Chemistry Reference
In-Depth Information
CHO
N
Me
2
N
cat, Et
3
N
1
,4-dioxane, 80 °C
LR
PhMe, reflux
S
S
S
N
58%
81%
S
O
OO
N
14
11
13
Me
Et
+
Br
-
N
Me
cat =
S
HO
12
Scheme 12 The use of a thiazolium ylide to prepare an unsymmetrically substituted 1,4-diketone
[
24
]
O
O
n
-BuLi, 1,3-dithiane
THF, -78 °C
65%
S
S
S
S
O
O
O
O
CuO, CuCl
2
H
2
O, Me
2
CO, reflu
x
LR
PhMe, reflux
S
S
H
H
60%
76%
S
OO
15
Scheme 13 The use of 2-lithio-1,3-dithiane to prepare a 1,4-keto-aldehyde and thence a
2-unsubstituted thiophene [
27
]
can be used for the addition of aldehydes to chalcones (aryl 2-arylethenyl ketones)
whilst attached to a solid support, generating 1,4-diketones; the subsequent
LR-induced thiophene-forming step can also be conducted whilst the
1,4-diketone is still attached to the support [
25
]. Symmetrical 1,4-diketones can
be generated from aldehydes, using thiazolium ylide catalysis, by reaction with
divinyl sulfone, a vinyl unit providing the central two carbons of the 1,4-diketone
unit [
26
].
The conjugate addition of the anion of 1,3-dithiane to a conjugated ketone,
followed by unmasking of the aldehyde, produces a 1,4-keto-aldehyde, e.g. 15,
the thiophene derived from which has a 2,4-disubstitution pattern, as is demon-
strated in Scheme
13
[
27
].
Catalytic carbonylation of methyl vinyl ketone using carbon monoxide and
(arylmethyl)zinc chlorides is yet another route to unsymmetrically substituted
1,4-diketones 16 that can then be used for thiophene synthesis (Scheme
14
)[
28
].
Even rather strained thiophenes can be prepared from 1,4-diketones, as
Schemes
15
and
16
[
29
,
30
] show, though somewhat less efficiently.
This last example also illustrates the use of bis(trimethylsilyl)sulfide in combi-
nation with trimethylsilyl triflate at room temperature for the thionation; however,
the use of this reagent combination [
31
] for thiophene synthesis has not displaced
