Carbanions in Carbohydrate Chemistry: Approaches to Chemical Precursors of C-Nucleosides

The condensation of D-ribofuranosyl halides containing participating (benzoate) and non-participating (benzyl) substituents, with sodio dialkyl malonates and sodio triethyl 1,1,2-ethanetricarboxylate is described. In the presence of participating groups at C-2, the major and sometimes exclusive products were the 1,2-ketal derivatives. Both α- and β-anomeric D-ribofuranosyl malonates were formed in the non-participating series. The possibility of epimerization (anomerization) was experimentally demonstrated.Similar results were obtained with the more highly functionalized tricarbethoxy carbanion. For the participating series however, 20% of C-glycoside was obtained. Condensations with sodio diethyl malonate were also done in the D-arabino series (O-benzyl protecting group) and the anomeric C-glycosyl compounds were isolated and characterized. The mechanistic aspects of these condensations are discussed.