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OBn
Ph
BnO
Ph
O
O
BnO
SPh
+
OBn
BnO
O
O
NIS, TMSOTf
MS 4 Å, MeCN, -40°C
O
O
O
O
TrocHN
O
BnO
O
TrocHN
O
OBn
HO
63% (
α/β
= 1:1.5)
O
F
BnO
OBn
F
O
OBn
BnO
OBn
OH
OH
OH
HO
HO
HO
O
O
AcHN
O
HO
O
O
HO
Globo-H
O
O
OH
O
OH
O
OH
HN
C
18
H
37
O
O
OH
O
OH
HO
O
C
13
H
27
HO
HO
OH
OH
HO
NHCbz
HO
OBn
OBn
NIS, TMSOTf
MS 4 Å, MeCN, -40°
C
HO
OBn
OBn
O
O
O
BnO
O
OBn
O
BnO
O
OBn
BnO
O
SEt
BnO
NHCbz
67% (
β
only)
OBn
OBn
SCHEME 12.17
Nitrile effect illustrated by Boons' approach to Globo-H.
12.3.3. 1,2-Anhydro Sugars
Another approach to 1,2-
trans
-equatorial glycosides fromDanishefsky and Bilodeau
makes use of 1,2-anhydro sugars and has been termed the glycal assembly method.
In this chemistry, 1,2-anhydro sugar, or glycal, is selectively oxidized at low
temperaturewith dimethyldioxirane to give a 1,2-epoxy sugar, which is then activated
for displacement by the acceptor alcohol with ZnCl
2
in THF [64] (Scheme 12.18).
When the acceptor is itself a hydroxyl glycal, only the protection of the alcohol
resulting from the opening of the intermediate oxirane is required before a further
glycosidic bond forming reaction can be conducted. Both the conversion of the glycal
to the anhydro sugar and the subsequent ring opening are typically highly selective
and lead to the formation of 1,2-
trans
-equatorial linkages, such as the
-galactosides
illustrated. Conditions have been developed, however, that allow access to 1,2-
cis
-
b
O
OTIPS
OTIPS
OTIPS
HO
O
O
1. DMDO
O
O
O
O
O
O
O
2. ZnCl
2
, THF
OH
HO
OTIPS
O
HO
OH
OH
OH
HO
HO
HO
O
AcHN
O
HO
O
O
Globo-H
O
HO
O
O
O
OH
OH
HN
C
18
H
37
C
13
H
27
O
O
OH
O
OH
O
HO
O
OH
HO
HO
OH
OH
OTIPS
O
O
OTIPS
O
O
O
1. DMDO
O
O
O
OTIPS
O
OH
2. ZnCl
2
, THF
O
BnO
OTIPS
HO
O
BnO
SCHEME 12.18
Glycal assembly method.
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