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Rh(II) Carbene-Mediated Synthesis of Methyl α- and β-Ketopyranosides: Preparation of Carbene Precursors, Quaternarization of the Anomeric Position, and Ring Opening of γ-Lactones

Methyl α- and β-ketopyranosides were efficiently prepared by carbene-mediated quaternarization of the anomeric position of corresponding aldopyranosides. Preparation of carbene precursors proved to be tedious and required a two-step procedure involving first bromoacetylation, followed by diazo-transfer with N,N′-ditosylhydrazine and DBU. Selective functionalization of the anomeric C-H bond was then achieved under Rh2(OAc)4 or Rh2(acam)4 catalysis. Finally, ring opening of the resulting γ-lactones delivered α- and β-ketopyranosides with the anomeric position functionalized by an independent chain.