecular hydrogen bond involving a nitro group oxygen of one monosaccharide residue and the C-4 hydroxyl group of the adjacent residue in the crystal lattice. This interaction results in a helical crystal packing. A series of nucleophilic displacement reactions was carried out on the triflate derivative of 4 in an attempt to introduce an axial carbon-oxygen bond at C-4 required for rubranitrose. Displacements with acetate and propionate gave as products the monosaccharide esters with the desired D-xylo configuration.