Biomedical Engineering Reference
In-Depth Information
1. Ph
2
SO, Tf
2
O, TTBP, CH
2
Cl
2
-60 to 0°C (72%)
MeO
2
C
RO
OBn
MeO
2
C
HO
OBn
O
O
HO
N
3
O
N
3
+
BnO
BnO
2. NH
2
NH
2
·H
2
O pyridine/AcOH (99%)
3
3
SPh
R = Lev or Bn
(β/α
= 10:1)
1. Benzyl 2-(hydroxymethyl)benzoate
Ph
2
SO, Tf
2
O, TTBP, CH
2
Cl
2
-60 to 0°C
2. H
2
, Pd/C, NH
4
OAc, EtOAc, MeOH
OBn
MeO
2
C
HO
O
OBn
MeO
2
C
RO
BnO
OBn
MeO
2
C
OBn
MeO
2
C
RO
O
SPh
O
O
O
O
BnO
BnO
Tf
2
O, DTBMP
CH
2
Cl
2
, -60 to 0°C
BnO
CO
2
H
SPh
A R = Lev (65%,
β/α
>10:1)
A
R = Bn (65%,
β/α
>10:1)
OBn
MeO
2
C
HO
O
O
N
3
BnO
3
1. BSP, Tf
2
O, TTBP, CH
2
Cl
2
,
-60 to 0°C (51%,
MeO
2
C
HO
OBn
OBn
MeO
2
C
MeO
2
C
O
OBn
β/α
>10:1)
O
O
O
O
A
O
N
3
BnO
2. NH
2
NH
2
O pyridine/AcOH (78%)
·H
BnO
BnO
3
MeO
2
C
RO
OBn
OBn
MeO
2
C
O
O
O
BnO
BnO
BSP, Tf
2
O, TTBP
CH
2
Cl
2
, -60 to 0°C
SPh
MeO
2
C
BnO
OBn
MeO
2
C
OBn
OBn
MeO
2
C
O
O
O
O
O
O
N
3
BnO
BnO
BnO
69%
3
4
(
β/α
>10:1)
OH
HO
2
C
HO
OH
HO
2
C
OH
HO
2
C
O
O
Series of deprotection
O
O
O
O
NH
2
HO
HO
HO
76%
3
4
SCHEME 12.42
Application to the synthesis of a
b
-alginate.
-product. The alternative
4
H
3
half-chair conformer
of the oxocarbenium ion is strongly disfavored due to misplacement of all sub-
stituents [81]. The second hypothesis, supported by low-temperature NMR, involves a
rare equatorial anomeric triflate. Whereas axial anomeric triflates have been fre-
quently characterized by NMR since they were first reported in 1997 [22], van der
Marel and coworkers' report constitutes the first experimental evidence in support of
the existence of equatorial triflates that had previously been evoked only as possible
intermediates in the literature [124].
Exploiting these studies, the authors described the synthesis of a fully
conformation, both leading to the
b
-alginate
trisaccharide [122a] andmost notably of a set of oligomers ofmannuronic acid alginates
[125]. Successive glycosylations (Scheme 12.42) relying on this robust
b
-selective
method allowed the synthesis of mono- to hexamer alginates. It is noteworthy that the
stereochemical outcome of the reaction is independent of the size of the acceptor
employed. The synthesis illustrated features an interesting use of Kim's method [126],
which is thus seen to be orthogonal to this activation of the thioglycoside.
b
12.7.
-ARABINOFURANOSIDES
b
Arabinofuranoses are predominantly found in bacteria and plant cell walls, where
they play diverse biological roles such as protecting the cell from aggression and
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