Formal Glycosylation of Quinones with exo -Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover.

The intermolecular C-O coupling reaction of 1,4-quinones with exo -glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O -ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C -glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.