Electrochemical glycosylation - Carbohydrate Chemistry: Chemical and Biological Approaches - page 162

Two decades later, a similar approach was developed starting from
hydroquinone bis-glycosides. 7 In a divided cell, upon electrochemical
oxidation of 11 in the presence of tetra-O-benzyl- D -glucopyranose 12, the
trehaloses 13 were recovered in modest yield and selectivity. Unfortu-
nately, no details (n. of equiv., electrolyte, temperature, time) about the
electrosynthesis of 13 were provided in the publication. On the other
hand, higher yields and selectivity were observed when an analogue of 11
bearing three methyl groups onto the dialkoxyphenyl ring was glycosi-
dated using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the
promoter. These results indicate that the hydroquinone O-glycosides are
better suited for chemical than electrochemical glycosylation.

2.2 Electrochemical activation of S-glycosides
Some years after the seminal contribution of Noyori and Kurimoto, 6 the
groups of Sina¨ and Amatore 8 and those of Lubineau and Balavoine 9
independently described the anodic glycosylation of aryl 1-thioglycosides,
taking advantage of their lower oxidation potentials in respect to aryl
O-glycosides. The first experiments described by Sina¨ and co-workers
were conducted in a divided cell at a constant current using Bu 4 NBF 4 as
supporting electrolyte, acetonitrile as the solvent, and in the presence of
potassium carbonate in the anodic compartment. It was found that the
perbenzylated phenyl S-glycoside 15 gave better yields of glycosylation
with MeOH (84%, b/a ratio = 3 : 1) or 17 (63% yields, b/a ratio = 4 : 1) when
compared to the peracetylated derivative 16 (50% for the two acceptors)
which, however, afforded exclusively the b- D -glycosides. 8

17

15
16

18
19

Lubineau team chose to carry out the glycosylations in an undivided
cell at a constant potential in the presence of LiClO 4 as the supporting
electrolyte, without any additive to neutralize the acid formed during the
electrochemical process. Under these conditions, with methanol as the
acceptor, the peracetylated glucosyl donor 16 led to a 13 : 87 a/b mixture
of methyl glucosides in a low yield (16%) whereas using the unprotected
phenyl thioglucoside 20 as the donor, a 4 : 6 a/b mixture was obtained in