Chemistry Reference
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Table 2 Iron triflate-catalyzed glycosylation of alcohol 77 with acetate 1 under various
conditions.
OAc
OAc
BnO
OMe
catalys
t
CH
2
Cl
2
,
μ
w, 80 °C
O
NHAc
O
NHAc
BnO
AcO
OBn
OBn
AcO
OMe
OAc
O
AcO
AcO
OBn
OBn
O
OH
O
1
77
78
Donor :
Acceptor
Time
(min)
Yield
(%)
Entry
Catalyst
TTBP
1
1 : 2
Fe(OTf)
3
6.2DMSO (15 mol%)
30
-
82
2
2 : 1
Fe(OTf)
3
6.2DMSO (15 mol%)
30
-
92
3
2 : 1
Fe(OTf)
3
6.2DMSO (15 mol%)
45
2 equiv
98
4
1 : 2
Fe(NTf
2
)
3
6.2DMSO (15 mol%)
30
-
51
5
2 : 1
FeCl
3
(15 mol%)
30
-
31
Tf
=
trifluoromethanesulfonate; TTBP
=
2,4,6-tri-tert-butylpyrimidine.
Table 3 Iron triflate-catalyzed glycosylation using acetate donor 1.
a
Entry Donor (D) Acceptor (A)
Product
D : A Time Yield (%)
OTBDPS
OAc
OTBDPS
Bn
O
BnO
O
AcHN
O
AcO
O
HO
1
1
AcO
2 : 1
1 h
70
OBn
BnO
OMe
OMe
79
80
OBn
OMe
OAc
Bn
BnO
O
BnO
HO
O
AcHN
AcO
O
20 (100)
b
2
1
BnO
AcO
2 : 1
3 h
O
OBn
OMe
81
82
3
1
81
82
4:1 16h 25
a
Conditions: 15 mol% of Fe(OTf)
3
6.2DMSO, 2 equiv of TTBP in CH
2
Cl
2
under microwave
irradiation at 80 1C.
b
Yield based on recovered acceptor. TBDPS
=
t-butyldiphenylsilyl.
b-1,4-glycosidic linkage between two
D
-glucopyranosyl units (entries 2, 3,
Table 2). Under all conditions attempted, very moderate yields were
obtained (20-25%), with a quantitative recovery of the acceptor 81
(entry 2, Table 3), however.
In the direct synthesis of b-pyranosides starting from GlcNAc deriva-
tives, oxazolinium ion 76 is assumed to be an intermediate, explaining
the high b-stereoselectivity. The glycosylation results with the less
nucleophilic 4-OH acceptor 81 by oxazoline 2 compared to b-acetate 1
(entry 2, Table 4 vs. entry 2, Table 3, 13 vs. 20% yield) were, however,
different suggesting another route to the glycoside.
To study this possibility, glycosylation with iron(
III
) triflate was
examined by varying the electronic and/or the steric properties of the N-
substituent in
D
-glucosaminyl donors 83-85. This was done taking the
glycosylation of primary alcohol 77 under the optimized conditions (entry
3, Table 4) as a reference test. Similarly to 1, tolylamide 83 provided the
expected glycoside 86, while trichloroacetamide 84 or pivaloyl amide 85
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