Synthesis of α-arylglycosides by Ni-photoredox arylation of sugars with an organic photocatalyst

Herein we report a Ni-photoredox catalytic methodology for the synthesis of C-aryl glycosides via arylation of monosaccharides unprotected at the anomeric position. In our work, the iridium-complex previously employed as photocatalyst in this kind of Ni-photoredox arylation of alcohols, was replaced by 5CzBN – a readily available donor-acceptor cyanoarene photocatalyst. Six benzyl protected monosaccharides showed moderate to good reactivity and, in most cases, an excellent selectivity for the α-C-aryl glycoside product (α/β ≥ 10:1). Electron-rich and electron-poor aryl bromides were found equally suitable for the reaction, affording the corresponding products with comparable yields. Stern-Volmer experiments and yield trends are supportive of a mechanism involving reductive quenching of the photocatalyst with a sugar-NHC adduct, generating a glycosyl radical which then enters a Ni-catalytic cycle.