Chemistry Reference
In-Depth Information
Scheme 5
of the propargyl group of the
D
-ribf 23a in the presence of propargyl
pyranoside 26 allowed the formation of propargyl disaccharide 27
(Scheme 5).
10
Armed/disarmed strategy plays a very important role in the synthesis of
oligosaccharides, which takes advantage of differential reactivity of C-2
protecting groups such as alkoxyls and acyloxyls. Hotha et al. studied
armed/disarmed effects of propargyl glycosides in the presence of a
catalytic amount of AuBr
3
in the hope of realizing sequential glycosyla-
tion. Under the AuBr
3
-catalyzed condition, benzyl protected propargyl
mannopyranoside glycosyl donor (28, armed) smoothly underwent gly-
cosidation with benzoyl protected propargyl mannopyranoside aglycone
(29, disarmed) to afford the expected disaccharide 30, which was further
converted into the per-O-benzylated glycosyl donor 31 (armed) and sub-
jected to the disarmed aglycone 29. However, the anticipated trisaccharide
was not detected. The major product proved to be 30, with the 1,6-anhydro
derivative 32
11
as a minor product. The failure to produce the expected
trisaccharide was ascribed to the initial cleavage of the interglycosidic
bond of 31 by the oxophilic AuBr
3
(Scheme 6). Fine tuning of the protecting
group of 31 into methyls (partially or completely) allowed the formation of
trisaccharides in extremely low yields.
12
Recently, the synthesis of trisac-
chrides has been achieved by applying the armed/disarmed strategy using
1-ethynylcyclohexanyl glycosides in the presence of 5 mol% each of AuCl
3
and AgSbF
6
at 25 1C in moderate yields.
13
Hotha and co-workers also found that using thiols as aglycones and
AuBr
3
as a catalyst, propargyl glycosides 33 could be converted into
thioglycosides, which are an important kind of glycosyl donors for gly-
cochemists (Scheme 7). Propargyl mannoside offered a-thioglycosides
with 1,2-trans stereoselectivity (34), while propargyl glucoside and
Search WWH ::
Custom Search