Reactions of nitro sugars. XI. Formation of 2,3-unsaturated hexonolactones by denitration of methyl deoxynitroglycosides

Treatment of methyl 4,6-O-benzylidene-3-deoxy-3-nitro-β-D-glucopyranoside (1) and its α-anomer (3), as well as of methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-β-D-erythro-hex-2-enopyranoside (2) and its α-anomer (4), with basic aluminum oxide in refluxing toluene, leads to loss of the nitro and glycosidic methoxyl groups. As a common product an α,β-unsaturated lactone, 4,6-O-benzylidene-2,3-dideoxy-D-erythro-hex-2-enono-1,5-lactone (5), was isolated in crystalline form. The β-D-manno isomer (6) of 1 incurs epimerization to 1 and partial conversion to 5. Methyl 4,6-O-benzylidene-3-deoxy-3-nitro-β-D-galacto-pyranoside (7) and methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-β-D-threo-hex-2-enopyranoside (8) similarly yield a crystalline lactone, 4,6-O-benzylidene-2,3-dideoxy-D-threo-hex-2-enono-1,5-lactone (9). The mechanism of this novel reaction is discussed, and certain differences in reactivity in the gluco and galacto series are pointed out. Base-catalyzed additions of methyl alcohol, ethyl alcohol, and piperidine to 8 afford methyl 4,6-O-benzylidene-3-deoxy-2-O-methyl-3-nitro-β-D-galactopyranoside (10), its 2-O-ethyl analogue (11), and methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitro-2-piperidino-β-D-galacto-pyranoside (12), respectively.