Electrochemical glycosylation - Carbohydrate Chemistry: Chemical and Biological Approaches - page 163

73% yield. 9 Remarkably, no formation of self-condensation products was
observed in the latter case. Furthermore, the use of p-methylphenyl a-
and b- D -thioglucosides was also explored because the p-methyl group was
expected to induce a greater stabilisation of the intermediate radical
cation. Better results were indeed obtained since the thioglycoside
21 gave the corresponding methyl O-glucoside in 85% yield and the
anomer 22 afforded the same compound in 89% yield.
After these preliminary explorations, both teams continued to
study the use of thioglycosides as glycosyl donors in electrochemical
glycosylation. Sina¨ and co-workers developed the electrosynthesis of
disaccharides from phenyl and ethyl 1-thioglucosides bearing partici-
pating or non-participating protecting groups and the primary (6 and 17)
or secondary (24) sugar alcohols. 10 As expected, the glucosylations took
place with complete b-selectivity using the acetylated (16) or benzoylated
(23) thioglucosides, yields being generally higher when the benzoylated
derivative was used as the donor. When the perbenzylated phenyl
S-glucoside 15 or ethyl S-glucoside 25 were employed as the glucosyl
donor in CH 3 CN, b-selectivity (a/b = 1 : 3 to 1 : 4) was also observed with
the three sugar alcohols (6, 17, and 24). This selectivity could be sig-
nificantly improved to 1 : 22 by decreasing the temperature to 30 1Cas
demonstrated for the synthesis of disaccharide 7 from 15. 10
Under slightly modified conditions ( 20 1C, 100 mA constant current
or 1.7 V controlled potential), 7 was prepared on a 2.55 g (52%) or 3.05 g
(62%) scale as the pure b- D anomer. 11 The preparative electroglycosyla-
tion of non-sugar alcohols (pent-4-enol, iodoethanol, bromoethanol, and
3-bromo-1-propanol) was also carried out on a 20 to 60 g scale of the
glycosyl donor 15. 11 Interestingly, changing the solvent from CH 3 CN to
CH 2 Cl 2 showed that the selectivity of the glycosylation of 6 with the
perbenzylated ethyl S-glucoside 25 could be reversed (b/a = 3.5 : 1 in
CH 3 CN, b/a = 1:6inCH 2 Cl 2 , divided cell). 10
Following their idea of tuning the oxidation potential by introducing
substituents onto the phenyl group, Lubineau and co-workers performed