Electrochemistry of chalcogenoglycosides. Rational design of iterative glycosylation based on reactivity control of glycosyl donors and acceptors by oxidation potentials

Electrochemical properties of various para-substituted phenylthio-, phenylseleno-, and phenyltelluroglucopyranosides bearing acetyl, benzoyl, and benzyl protecting groups have been investigated to estimate the reactivity of chalcogenoglycosides toward electrochemical glycosylations. The oxidation potential of the chalcogenoglycosides shows good correlation with the ionization potential of chalcogen atoms, and decreases in the order thio-, seleno-, and telluroglycosides. It is also affected by the para-substituents, and the substitution effect correlates very well with the HOMO energy of para-substituted benzenechalcogenol and with the Hammett sigma(p)(+) value. Electrochemical glycosylation of telluroglycosides has been examined, and it was found that the use of an undivided cell is more effective than the use of a divided cell. Selective activation of the chalcogenoglycosides in bulk electrolysis based on their oxidation potentials has been examined, and the relative reactivity of the telluroglycosides can be estimated from their oxidation potentials. However, the relative reactivity of selenoglycosides in the preparative glycosylation was rather insensitive to the oxidation potential values.