Biomedical Engineering Reference
In-Depth Information
CH
2
Cl
2
, followed by treatment with
para
-toluenesulfonic acid (
p
-TsOH) to remove
the anisylidene protecting group. Spiroketal
79
prefigures the C15-C27 fragment
of the targeted molecule. The Au(I)-catalyzed reaction of ynetriol
80
,which
possesses an hydroxyl functionality at the propargylic position C32, selectively
produced spiroketal
in 65% yield, following a mechanism similar to that
proposed by Trost and coworkers. Spiroketal
81
81
prefigures the C28-C38 fragment
of okadaic acid
82
.
4.2.3. Other Transformations
In 2007, Skouta and Li reported an intriguing transformation allowing the formation
of isoflavanones
87
by the Au(I)-catalyzed coupling reaction between a salicylalde-
hyde
(Scheme 4.25) [22]. This transformation that
generally requires a high temperature (150
C) and a long reaction time (36 h) was
proposed to proceed via an initial oxidative addition of the aldehyde C-H bond to the
Au(I) catalyst. A further complexation of the arylacetylene with the thus generated
Au(III) species
83
and an arylacetylene
84
85
subsequently furnishes the
a
,
b
-unsaturated ketone
86
after an
hydroauration/reductive elimination sequence. Isoflavanone
was finally obtained
by a conjugated addition of the hydroxyl group to the unsaturated ketone. This
unusual gold-catalyzed transformation has been applied to the expedient syntheses of
several isoflavonoid pterocarpans of type
87
88
[22b].
As seen previously, the great majority of the gold-catalyzed transformations,
which have been applied to the synthesis of oxygen-containing heterocycles, involves
the creation of one or several new C-O bond(s). An example that illustrates the
formation of an oxygen-containing heterocyclic structure by the creation of two new
O
AuCN (1 mol%)
Bu
3
P (25 mol%)
toluene, 150°C, 36 h
O
R
2
R
2
H
R
1
R
1
- Au(I)
OH
O
52-75%
83
84
87
O
Ar
Au(I)
OH
O
O
O
[Au]
86
H
H
R
1
[Au]
[Au]
R
1
[Au]
Ar
OH
H
H
Ar
85
1. NaBH
4
THF/MeOH
rt, 2 h
2. BF
3
.OEt
2
MOMO
OMOM
O
O
see above
R
R
+
O
R = H (73%)
R = Cl (70%)
R
O
O
H
R = H (91%)
R = Cl (53%)
(±)-Pterocarpan
88
(R = H)
OH
SCHEME 4.25
Synthesis (
)-pterocarpan by Skouta and Li.
Search WWH ::
Custom Search