Biomedical Engineering Reference
In-Depth Information
R
R
O
O
R
R
O
O
Ph
Ph
N
N
CO
2
M
e
h
ν
Ph
N
Ph
N
+
Me
O
N
O
N
Me
N
N
O
Me
O
Me
CO
2
Me
CO
2
Me
159a
(R = H)
159b
(R = CO
2
Me)
159c
(R = CH
2
OAc)
160a
(R = H)
160b
(R = CO
2
Me)
160c
(R = CH
2
OAc)
161a
(R = H; 59%)
161b
(R = CO
2
Me; 52%)
161c
(R = CH
2
OAc; 37.5%)
162a
(R = H; 12%)
162b
(R = CO
2
Me; 10%)
162c
(R = CH
2
OAc; 7.5%)
OTBS
O
OTBS
OTBS
Ph
O
O
O
N
Ph
h
ν
Ti(O
i
-Pr)
4
EtOH, reflux
62%
N
O
N
Me
Ph
N
+
N
SO
2
O
N
Me
45%
S
N
O
N
Me
O
O
O
CO
2
Et
163
164
165
166
O
OMe
N
N
Me
H
CO
2
H
Quinocarcin
SCHEME 13.34
13.2.5.3. Nonstabilized Azomethine Ylides Several groups have developed
approaches to the generation of nonstabilized azomethine ylides for cycloaddi-
tions [65]. For example, Vedejs and Martinez employed an imidate-derived azo-
methine ylide in their synthesis of retronecine. Exposure of lactam
167
to methyl
triflate produced iminium salt
168
(Scheme 13.35) [66]. The silane group in
168
was
then treated with CsF to give ylide
169
that underwent cycloaddition with methyl
acrylate giving
170
in 51% yield. A series of oxidation state manipulations and
deprotection gave (
)-retronecine.
A few years later, Pandey and Lakshmaiah devised a strategy that used
pyrrolidine
171
, available from optically active 4-hydroxy-
L
-proline, as the azo-
methine ylide precursor (Scheme 13.36) [67]. In this approach, the desilylation of
171
BnO
BnO
BnO
CH
3
OTf
CsF
O
OMe
OMe
N
N
N
TMS
TMS
167
168
169
CO
2
Me
51%
OH
CO
2
Me
HO
BnO
H
steps
N
N
(±)-Retronecine
170
SCHEME 13.35
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