Recent advances in gold-catalyzed glycosylation - Carbohydrate Chemistry: Chemical and Biological Approaches - page 142

Scheme 3
2.4 Gold(III)-catalyzed glycosidation with propargyl glycoside donors
In 2006, Hotha and Kashyap proved per-O-benzylated glucoside 7a,in
which a propargyloxyl is positioned at C-1, to be a novel and stable gly-
cosyl donor for specific anomeric activation and glycoside syntheses
catalyzed by AuCl 3 . Aglycones such as 4-penten-1-ol, menthol, benzyl al-
cohol, 3-chloropropanol, cholesterol, and sugar aglycone might be used
as glycosyl acceptor to afford glycosides (including disaccharide) as a/b
mixtures and in most cases the yields are good. per-O-Acetylated (7b)or
per-O-benzoylated propargyl glucosides (7c) did not give such transgly-
cosylated products, indicating the poor reactivity of disarmed donor in
this type of Au(III) catalyzed glycosylation. Indeed, per-O-benzylated
propagyl galactoside and mannoside both are ecient glycosyl donors
for this glycosylation strategy (Scheme 4). 6
A plausible mechanism for this AuCl 3 catalyzed glycosylation reaction
was proposed (Fig. 1). First the alkynophilic AuCl 3 is coordinated with the
propargyl group of the substrate 15 to form a complex 16 (step A), fol-
lowed by the formation of a gold carbene s-complex 17 (step B). Spon-
taneous cleavage of intermediate 17 gives the key oxycarbenium ion 18,
accompanied with an alkenyl gold complex 21 (step C). Trapping of
oxycarbenium ion 18 by various aglycones yields glycosides 20 (step D).
On the other hand, acid-mediated protodemetalation of 21 regenerates
AuCl 3 for the catalytic cycle, simultaneously extruding methylene-
oxirane 22, which promptly isomerizes to cyclopropanone (step E). 6
The successful use of Au(III)X 3 to catalyze the glycosylation reaction of
propargyl glycopyranosides as glycosyl donors with aglycones prompted
Hotha et al. to investigate the potential of using propargyl furanosides as
glycosyl donors under similar conditions. The compound per-O-benzy-
lated propargyl furanosides 23 ( D -ribf, D -xylf, D -araf, and D -lyxf) were used
as the substrates and monosaccharide 24 was used as an aglycon. In the