Synthesis of C-pento, -hexo-, and -heptulo-pyranosyl compounds via radical CC bond-formation reactions

C‐Glycosidations of a 2‐Ketohexosyl Bromide with Electrophilic, Radical, and Nucleophilic Anomeric Carbons

“…This course is in fact not unexpected, since even the prototype nucleophilic glucopyranosyl radical, generated from acetobromoglucose, [17] produces the hydrogen transfer product 1,5-anhydroglucose tetraacetate in a yield of over 20% on reaction with acrylonitrile, [17b] and the 2-oxoglycosyl radical 8 is clearly less nucleophilic, if not essentially electrophilic due to its push-pull Ϫ or capto-dative [18] Ϫ substitution: an electron-withdrawing group such as a carbonyl moiety next to a carbon radical entails a stabilization by some 10Ϫ12 kcal/mol in relation to a ''pure'' C-radical, [19] and the additional presence of a cycloalkoxy group allows for a variety of mesomeric forms (i.e., 8a o 8b o 8c, Scheme 3). This is likely to result in a planar arrangement of O5ϪC1ϪC2ϪC3 (i.e., a 4 H 5 half chair form rather than a mere 4 C 1 -type chair geometry as in 8a).…”

“…Nomenclature: For simplicity, the anomerically C-extended products described here are designated as C-glycosides rather than as heptos-3-ulose (16,17), octulose (20), or nonulosonic acid (10) derivatives, as officially allowed. [26] Accordingly, the C-aglycon residue (with prime numbering) is considered to be substituted by a C-hexopyranosyl moiety (standard numbering, C-1 to C-6).…”

“…[17] However, attempts to induce radical-induced coupling of ulosyl bromide 1 with acrylates or acrylonitrile under a variety of conditions failed, since the 2-oxoglycosyl radical 8, generated by Bu 3 SnH/hν or AIBN, is quantitatively trapped by hydrogen even in acrylonitrile as the solvent, to afford the known [3b,3c] 1,5-anhydro--fructose tribenzoate (6).…”

“…[21] Scheme 3 That the free radical-mediated C-glycosidations of ulosyl bromide 1 to 10 and 11 proceeded α-selectively, as already observed for the deuteration (Ǟ 7), followed most simply from their reduction products with BH 3 /pyridine, 12 and 13, which proved to have α--gluco configurations on the basis of their pyranoid ring J 1,2 couplings of 5.3Ϫ5.4 Hz, as usually found for α-C-alkyl or allyl glucosides. [17,22] Another preparatively useful ensuing reaction of C-glycosid-2-uloses of type 10 or 11 derives from their propensity to eliminate benzoic acid from the 3,4-positions. Simple treatment of 10 with, for example, sodium hydrogen carbonate in aqueous acetone at ambient temperature, smoothly afforded the enolone ester 9 (85% isol.…”

Synthesis of a Carbohydrate-Centered C-Glycoside Cluster1

“…15 This reaction furnished methyl ester 3, which in turn was saponified 16 to give the corresponding acid 4 in situ. This has to be a fast reaction in order to avoid decomposition of the expensive reagent.…”