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O
OH
(TMS)
2
S
TMSOTf
O
O
RO
SH
RO
Gluco-
R = OBn,
88%,
α
-anomer only
Galacto-
R = OBn,
90%
α
-anomer only
Gluco-
R = OAll,
78%
α
-anomer only
Gluco-
R = 2, 3-OAll, 4- OBn 85%
α
-anomer only
Galacto-
R = 2, 3-OAll, 4- OBn 92%
α
-anomer only
Zhu synthesis of
-glycosyl thiols from 1,6-anhydro sugars [27, 28].
SCHEME 2.6
with catalytic amount of trimethylsilyl triflate at elevated temperature proved to be
essential for the opening of the 1,6-anhydro ring with an excellent stereoselectivity.
The methodology is based on the previous application of BTMSS in the acid-catalyzed
ring opening of
O
-benzyl protected 1,2-anhydro-glucopyranose (Brigl's anhydride
benzyl analog) originally developed by the Schmidt group [29].
Bundle and coworkers [30] reported a simple strategy of nucleophilic displace-
ment of the anomeric
-bromide of peracetylated D-mannopyranose with thioacetate
prepared
in situ
by mixing thioacetic acid and potassium
tert
-butoxide. This pro-
tocol leads exclusively to the formation of the strategically very important 1-thio-
−
mannopyranoside as depicted in Scheme 2.7.
Knap and coworkers [31, 32] (Scheme 2.8) reported an efficient strategy for the
synthesis of
-GalNAc thiols. The procedure is based on the treatment
of 2-acetamido-2-deoxy-1,3,4,6-tetra-
O
-acetyl-
-GlcNAc and
-D-galactopyranose or glucopyra-
nose with Lawesson's reagent in toluene at 80
◦
C with the formation of thiazoline
derivatives, followed by hydrolysis under acidic conditions (TFA in wet MeOH). The
hydrolysis occurred exclusively at the iminium carbon and produced
-GlcNAc and
-GalNAc thiols in quantitative yields.
OAc
OAc
OAc
OAc
O
AcSH, t-BuOK
DMF-THF
O
AcO
AcO
Ac
AcO
SAc
Br
NaOMe/MeOH
OH
OH
OH
RI
OH
O
HO
HO
O
SR
H
HO
SNa
R= CH
3
, (CH
2
)
6
CH
3
SCHEME 2.7
Bundle synthesis of 1-thio-
-mannopyranose [30].
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