Biomedical Engineering Reference
In-Depth Information
O
SPh
O
SPh
Si
O
Si
O
OLev
OH
BnO
BnO
O
O
BnO
BnO
O
O
O
O
AcO
1. NIS, AgOTf, CH
2
Cl
2
MS 4 Å, -20°C (67%)
HO
NIS, AgOTf, CH
2
Cl
2
MS 4 Å, -20°C
BnO
AcO
BnO
AcO
O
O
BnO
O
BzO
O
OBz
89%
HO
2. Et
3
N, CH
2
Cl
2
(80%)
O
O
OBz
OBz
O
O
Si
O
O(CH
2
)
3
N
3
O(CH
2
)
3
N
3
BzO
O
BzO
OBz
OBz
(β
only)
OH
BnO
OH
O
O
O
Si
OH
O
BnO
OH
O
O
O
Deprotection steps
O
O
O
AcO
HO
AcO
BnO
OH
OH
O
O
HO
BnO
O
O
OBz
O
OH
O
HO
O
OH
BzO
O
O
O
Si
O
O(CH
2
)
3
N
3
O(CH
2
)
3
NH
2
OH
O
BzO
HO
OBz
OH
(β
only)
SCHEME 12.45
3,5-
O-
Bis(
tert
-butyl)silylene acetal-protected donors in arabinogalactan
synthesis.
toward cleavage. The 3,5-
O-
bis(
tert
-butyl)silylene acetals, with their longer Si-O
bonds, proved to be much easier to prepare and considerably more stable. With such
systems in hand, a number of activation methods were investigated independently
by Boons and coworkers [129] and Crich and coworkers [130]. Among them, the use
of NIS and silver triflate was found to be optimal and provided a number of
b
-arabinofuranosides with good yield and selectivity. Using this approach, Boons
and coworkers synthesized a fragment of an arabinogalactan (Scheme 12.45), an
important constituent of primary plant cell walls that plays an important role in the
differentiation of young plant cells.
Boons' explanation for the observed
-selectivity with these donors relies on
the intermediacy of oxocarbenium ions (Scheme 12.46), which have significant
double bond character between endocyclic oxygen and C1 and place these two atoms
and C2 and C4 in one plane. Consequently,
L
-furanoside-derived oxocarbenium ions
can adopt two possible low-energy conformations in which C3 is either above (
3
E
)or
below the plane (
E
3
) of C4,
O-
4, C1, and C2. The analysis of the Newman projection
of the
3
E
conformer shows that nucleophilic attack from the
b
-face would suffer
significant steric interactions froman eclipsedC2 substituent, contrary to the
b
face on
which attack will encounter staggered substituents. In contrast, nucleophilic attack
from the
a
-face of the
E
3
conformer is predicted to be disfavored because it will
experience an eclipsed H2.
a
3
E
conformer
1,2-
trans
attack
straggered, favored
E
3
conformer
1,2-
trans
attack, eclipsing
interactions, disfavored
α
face
H
2
C
3
H
2
O
O
O
O
SPh
H
1
C
1
O
C
4
H
1
C
1
O
C
4
O
O
Si
Si
BnO
BnO
OR
O
2
C
3
O
2
1,2-
cis
attack, eclipsing
interactions, disfavored
β
face
1,2-
cis
attack
straggered, favored
SCHEME 12.46
Rationalization of
b
-selectivity.
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