Recent advances in gold-catalyzed glycosylation - Carbohydrate Chemistry: Chemical and Biological Approaches - page 150

Scheme 15
Fig. 2
used as aglycones; (3) Au(I) is used catalytically; (4) the yields are
generally excellent and specific stereoselectivity could be attained in
numerous cases; (5) the reaction condition is mild and orthogonal
to other glycosylation protocols. These merits were convincingly
exemplified by the ecient total synthesis of the acid labile betulin
and betulinic acid derivatives. 29
The mechanism of this Au(I)-catalyzed glycosylation reaction with
glycosyl ortho-alkynylbenzoates as donors was unambiguously estab-
lished. First alkynophilic Au(I) is coordinated with the carbon-carbon
triple bond of the donor (78) to form a complex (79, step A), followed by
the cleavage of the glycosidic bond to form the sugar oxocarbenium (80)
and the isochromen-4-yl-gold(I) s-complex (83a, step B). Attack of 80 by
an aglycone (81) generates the glycoside (82, step C). The crystal structure
of 83 was obtained, which allowed a deeper insight into the catalytic
mechanism. It was critical to discover that a strong protic acid such as
HOTf is required to convert Au(I) complex 83 into isocoumarin 84 and
simultaneously regenerates the active Au(I) species for the catalytic cycle
(step D). This finding has enabled the use of low loading in the Au(I)
catalyst in the presence of a strong protic acid (Fig. 2). 28,30-32
They also discovered that by use of sialyl 2-ortho-hexynylbenzoate 85
as a donor, Ph 3 PAuNTf 2 catalyzed the challenging direct sialylation of