C-GLYCOSIDE SYNTHESES .2. HENRY CONDENSATIONS OF 4,6-O-ALKYLIDENE PYRANOSES WITH A 1,3-PROTON TRANSFER CATALYST - A ROUTE TO BLOCKED AMINOMETHYL-C-GLYCOSIDES

In the presence of a novel 1,3-proton transfer catalyst {2-hydroxypyridine (2-HP)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/ molecular sieves}, 4,6-O-benzylidene-D-glucopyranose (1), 4,6-O-isopropylidene-D-mannose (12), and 4,6-O-isopropylidene-D-glucose (16) undergo Henry condensations with nitromethane in THF to give acetal protected nitromethyl C-glycopyranosides (2, 13, and 17, respectively), which were characterized as their O-acetyl derivatives (5, 15, and 18, respectively). The Henry product from 4,6-O-benzylidene-Dglucopyranose could be reduced, with retention of the 4,6-O-benzylidene protecting group, by a specially prepared form of elemental iron in aqueous tetrahydrofuran under CO2 to aminomethyl-C-glycopyranoside (16), characterized as N-acetyl, peracetyl, and N-Cbz derivatives (7, 8, 9, 10), and converted with diazonium salt to a triazene derivative (11). Nitroalkenes are only mechanistic intermediates in our condensations with nitromethane, but they undergo Michael additions with a second mole of nitromethane to give novel 5,7-O-alkylidene-1,2-deoxy-1 nitro-2-nitromethyl-D-heptitols (3 and 14) as side-products.