Reactions of nitro sugars. IX. The synthesis of branched-chain dinitro sugars by Michael addition

The reaction of methyl 2-O-acetyl-4,6-O-benzylidene-3-deoxy-3-nitro-β-D-glucopyranoside (1) or of methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-β-D-erythro-hex-2-enopyranoside (2) with nitroalkanes led to branched-chain dinitro sugar derivatives, namely methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-2-nitroalkyl-β-D-glucopyranosides (3). The nitroalkanes employed included nitromethane, nitroethane, 1-and 2-nitropropane, and methyl nitroacetate. The three first-mentioned nitroalkanes were added in similar fashion to methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-β-D-threo hex-2-enopyranoside (5) to give methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-2-nitroalkyl-β-D-galactopyranosides (6). Analogous reactions of 1 and 5 with diethyl malonate produced the corresponding branched-chain glycosides 7 and 8. The reactions appear to be highly stereoselective as far as the configurations at C-2 and C-3 in the products are concerned. However, there is evidence for the formation of side-chain epimers in three cases where the nitroalkyl side chain possesses an asymmetric carbon.