Biomedical Engineering Reference
In-Depth Information
I
I
OBn
OBn
CN
CN
OMe
1. Ph
2
SO, Tf
2
O
TTBP, CH
2
Cl
2
, -20°C
1. Bu
3
SnH, AIBN
Xylene, reflux
OBn
OBn
O
O
O
O
O
O
O
O
NAPO
NAPO
2.
2. DDQ
OMe
O
57%
(over 2 steps)
SPh
O
O
HO
O
O
BnO
CN
O
OMe
OBn
80% ( only)
O
O
O
O
HO
O
O
CN
BnO
O
OBn
OMe
OBn
1. BSP, Tf
2
O
TTBP, CH
2
Cl
2
, -60°C
BnO
OBn
O
O
2.
3. DDQ
O
O
Ph
O
O
OBn
O
O
O
Ph
NAPO
O
O
88%
O
O
SPh
HO
HO
OMe
HO
OH
SPh
O
O
HO
1. BSP, Tf
2
O
TTBP, CH
2
Cl
2
, -60°C
O
HO
OH
O
O
2.
3. Full deprotection
O
BnO
HO
HO
OH
O
O
73%
O
O
HO
HO
OH
SCHEME 12.14
Synthesis of a tetrasaccharide from
P. shigelloides
.
A second generation method for rhamnopyranoside formation makes use of
a radical nitrile transfer reaction to access the acetal radical that then undergoes
the customary highly selective radical fragmentation reaction [52]. This method
possesses the advantage over the earlier thioester-based one of the employment of
a more broadly stable radical precursor that allows greater flexibility in the range
of reaction conditions to which it is stable. This sequence was employed in the
synthesis (Scheme 12.14) of a tetrasaccharide subunit of the lipopolysaccharide from
Plesiomonas shigelloides
that contains a most unusual 6-deoxy-
-
D
-mannohepto-
pyranoside unit [53]. The same fragmentation reaction was also employed in the
synthesis of a
b
-
D
-rhamnan from
E. hermanii
ATCC 33651 [54].
Yet another method involves the synthesis of a 6-deoxy-6-sulfanyl-4-
O
,6-
S
-
acetal as a control unit for the glycosylation reaction, followed by desulfurizationwith
Raney Nickel. The inclusion of an electron-withdrawing group, the cyano group, in
the acetal was necessary to preclude activation of the C6 sulfur during the glyco-
sylation step [55]. Raney Nickel desulfurization of the coupled product occurred in
76% yield, providing the
b
-rhamnose derivative [55] (Scheme 12.15).
b
CN
CN
OMe
1. BSP, TTBP
Tf
2
O, CH
2
Cl
2
, -78°
C
H
2
Raney Nickel
OMe
OH
OBn
OBn
S
Ph
S
O
O
O
Ph
HO
O
O
O
O
O
O
HO
BnO
76%
2.
OMe
BnO
O
O
O
SPh
O
O
HO
O
O
89% (
β
only)
SCHEME 12.15
Desulfurization route to
b
-rhamnopyranosides.
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