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In-Depth Information
t
-Bu
t
-Bu
O
O
O
t
-Bu
t
-Bu
S
t
-Bu
t
-Bu
t
-Bu
Al
2
O
3
,CH
2
Cl
2
O
OS
+
98%
S
Al
2
O
3
,CH
2
Cl
2
t
-Bu
t
-Bu
t
-Bu
O
O
O
O
36
O
Al
2
O
3
, DDQ
-SO
2
t
-Bu
t
-Bu
100%
t
-Bu
t
-Bu
O
37
OH
O
OH
O
t
-Bu
t
-Bu
t
-Bu
t
-Bu
Al
2
O
3
[O]
-SO
SO
SO
t
-Bu
t
-Bu
t
-Bu
t
-Bu
O
OH
OH
O
36
38
39
Scheme 26
Preparation of 2,3,6,7-tetra-
t
-butylanthraquinone by using a Diels-Alder reaction
catalyzed by silica gel (alumina) and the proposed mechanism [
40
]
O
H
2
O
2
,CF
3
COOH
CH
2
Cl
2
,20°C,2.5h
O
+
N
Ph
N
Ph
SO
S
75%
O
O
Scheme 27
Oxidative Diels-Alder cycloaddition of thiophene with
N
-phenylmaleimide using
H
2
O
2
/CF
3
CO
2
H as oxidant [
41
]
O
O
X
X
X
m
-CPBA, CH
2
Cl
2
X=H,CH
3
or
X, X =
S
N
Ph
+
N
Ph
SO
12% - 49%
X
S
O
O
Scheme
28
Oxidative Diels-Alder
cycloaddition
of
thiophene
derivatives with
N
-
phenylmaleimide using
m
-CPBA as oxidant [
22
,
42
]
Silica gel or alumina was used as catalyst for the Diels-Alder reaction of 3,4-di-
tert
-butylthiophene 1-oxide with 1,4-benzoquinone to produce 6,7-di-
tert
-butyl-
1,4-naphthoquinone
36
and bis-adduct 2,3,6,7-tetra-
tert
-butylanthraquinone
37
[
40
]. The silica gel (alumina)-catalyzed enolization of
38
to form
39
may play an
important role in the formation of
36
(Scheme
26
).
Oxidation of thiophene or thiophene derivatives with various oxidants in the
presence of dienophile
N
-phenylmaleimide always leads to the Diels-Alder cyclo-
addition products. Typical oxidants H
2
O
2
/CF
3
CO
2
H[
41
],
m
-CPBA [
22
,
42
],
m
-
CPBA/BF
3
·Et
2
O complex [
13
], and TPHP (
tert
-butyl hydroperoxide) [
43
] are
shown in Schemes
27
,
28
,
29
, and
30
. The rhenium-catalyzed reactions are of
