Chemistry of 1-Alkoxy-1-glycosyl Radicals:  Formation of β-Mannopyranosides by Radical Decarboxylation and Decarbonylation ofmanno-Heptulosonic Acid Glycoside Derivatives

Abstract

A method for the preparation of highly enriched beta-mannopyranosides is described. A glycosyl donor 28 is prepared from tetraallyl mannonolactone by standard means and coupled to a number of primary carbohydrate alcohols, resulting in the isolation in excellent yields of axial disaccharides. Following exchange of the allyl groups for acetyl esters, the furan is oxidatively cleaved with catalytic RuO(2) and NaIO(4) and the resulting acid subjected to the Barton decarboxylation. Coupling of 28 to a secondary alcohol, methyl 2,3-isopropylidene-alpha-L-rhamnopyranoside, resulted in an apparent inversion of anomeric stereochemistry and isolation of an equatorial disaccharide.

Chemistry of Acyl Radicals

“…215 Thus, the reaction of 198 in the presence of methyl vinyl ketone (199) gave a mixture of saturated and unsaturated products 200 and 201 (Scheme 150). A similar photolysis with ethyl acrylate or styrene gave mainly unsaturated products.…”

“…152 When the decomposition reaction of complex 200 was conducted in the presence of methyl vinyl ketone (199), 1,4-diketone 202 was formed (Scheme 156). Boron enolate 201 is proposed as the precursor for 202.…”

Enhanced Diastereoselectivity in β-Mannopyranosylation through the Use of Sterically Minimal Propargyl Ether Protecting Groups

“…The extract was washed several times with sat NaHCO 3 , and dried over Na 2 SO 4 . Evaporation of the solvent in vacuo gave an oil, which was chromatographed on a flash silica gel column (hexane:ethyl acetate; 4:1) to give 12 (0.34 g, 93%) as a white solid MP 128 ºC [α] 27…”

Radicals and Carbohydrates

“…200 These radicals were prepared from hemithio ortho esters such as 159 by treatment with nBu 3 SnH, observing that hydrogen quenching led preferentially to the β anomer in high yields (Scheme 22). Moreover, Crich 201 proved that the Barton disaccharide 161 with inversion of configuration at C-1 j . Examples of these decarboxylations in the 2ulofuranosonic acid series have also been reported by Crich and Ritchie 202 and Garner.…”