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Table 4 Iron triflate-catalyzed glycosylation using acetate 1, oxazoline 2 and different
2-acetamide-
b
-acetate donors 83-85.
a
OAc
OAc
OAc
Acceptor
cat. Fe(OTf
)
3
O
AcO
O
NH
BnO
O
NH
AcO
OMe
AcO
AcO
O
AcO
OAc
AcO
OBn
OBn
O
N
CH
2
Cl
2
,
μ
w, 80 °C
O
O
R
O
R
R
1
78
: R = CH
3
83
: R = CH
3
86
: R = PhCH
3
87
: R = CCl
3
88
89
: R = PhCH
3
84
: R = CCl
3
85
: R = C(CH
3
)
3
: R = PhCH
3
90
: R = CCl
3
91
: R = C(CH
3
)
3
: R = C(CH
3
)
3
Entry Donor Acceptor Product (yield)
1 oxazoline 2 77 78 (99%)
b
2 oxazoline 2 81 82 (13%)
c
3
1
, R = CH
3
77 78
(98%)
b
4 1,R
=
CH
3
none 1a-1 (100%)
d
5 83,R
=
PhCH
3
77 86 (82%)
6 83,R
=
PhCH
3
none 89 (62%)
7 84,R
=
CCl
3
77 87 (
o
5%)
8 84,R
=
CCl
3
none nr
e
9 85,R
=
C(CH
3
)
3
77 nr
e
a
Conditions: 2 equiv of donor, 15 mol% of Fe(OTf)
3
6.2DMSO in CH
2
Cl
2
under microwave
irradiation at 80 1C.
b
45 min.
c
3h.
d
1/1a ratio of 4/1.
e
No reaction.
1
92
93
3
94
Scheme 20 Possible mechanism for the iron(III) triflate-catalyzed glycosylation using
2-acetamido-2-deoxy-
b
-glucopyranosyl donor 1. TTBP
=
2,4,6-tri-tert-butylpyrimidine.
were completely ineffective.
74
Without the nucleophile, oxazolines 2, 90
and 91 were not detected except for oxazoline 89 from the tolylamide 83.
These experiments suggest that glycosylation does not necessarily require
the formation of the oxazolinium intermediate. The glycosylation may
not only involve activation of the anomeric acetate but also a pre-
complexation of the catalyst by a suitable amide group such as the
acetamide found in 1 (see 92, Scheme 20) or tolyl amide 83. Effective
amide pre-complexation of Fe(OTf)
3
may be prevented for electronic
(NHTCA, donor 84) or steric (NHPiv, donor 85) reasons thus blocking
glycoside formation as experimentally observed. Alpha-ionic pair 93 from
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