We describe herein a convenient strategy for the construction of C,C-glycoside building blocks via the intermediacy of tertiary pseudoanomeric radicals. Application of an iron-mediated hydrogen atom transfer/Michael−Giese coupling enables the anomeric quaternization of readily available exo-glycals with good to complete stereocontrol in the pyranose and furanose series. Carefully optimized conditions allow the use of challenging trisubstituted derivatives prone to undergo further elaboration to stable neoglycoconjugates. Preliminary results for direct C-disaccharide synthesis are also discussed.
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.
We report herein the development of a stereodivergent route towards polyhydroxylated bicyclic azetidine scaffolds, namely 6-azabicyclo[3.2.0]heptane derivatives. The strategy hinges on a common bicyclic β-lactam precursor, which is forged by way of a rare example of a cationic Dieckmann-type reaction, followed by IBX-mediated desaturation. Substrate-controlled diastereoselective oxidations then allow the divergent preparation of novel iminosugar mimics.
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