Biomedical Engineering Reference
In-Depth Information
OBn
BnO
O
Boons and Zhu
BnO
OBn
BnO
BnO
O
OBn
HO
OBn
BnO
SEt
OBn
O
OBn
OBn
O
O
BnO
O
OBn
O
O
OBn
O
O
BnO
BnO
O
NHCbZ
IDCP, MS 4 Å
CH
2
Cl
2
/Et
2
O
71%
BnO
NHCbz
OBn
OBn
OH
OH
OH
HO
HO
HO
O
AcHN
O
HO
O
O
O
HO
O
O
OH
O
OH
O
HN
C
18
H
37
OH
O
O
OH
O
HO
O
C
13
H
27
OH
HO
HO
OH
OH
Danishefsky and coworkers
BnO
OBn
BnO
OBn
O
BnO
HO
OBn
OBn
O
F
BnO
O
O
O
OBn
OBn
OBn
O
BnO
BnO
AgClO
4
, SnCl
2
DTBP, Et
2
O
54%
O
O
OBn
O
BnO
BnO
OBn
(α/β
= 54:18)
SCHEME 12.24
Boons' and Danishefsky's approaches to the
a
-galactoside moiety of
Globo-H.
coworkers (Scheme 12.20) [60] and delivered in all cases the desired compound with
good
a
-selectivity.
12.4.2. Conformational Constraint by a 4,6-O-Acetal
It has been noted that the presence of a 4,6-
O-
benzylidene acetal in glucopyranosyl
thioglycosides and sulfoxides carrying ethers at
O-
2 and
O-
3 (Scheme 12.25) allows,
by preactivation methods, the formation of
-glucopyranosides with high selectivity
[86]. The difference in the influence of 4,6-
O-
acetal group between manno- and
glucopyranosides has been rationalized on the basis of the divergences in the evo-
lution of
O-
2-C2-C3-
O
-3 torsional interaction as the covalent donor proceeds toward
the glycosyl cation [21,87].
In the galactopyranosyl series, the presence of a 4,6-
O-
benzylidene acetal is
also highly
a
-directing, as illustrated in the last step of the synthesis of the tetra-
saccharide repeating unit of the lipopolysaccharide from
E. hermanii
ATCC 33650
(Scheme 12.13) [51].
a
PhSOTf, DTBMP
CH
2
Cl
2
, -78°C
O
O
O
O
Ph
Ph
O
OBn
O
BnO
O
O
O
SPh
BnO
BnO
O
O
O
O
O
O
α
or
β
(α/β
> 95:5)
O
HO
O
70-80%
SCHEME 12.25
Benzylidene-directed synthesis of
a
-glucopyranosides.
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