Electrochemical glycosylation - Carbohydrate Chemistry: Chemical and Biological Approaches - page 169

the aryl group and the sugar hydroxyls protecting groups. Aiming
to develop a programmable one-pot glycosylation strategy based on
suitable combinations of donors (e.g. 75), donor-acceptors (e.g. 76 and
77), and acceptors (e.g. 73), the synthesis of 1-thiodisaccharide 78 and
1-selenodisaccharide 79 in an undivided cell was achieved. The results
proved that the Se-glycosides were less selectively activated than the
corresponding thioglycosides.
The groups of Fairbanks and Compton were also interested in
the selective activation of S-andSe-glycosides. In a first paper 27
they carefully investigated the electrolysis of six aryl thioglycosides
and selenoglycosides as well as a phenyl O-glycoside using the fast
scan technique in order to determine their formal (thermodynamic)
potentials. This study allowed the validation of the use of the slow scan
cyclic voltammetry as a simpler method for the determination of the
glycosyl donor reactivity. Actually, square wave and cyclic voltammetry
are widely employed to measure the peak potential, i.e.thepotential
at which the peak of oxidation wave occurs, which is determined by
the formal potential, mass transport to the electrode, electrode
kinetics of the electron transfer, and the follow-on homogeneous
kinetics. The discovery, during the above-mentioned investigation,
that the peak oxidation potential for perbenzoylated phenyl Se-glycoside
was very close to that measured for the perbenzylated analogue, clearly
indicated that a selective electrochemical glycosylation based on
reactivity tuning due to protecting group effects would not be effective.
Therefore, the approach relying on the selective oxidation of
selenoglycoside donors in the presence of thioglycoside acceptor-
donor, was followed by Fairbanks, Compton and their co-workers. 28
The phenyl Se-glycoside 68 was first coupled with 17 in CH 3 CN
containing n-BuN 4 ClO 4 as the electrolyte, which was preferred to the
tetrafluoroborate salt, to give the disaccharide 80 in excellent yield as
the pure b- D anomer. Then, the same donor 68 was submitted to
electrolysis in the presence of the S-glycoside alcohol 76 to afford the
corresponding disaccharide 81 in modest yield. The latter compound
was coupled to the acceptor 17 using a higher oxidation potential (2.0 V
vs. 1.7 V) to give the trisaccharide 82 in high yield and with total b- D
selectivity. 28