Biomedical Engineering Reference
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H
OMe
H
AcO
AcO
(XPhos)AuNTf
2
(1 mol%
)
CH
2
Cl
2
/MeOH 10:1
rt, 6 h
AuL
+
78%
197
198
HO
OH
HO
Grubbs II
(10 mol%)
CH
2
Cl
2
, 40°C, 1 h
100%
OMe
OMe
AcO
AcO
H
H
H
HO
O
HO
O
OH
H
OH
201
200
199
SCHEME 4.58
Stereoselective formation of functionalized cyclopentenes by Gagosz and
coworkers.
formation of functionalized cyclopentenes through a Au(I)-catalyzed alkoxy-
cyclization of 1,5-enynes, such as of type
197
(Scheme 4.58) [55]. In this case,
the cationic intermediate species
, which is formed by the nucleophilic addition of
the alkene on the gold-activated alkyne, is intercepted by methanol to produce
cyclopentene
198
199
. A subsequent ring-closing metathesis furnished the 5,7-fused
bicyclic product
200
, whose structure is found in various guaianolide natural products
such as
201
.
4.4.2. Cyclizations Involving the Formation of Several
New C-C Bonds
4.4.2.1. Enyne Cyclizations In addition to oxygen or nitrogen nucleophiles,
carbon nucleophiles such as alkenes or arenes can be used to intercept the cationic
species generated during the course of a gold-catalyzed reaction. Such a process can
lead to the formation of polycarbocyclic structures from linear substrates by the
sequential formation of two or several new C-C bonds. These transformations are
generally highly stereoselective as exemplified by the enantioselective formation of
tetracyclic compound
203
from polyenyne
202
, which was reported by Toste and
coworkers (Scheme 4.59) [56].
Alternatively, when an enyne is used as the substrate, the cationic intermediate
can be directly trapped by the vinyl gold species generated via the addition of the
alkene moiety on the gold-activated alkyne. For instance, the gold-catalyzed cycloi-
somerization of a 1,5- or 1,6-enyne
204
bearing a hydroxyl or acyloxy functionality at
OMe
OMe
EtO
2
C
EtO
2
C
MeO-DTBM-BIPHEP(AuCl)
2
(3 mol%)
AgSbF
6
(3 mol%)
m
-Xylene, rt
EtO
2
C
EtO
2
C
OMe
OMe
H
H
61%
202
203
(ee = 97%)
SCHEME 4.59
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