Chemistry Reference
In-Depth Information
EtO
2
C
EtO
2
C
CO
2
H
O
O
(Ph
3
P)AuCl, AgSbF
6
EtO
2
C
EtO
2
C
H
6
.
1
8
2
6
1
8
3
Scheme 6.87
OMe
OMe
Ph
3
PAuNTf
2
O
H
OH
6
1
8
H
6
.
1
5
Scheme 6.88
PtCl
2
, AgOTf
+
CO
2
Et
CO
2
Et
6
.
1
6
1
8
6.187
Scheme 6.89
AuL
+
MeO
MeO
MeO
AuL
MeO
O
MeO
O
MeO
O
6.189
6.190
6.191
H
+
MeO
AuL
MeO
MeO
•
O
MeO
MeO
O
MeO
O
6.193
5-exo product
6.192
6.194
6-endo product
Scheme 6.90
Tandem cyclization reactions to form polycyclic structures have also been reported (Schemes 6.87 and
6.88).
96
Some of these reactions can strongly resemble the polyene cyclizations that lead to steroids.
97
Further
ene-yne reactions can be found in Section 6.2.5.
Alkynes can also be activated to attack by carbon nucleophiles. Examples include the reaction between
propiolate esters
6.187
and arenes (Scheme 6.89).
98
In intramolecular reactions, both
exo
- and
endo
-mode cyclizations may be observed. In a study of chromene
formation,
endo
-cyclization giving the chromene
6.194
was generally the preferred pathway; the observation
of the 5-
exo
product
6.193
was attributed to fragmentation of the organogold intermediate
6.191
prior to
protonation, to give an allenic intermediate
6.192
, followed by cyclization (Scheme 6.90).
99
