Synthesis of α-c-arylglycosides by ni-photoredox catalysis

Photocatalysis has proved to be a valid instrument to overcome the classic problems related to functionalization of carbohydrates, enabling new pathways of reactivity and leading to high-value intermediates. Herein we report a Ni-photoredox catalytic strategy for the synthesis of C-aryl glycosides through the direct arylation of monosaccharides unprotected at the anomeric position. Ir complexes have been previously employed as photocatalyst in the Ni-photoredox arylation of alcohols. In this work, we replaced the Ir photocatalyst with 5CzBN, an easily accessible (and cheaper) donor-acceptor cyanoarene (Scheme 1). Six O-benzyl monosaccharides unprotected at the anomeric position showed moderate to good reactivity with a variety of arylbromides 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 support a mechanism involving reductive quenching of the photocatalyst with a sugar-NHC adduct, generating a glycosyl radical which then enters a Ni-catalytic cycle.