One-Step Aqueous Synthesis of Glycosyl Pyridinium Salts, Electrochemical Study, and Assessment of Utility as Precursors of Glycosyl Radicals Using Photoredox Catalysis.

A single step method for the production of unprotected glycosyl pyridinium salts has been developed involving treatment of the unprotected sugar with a pyridine, triethylamine, and either 2-chloro-1,3-dimethylimidazolinium chloride (DMC) or 2-chloro-1,3-dimethyl-1 H-benzimidazol-3-ium chloride (CDMBI) as an activator, in aqueous solution. Reaction efficiency is sensitive to steric effects, and in particular, ortho-substitution of the pyridine ring significantly decreased conversion to product; para-substitution of the pyridine ring is well tolerated. Cyclic voltammetry reveals that glycosyl pyridinium salts possess reduction potentials in the range of -0.9 to -1.4 V versus the standard calomel electrode, which are modulated by the electron effects of ring substituents. However, glycosyl pyridiniums are not found to be useful precursors for the production of glycosyl radicals under photoredox conditions.