Biomedical Engineering Reference
In-Depth Information
[Au]
[Au]
O
O
O
O
[2+2+2]-cycloaddition
Application to the synthesis of Inglerins
Echavarren et al. :
AuL
AuL
[(IPr)Au(NCPh)]
+
SbF
6
-
(3 mol%)
CH
2
Cl
2
, rt, 5 h
58%
OH
O
OSET
OSET
OSET
O
H
H
O
HO
OH
- AuL
+
Ph
O
H
O
O
OSET
Englerin A
50
(OR = )
O
OH
steps
O
O
Englerin B
220
(OR = OH)
H
H
OR
OH
Ma et al. :
steps
OH
AuCl (10 mol%)
CH
2
Cl
2
, rt, 0.3 h
48%
O
H
OH
O
SCHEME 4.64
Synthesis of englerins A and B by Echavarren and coworkers and Ma and
coworkers.
was formed in a single operation from propargylic ester
217
by a transannular [4
þ
3]-
cycloaddition proceeding on intermediate allene
.
The total syntheses of the guaianolides englerin A
218
50
[63,64] and englerin B
220
[64], independently reported by Echavarren and coworkers and Ma and co-
workers, are undoubtedly among the best representative examples of complex gold-
mediated transformations. The syntheses of these natural products are based on the
possibility to perform a [2+2+2]-Au(I)-catalyzed intramolecular cycloaddition
between an alkyne, an alkene, and a ketone (Scheme 4.64). The most impressive
feature of this approach is that the tricyclic core structure of the targeted molecules is
assembled in a single operation from a linear substrate with complete control of the
stereoselectivity. No less than three new bonds and three new asymmetric centers
(two being quaternary) are formed during the process. The gold-catalyzed
cycloisomerization key steps in the synthesis of englerin A
220
are shown in Scheme 4.64. The same approach was used by Echavarren and
coworkers for the total synthesis of the parent molecule (
50
and englerin B
þ
)-orientalol [65].
Hashmi's phenol synthesis, which corresponds to the gold-catalyzed cycloi-
somerization of a furan-yne substrate
bearing
a phenol ring, is one of the earliest discoveries made in the field of modern
221
into a fused bicyclic compound
223
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