Furan-fused 3-sulfolene as a novel building block: Intermolecular Diels-Alder reactions of 4H,6H-thieno[3,4-c]furan 5,5-dioxide

Article abstract of DOI:10.1039/a604600b

A furan-fused 3-sulfolene 4H,6H-thieno[3,4-c]furan 5,5-dioxide 1, can be used as a bis-diene, reacting sequentially with a variety of dienophiles to construct four types of skeleton, depending on the dienophile and the reaction conditions. The 3-sulfolene

Full text of DOI:10.1039/a604600b

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Furan-fused 3-sulfolene as a novel building block: intermolecular
Diels–Alder reactions of 4H,6H-thieno[3,4-c]furan 5,5-dioxide
Takayoshi Suzuki, Kan Kubomura and Hiroaki Takayama *
Faculty of Pharmaceutical Sciences, Teikyo University, Sagamiko, Kanagawa 199–01, Japan
A furan-fused 3-sulfolene 4H ,6H -thieno[3,4-c]furan 5,5-dioxide 1, can be used as a bis-diene, reacting
sequentially with a variety of dienophiles to construct four types of skeleton, depending on the
dienophile and the reaction conditions. The 3-sulfolene moiety of the furansulfolene not only functions
as the s-cis-diene part in D iels–Alder reactions, but also extends the range of the D iels–Alder reactions of
the furan moiety with various dienophiles through its desulfonylation to form the s-cis-diene.
ized. Thus, the furansulfolene 1 is a useful building block for the
construction of polycyclic polyfunctional systems.
Introduction
In the course of our studies on the chemistry of 3-sulfolene,1 we
have been interested in five-membered heteroaromatic ring-
fused 3-sulfolenes and have synthesized the previously
unknown furan-fused 3-sulfolene, 4H,6H-thieno[3,4-c]furan
5,5-dioxide 1.2 This compound contains furan and 3-sulfolene
moieties, which can sequentially react with dienophiles to con-
struct four types of skeleton, depending on the dienophile and
the reaction conditions (Scheme 1).
Results and discussion
Diels–Alder reaction of the furansulfolene 1
The Diels–Alder reaction of the furansulfolene 1 with a variety
of dienophiles has been studied and the results are summarized
in Table 1.
The furansulfolene 1 reacted with dimethyl acetylenedicarb-
oxylate (DMAD) (3 mol equiv.) in benzene (sealed tube) at
150 ЊC (1 h), to give two types of cycloadduct: the mono-
cycloadduct 4a bearing two methylene groups (type A) and the
bis-cycloadduct 6a (type B) (entry 1). Even at lower temperature
(120 ЊC or room temperature), the same cycloadducts were
obtained. Essentially the same type of reaction was observed
with dimethyl fumarate as a dienophile. With other ethylenic
dienophiles such as dimethyl maleate, cycloaddition proceeded
at 150 ЊC to afford a new type of monocycloadduct 9a (type C),
in addition to the type A monocycloadduct 5a (entry 10). With
fumaronitrile, the formation of the type C adduct predomin-
ated over that of the type A adduct. Furthermore, the furan-
sulfolene 1 added to maleic anhydride at room temperature
(72 h) to yield the fourth type of cycloadduct (type D),
compound 3d, as the sole product (entry 16).
E
E
E
E
O
E
E
O
O
i
– SO2
ii
O
S
O2
S
O2
E′
E′
Dienophile
Dienophile
( )
1
alkyne
alkene
2
4
5
alkyne′
alkene′
6
7
3
Type D
Type A
Type B
–alkene
O
O
a EE = E′E′ = CO2Me
b EE = E′E′ = CN
Thus, the furansulfolene 1 reacts with various dienophiles to
give four types of cycloadduct, depending on the dienophiles
and the reaction conditions, and even reacts with dimethyl
fumarate and dimethyl maleate, whose adducts with furan have
not been isolated under thermal conditions, to afford Diels–
Alder adducts in good total yields. Both the successful Diels–
Alder reaction of the furansulfolene 1 with these unfavourable
dienophiles and the current interest in efficient construction of
clinically important polycyclic quinones, such as anthra-
cyclinones and pradimicinone A (an anti-HIV agent) prompted
us to examine the reaction of compound 1 with quinones,
whose Diels–Alder adducts with furan are thermally unstable
and either decompose back to furan and the dienophile or are
cleaved to Michael-type adducts. At 20 kbar,† furan undergoes
cycloaddition to p-benzoquinone, to give in addition to the
recovered starting materials (71%), a mixture of endo- and exo-
1:1 adducts (14 and 15%), which are unstable and revert to the
starting materials at normal pressure.5 Strikingly, the reaction
of compound 1 with p-benzoquinone (2 mol equiv.) proceeded
at 120 ЊC (3 h, benzene, sealed tube) and afforded exo-5e as the
single product (78% isolated yield) with recovery of substrate 1
(entry 17). Furthermore, treatment of compound 1 with 1,4-
naphthoquinone (2 mol equiv.) at 150 ЊC for 4 h gave com-
pound 5f together with a small amount of a type B adduct.
c EE = E′E′ = CO-N(Ph)-CO
d EE = E′E′ = CO-O-CO
e EE = E′E′ =
E′
E′
( )
9
8
O
Type C
O
O
f EE =
g EE =
O
O
OH
Scheme 1 Reagents: i, dienophile; ii, dienophileЈ
In view of the well known weak reactivity of furan as a diene
and the retro-Diels–Alder reactions of the adducts of alkenyl
dienophiles, such as dimethyl fumarate, dimethyl maleate,
and p-benzoquinone, due to their inherent thermodynamic
instability with respect to either retrocycloaddition or re-
aromatization,3 it is noteworthy that the Diels–Alder adducts of
compound 1 with these unfavourable dienophiles (adducts with
furan can generally be obtained only under high-pressure con-
ditions4) can be isolated in good yields even under thermal
conditions. The key to making the equilibrium favourable for
the formation of the product is rapid extrusion of SO2 from the
initially formed adducts 3. Furthermore, the furan and 3-
sulfolene moieties of substrate 1 can both be readily functional-
† 1 kbar = 0.1 GPa.
J. Chem. Soc., Perkin Trans. 1, 1997
251

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Table 1 Diels–Alder reactions of furansulfolene 1 with dienophiles under thermal conditions
Products (yield, %)a
Reaction
Total
Entry
Dienophile (mol equiv.)
DMAD (3)
conditions
Type D
Type A
Type B
6a
Type C
yield (%)
1
2
sealed tube, 150 ЊC
1 h, benzene
sealed tube, 120 ЊC
1 h, benzene
sealed tube, 120 ЊC
1 h, benzene
atmosphere, 28 ЊC
7 days, CH2Cl2
atmosphere, 28 ЊC
7 days, CH2Cl2
atmosphere, 0 ЊC
90 days, CDCl3
sealed tube, 150 ЊC
2 h, benzene
sealed tube, 150 ЊC
2 h, benzene
atmosphere, 28 ЊC
48 h, CH2Cl2
4a
45
62
47
29
3
92
91
DMAD (3)
4a
6a
3
DMAD (1)
4a
40
6a
43
(51)b
93
4
DMAD (3)
4a
54
6a
39
5
DMAD (1)
4a
35
35
(61)b
100c
6
DMAD (3)
4a
100c
7
Dimethyl fumarate
(3)
Dimethyl fumarate
(1)
Dimethyl fumarate
(3)
Dimethyl maleate
(3)
5a
78
78
21
21
7a
11
11
89
transd
5a
transd
8
21
(72)b
0
transd
no reaction
9
(96)b
73
10
sealed tube, 150 ЊC
3 h, benzene
5a
63
53
10
61
51
10
72
9a
10
10
endod
exod
5a
cise
11
Dimethyl maleate
(3)
sealed tube, 120 ЊC
12 h, benzene
7a
29
11
18
90
endod
exod
5a
endod
exod
12
13
14
15
16
17
18
19
20
Dimethyl maleate
(1)
Dimethyl maleate
(3)
Fumaronitrile
(3)
N-Phenylmaleimide
(3)
Maleic anhydride
(3)
p-Benzoquinone
(2)
1,4-Naphthoquinone
(2)
1,4-Naphthoquinone
(2)
sealed tube, 120 ЊC
12 h, benzene
atmosphere, 28 ЊC
48 h, CH2Cl2
sealed tube, 150 ЊC
3 h, benzene
sealed tube, 120 ЊC
1 h, benzene
atmosphere, 28 ЊC
72 h, THF
sealed tube, 120 ЊC
3 h, benzene
sealed tube, 150 ЊC
4 h, benzene
sealed tube, 120 ЊC
4 h, benzene
72
(24)b
0
no reaction
(94)b
74
5b
36
9b
9f
38
7c
exo
82
82
82
62
3d 62
exo 62
5e
exo
5f
exo
5f
exo
5g
78
78
58
58
50
50
60
60
78
(22)b
62
4
50
(47)b
60
Juglone
(2)
sealed tube, 120 ЊC
4 h, benzene
exo
(13)b
Isolated yield. b Recovery of substrate 1. c Yield determined by 1H NMR spectroscopy. d The configuration of the two methoxycarbonyl groups on
the 7-oxabicyclo[2.2.1]heptanyl skeleton of 5a or 7a. e The configuration of the two methoxycarbonyl groups at C-5 and -6 of compound 9.
a
Under the same conditions, the reaction of compound 1 with
juglone afforded the desired product 5g (entry 20). The struc-
tures of all cycloadducts thus obtained were confirmed by H
perature higher than 150 ЊC, the retro-Diels–Alder reactions of
intermediates trans-7a and 6a to the corresponding type C
products were not observed (on heating at 210 ЊC for 3.5 h and
then 240 ЊC for 2 h, the recovery of substrate 6a was 96% and
that of compound trans-7a was 99%). The retro-Diels–Alder
reaction of tricycle 7a to compound 9a seems to be dependent
on the configuration of the ester groups on the ethano bridges.
1
and 13C NMR spectral analysis. The configurations of all
cycloadducts could be readily determined by inspection of the
1H NMR spectra; thus, the bridgehead protons of the cis-exo
isomers of the cycloadducts 5 and 7 appeared as singlets in the
expected region, and those of the cis-endo isomers appeared as
doublets.
Formation of type D adducts and their desulfonylation
The formation of the type A compounds 4 and 5 is the result of
spontaneous desulfonylation of the initially formed type D
adducts 2 and 3, respectively. To examine the formation of
the type D adducts, high-pressure conditions were employed
(Table 2).
The reaction of compound 1 and 3 mol equiv. of dimethyl
maleate in CH2Cl2 at 28 ЊC under a pressure of 12 kbar (1.2
GPa) for 48 h gave a single adduct, the cis-endo type D adduct
3a, in 81% yield. At lower pressures or with a smaller mol ratio
(1:1) of the dienophile, the yields of product 3 were low and
substrate 1 was recovered. Despite our success in the isolation
of product 3, all attempts to detect compound 2a failed,
presumably owing to rapid desulfonylation (Table 1, entries 4–6),
but the isolation of the adduct 10 (53%), a 1:1 adduct of com-
pounds 1a and 2a, when compound 1 was treated with DMAD
at 28 ЊC under a pressure of 1.2 GPa, suggested the formation
Retro-Diels–Alder reaction of type B adducts and formation of
type C adducts
A comparison of entries 10 and 11 suggested that the formation
of compound 9a could be explained by a retro-Diels–Alder
reaction in which the 7-oxanorbornene skeleton of intermedi-
ate 7a releases the unit alkene to afford the furan ring, driven by
both the restoration of aromatic character and the reduction
of steric strain. In accord with this, heating of the isolated
intermediate cis-endo-7a in benzene solution at 150 ЊC for 1 h
gave compound 9a as the exclusive product in 85% yield.
Under the same conditions, the retro-Diels–Alder reaction of
the cis-exo isomer to product 9a was slow (26% yield; the
recovery of cis-exo-7a was 74%). In both cases, no type A prod-
uct 5 was observed. In contrast to the ready retro-Diels–Alder
reaction of the cis-isomers to compound 9a at reaction tem-
252
J. Chem. Soc., Perkin Trans. 1, 1997

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Table 2 Diels–Alder reactions of furansulfolene 1 under high-pressure conditions
Products (yield, %)a
Reactions
Entry
Dienophile (mol equiv.)
conditions
Type D
Type A
Type B
Type C
Total yield (%)
1
2
3
4
Dimethyl maleate
(3)
Dimethyl maleate
(3)
Dimethyl fumarate
(3)
Dimethyl acetylene-
dicarboxylate (DMAD)
(3)
12 kbar, 28 ЊC
48 h, CH2Cl2
4 kbar, 28 ЊC
48 h, CH2Cl2
12 kbar, 28 ЊC
48 h, CH2Cl2
4 kbar, 28 ЊC
24 h, CH2Cl2
3a
81
82
endob 81
3a
endob 19
3a 41
transb 41
19
19
(75)c
78
5a
37
transb 37
(12)c
100
4a
97
6a
3
a
Isolated yield. b The configuration of the two methoxycarbonyl groups. c Recovery of substrate 1.
of intermediate 2a. In the rapid desulfonylation of species 2,
release of the high strain arising from two endocyclic olefin-
oxabridge repulsions6 in the oxanorbornadiene moiety of com-
pounds 2 should play an important role [eqn. (1)].
O
E
E
+
O
OH
O
cis-endo -5a
O
MeO2C
(2)
SO2
O
MeO2C
2a
E
O
– SO2
(1)
E
O
OH
O
11
MeO2C
E = CO2Me
MeO2C
4a
so any influence of the secondary orbital effect and steric effects
can be avoided.
While compound cis-endo-5a afforded bicycle 8a at 150 ЊC
for 3 h, both cis-exo- and trans-5a gave new bis-adducts, cis-
exo- and trans-12a, respectively (Scheme 2; Table 3). It is note-
O
O
MeO2C
MeO2C
SO2
SO2
E
E
E
E
10
O
i
In contrast to the rapid desulfonylation of 2, desulfonylation
of the oxanorbornene fused sulfolenes 3 required moderate
heating. Desulfonylation of cis-endo-3a took place at 80 ЊC (0.5
h) in benzene, and the desulfonylated product 5a was obtained
in quantitative yield. Even at room temperature, compound
trans-3a underwent desulfonylation to give the corresponding
type A product. These desulfonylations of the type D adducts 3
are considered to circumvent the unfavourable equilibrium
between compound 1 (furan) and the type D adducts 3 (7-
oxanorbornenes).
O
E
E
5a
Type B′
12a
– alkene
E
E
E
E
O
O
i
Reactivity of the type A adducts
The results shown in Tables 1 and 2 indicate that the cyclo-
addition of the second equivalent of the dienophile to
the monoadduct (type A), giving the corresponding bis-adduct
(type B), was slower than the addition of the first equiv-
alent of the dienophile to substrate 1, which yields the
monoadduct (type A). This differentiation would be very useful
for constructing polycyclic systems. For example, cis-endo-5a
reacted with juglone (1.1 mol equiv.) at 80 ЊC for 10 h to give a
pentacyclic quinone 11 in 58% isolated yield [eqn. (2)]. This
approach to pentacyclic quinones starting from the readily
available type A adduct with naphthoquinone may be fruitful in
constructing a number of important polycyclic polyfunctional
systems, including anthracycline derivatives.
E
E
6a
8a
Scheme 2 Reagent: i, DMAD
worthy that the rate of Diels–Alder reaction of compound cis-
exo-5a with DMAD is as slow as that of compound 4a with
DMAD. The slowness of the latter can be attributed to the
formation of a new endocyclic olefin (᎐ another endocyclic
᎐
olefin–oxabridge repulsion). Thus, when DMAD adds to diene
4a to afford adduct 6a, one would expect the new endocyclic
double bond to push the oxabridge back to a more symmetrical
position. This leads to extra strain. The cis-exo-ester groups on
diene 5a seem to play the same role in the reaction of com-
pound 5a with DMAD. The thermally unstable tricycle cis-
endo-12a was isolated when diene cis-endo-5a was treated with
DMAD under a pressure of 12 kbar (28 ЊC), and compound
cis-endo-12a underwent a retro-Diels–Alder reaction to give
Next, as we had obtained three isomers of compound 5a,
we examined the influence of the ester substituents of the
7-oxanorbornane frame on the Diels–Alder reaction. The
reactivity of compound 5a’s isomers was examined by employ-
ing DMAD (1.1 mol equiv.) as a dienophile; DMAD is linear,
J. Chem. Soc., Perkin Trans. 1, 1997
253

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Table 3 Diels–Alder reaction of the isomers of compound 5a with 1
mol equiv. of DMAD at 150 ЊC for 3 h
to column chromatography on silica gel and eluted with a
mixture of hexane and AcOEt (9:1).
Diels–Alder reaction of sulfolene 1 with DMAD at 150 ЊC
(entry 1 of Table 1). After column chromatography, compound
4a (48.2 mg, 45%) was obtained as needles, mp 86–87 ЊC [from
AcOEt–hexane (1:4)] together with compound 6a (61.1 mg,
47%) also as needles, mp 71–72 ЊC [from AcOEt–hexane (1:4)]
from substrate 1 and DMAD (0.12 cm3, 3 mol equiv.) at 150 ЊC
for 1 h by the general method.
Dimethyl 5,6-dimethylene-7-oxabicyclo[2.2.1]hept-2-ene-2,3-
dicarboxylate 4a, δH 3.83 (6 H, s), 5.35 (2 H, s, bridgehead H),
5.44 (2 H, s) and 5.45 (2 H, s); δC 52.37 (q), 85.16 (d), 106.00
(t), 140.14 (s), 143.27 (s) and 162.35 (s); m/z 236 (M+),
205 (M+ Ϫ OCH3, 3.0%), 176 (M+ Ϫ CO2CH3, 24.7) and 145
(M+ Ϫ CO2CH3 Ϫ OCH3, base) (Found: M+, 236.0689. Calc.
for C12H12O5: M, 236.0684).
Tetramethyl 1,4-epoxy-1,4,5,8-tetrahydronaphthalene-2,3,6,
7-tetracarboxylate 6a, δH 3.15 (2 H, m), 3.44 (2 H, m), 3.79 (6 H,
s), 3.83 (6 H, s) and 5.54 (2 H, s, bridgehead H); δC 27.02 (t),
52.35 (q), 52.39 (q), 86.33 (d), 132.57 (s), 145.33 (s), 152.56 (s),
163.13 (s) and 167.93 (s); m/z 347 (M+ Ϫ OCH3, 2.2%), 285
(M+ Ϫ OCH3 × 3, 24.1) and 205 (M+ Ϫ C6H6O4 Ϫ OCH3, base)
(Found: M+, 378.0986. Calc. for C18H18O9; M, 378.0949).
Diels–Alder reaction of sulfolene 1 with dimethyl fumarate at
150 ЊC (entry 7 of Table 1). After column chromatography,
compound trans-5a (59.3 mg, 78%) and compound trans-7a
(13.9 mg, 11%) were both obtained as oils from sulfolene 1 and
dimethyl fumarate (138.0 mg) at 150 ЊC for 1 h by the general
method.
Products (yield, %)a
Total
Entry
5a
12a
8a
6a
yield (%)
1
2
3
cis-endo
trans
cis-exo
0
84
39
0
0
0
37
84
94
trans 55
cis-exo 10
47
(9)b
34
cf.
4a
0
34
(35)b
Isolated yield. b Recovery of the diene 5a.
a
bicycle 8a (quant.) at 150 ЊC (1 h). These results, together with
the fact that compound 8a does not react with the alkene dieno-
philes to give any trace of adduct 12a under the same condi-
tions, suggested that the Diels–Alder reactivity of the s-cis-
butadiene moiety grafted onto 7-oxanorbornane is affected by
the remote ester groups of the bicyclic skeleton. The effect of
the cis-exo-methoxycarbonyl groups of the 7-oxanorbornene
skeleton is observed both in the reactions of the type A adducts
with the second dienophiles and in the retro-Diels–Alder reac-
tions of the type B adducts to afford the corresponding type
C compounds. In these cases, the cis-exo-methoxycarbonyl
groups of compounds 3a and 5a should play the same role as do
the endocyclic olefins of 4a and 6a.
Conclusions
The 3-sulfone function of the furansulfolene 1 not only acts as
an s-cis-diene in Diels–Alder reactions, but also extends the
scope of the Diels–Alder reactions of the furan moiety with
various dienophiles through its desulfonylation to form an s-
cis-diene. So, the furansulfolene, 4H,6H-thieno[3,4-c]furan 5,5-
dioxide 1, can be used as a bis-diene which can sequentially
react with two different dienophiles to afford polycyclic poly-
functional systems. Remote substituent effects are caused by
methoxycarbonyl groups in compounds 5a and 7a, and the
largest retardation effect is seen with cis-exo-methoxycarbonyl
groups.
Dimethyl 5,6-dimethylene-7-oxabicyclo[2.2.1]heptane-trans-
2,3-dicarboxylate trans-5a, δH 3.29 (1 H, d, J 4.88), 3.68 (3 H, s),
3.72 (1 H, s), 3.75 (3 H, s), 4.99 (1 H, s, bridgehead H), 5.07 (1
H, d, J 4.88, bridgehead H), 5.13 (1 H, s), 5.15 (1 H, s), 5.30
(1 H, s) and 5.32 (1 H, s); δC 50.19 (d), 51.17 (d), 52.21 (q), 52.54
(q), 82.36 (d), 84.54 (d), 103.00 (t), 104.46 (t), 143.66 (s), 145.28
(s), 170.31 (s) and 172.12 (s); m/z 238 (M+, 2.2%), 207 (M+ Ϫ
OCH3, 5.0), 179 (M+ Ϫ CO2CH3, 7.0) and 94 (M+ Ϫ C6H8O4,
base) (Found: M+, 238.0848. Calc. for C12H14O5: M, 238.0841).
Tetramethyl
1,4-epoxy-1,2,3,4,5,6,7,8-octahydronaphtha-
lene-trans-2,3-trans,6,7-tetracarboxylate trans-7a, δH 2.45–2.52
(2 H, m), 2.67–2.94 (4 H, m), 3.64–3.66 (1 H, m), 3.68 (1 H, s),
3.74 (6 H, s), 3.75 (3 H, s), 3.81 (3 H, s) and 5.02–5.04 (2 H, s,
bridgehead H); δC 23.81 (t), 23.97 (t), 24.90 (t), 26.24 (t), 40.81
(d), 41.03 (d), 41.15 (d), 41.61 (d), 47.57 (d), 47.82 (d), 48.82 (d),
48.97 (d), 51.94 (q), 51.99 (q), 52.05 (q), 52.09 (q), 52.11 (q),
52.45 (q), 52.47 (q), 81.45 (d), 81.69 (d), 84.08 (d), 84.43 (d),
138.70 (s), 139.35 (s), 140.72 (s), 140.89 (s), 170.59 (s), 170.79
(s), 172.47 (s), 174.14 (s), 174.16 (s), 174.32 (s) and 174.43 (s);
m/z 351 (M+ Ϫ OCH3, 2.9%), 323 (M+ Ϫ CO2CH3, 1.1), 238
(M+ Ϫ C6H8O4, 34.4) and 94 (M+ Ϫ C6H8O4 × 2, base) (Found:
M+, 382.1257. Calc. for C18H22O9: M, 382.1263).
Diels–Alder reaction of sulfolene 1 with dimethyl maleate at
150 ЊC (entry 10 of Table 1). After column chromatography,
compounds cis-endo-5a (40.4 mg, 53%), cis-exo-5a (7.4 mg,
10%) and 9a (7.8 mg, 10%) were all obtained as oils from sulfo-
lene 1 and dimethyl maleate (0.12 cm3) at 150 ЊC for 3 h by the
general method.
Experimental
Mps (Yanaco Micro Melting Point apparatus) are uncor-
1
rected. The H (400 MHz) and 13C (100 MHz) NMR spectra
were determined for CDCl3 solutions containing ~1% SiMe4 as
internal standard with a JEOL GSX-400 spectrometer while 1H
NMR (90 MHz) and 13C (22.5 MHz) were determined with a
JEOL GSX-90 spectrometer. J Values are given in Hz. Infrared
spectra were taken on a JASCO A-302 diffraction grating infra-
red spectrophotometer. Column chromatography was per-
formed on silica gel (Wakogel C-200). All reactions were con-
ducted under argon unless otherwise stated. High-pressure
equipment: a double-walled cylindrical pressure vessel (Hikari
Koattu Ltd., Hiroshima, Japan) was fitted with a piston
powered by a hydraulic ram, the whole being contained in a
press frame. Samples of up to 1.8 cm3 were placed in a poly-
(tetrafluoroethylene) (PTFE) cylindrical cell closed by a sliding
stopper. This was placed within the cylinder which was filled
with kerosene and the desired pressure was applied, monitored
by a calibrated strain gauge directly connected to the cylinder.
The temperature was controlled by an external heating jacket.
CH2Cl2 was distilled from CaH2 under argon.
Dimethyl
5,6-dimethylene-7-oxabicyclo[2.2.1]heptane-2-
endo,3-endo-dicarboxylate cis-endo-5a, δH 3.43 (2 H, d, J 2.44),
3.63 (6 H, s), 4.98 (2 H, d, J 2.44, bridgehead H), 5.10 (2 H, s)
and 5.42 (2 H, s); δC 48.50 (d), 51.74 (q), 82.00 (d), 105.02 (t),
143.79 (s) and 169.82 (s); m/z 238 (M+, 3.3%), 207 (M+ Ϫ
OCH3, 7.5), 179 (M+ Ϫ CO2CH3, 7.0) and 94 (M+ Ϫ C6H8O4,
base) (Found: M+, 238.0847. Calc. for C12H14O5: M, 238.0841).
Dimethyl 5,6-dimethylene-7-oxabicyclo[2.2.1]heptane-2-exo,
3-exo-dicarboxylate cis-exo-5a, δH 3.15 (2 H, s), 3.71 (6 H, s),
5.09 (2 H, s, bridgehead H), 5.16 (2 H, s) and 5.29 (2 H, s);
δC 51.56 (d), 52.30 (q), 83.03 (d), 103.10 (t), 145.71 (s) and
171.02 (s); m/z 238 (M+), 207 (M+ Ϫ OCH3, 0.8%) and 94
(M+ Ϫ C6H8O4, base) (Found: M+, 238.0849).
Diels–Alder reactions of sulfolene 1 with dienophiles under
thermal conditions; general method
A solution of sulfolene 1 (50.0 mg, 0.32 mmol), 4-methoxy-
phenol (10 mg, 0.25 mol equiv., as a polymerization inhibitor)
and the dienophile (3 mol equiv.) in benzene (1 cm3) was heated
in a sealed tube. After concentration, the residue was subjected
254
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Dimethyl
4,5,6,7-tetrahydrobenzo[c]furan-cis-5,6-dicarb-
anhydride (94.1 mg, 3 mol equiv.) in absolute tetrahydrofuran
oxylate 9a, δH 2.85 (2 H, dd, J 0.91 and 5.19), 2.88 (2 H, dd,
J 0.91 and 5.19), 3.18 (2 H, m), 3.70 (6 H, s) and 7.19 (2 H, s);
δC 20.62 (t), 40.95 (d), 51.96 (q), 118.80 (s), 137.75 (d) 173.14
(s); m/z 238 (M+, 11.5%) and 119 (M+ Ϫ CO2CH3 × 2, base)
(Found: M+, 238.0846).
Diels–Alder reaction of sulfolene 1 with dimethyl maleate at
120 ЊC (entry 11 of Table 1). After column chromatography,
compoundscis-endo-5a (39.2mg, 51%), cis-exo-5a (7.2mg, 10%),
cis-endo-7a (13.0 mg, 11%) as an oil and cis-exo-7a (21.8 mg,
18%) as another oil were obtained from sulfolene 1 and dimethyl
maleate (0.12 cm3) at 120 ЊC for 12 h by the general method.
(THF) (1 cm3) was stirred at room temperature for 72 h. After
concentration, the residue was purified by column chroma-
tography (silica gel; hexane–AcOEt 1:1) to give adduct cis-exo-
3d (51.3 mg, 62%) as an oil.
4,7-Epoxy-2,2-dioxo-1,3,4,5,6,7-hexahydrobenzo[c]thio-
phene-5-exo,6-exo-dicarboxylic acid anhydride cis-exo-3d, δH
3.29 (2 H, s), 3.86 (4 H, s) and 5.20 (2 H, s, bridgehead H);
δC 49.02 (d), 56.51 (t), 84.30 (d), 116.51 (s) and 168.21 (s); m/z
256 (M+, 2.2%), 192 (M+ Ϫ SO2, 8.9) and 96 (base) (Found:
M Ϫ SO2, 192.0419. C10H8O4 requires 192.0422).
Diels–Alder reaction of sulfolene 1 with p-benzoquinone at
120 ЊC (entry 17 of Table 1). After column chromatography,
adduct cis-exo-5e (50.5 mg, 78%) as a yellow oil and sulfolene 1
(11.0 mg, 22% recovery) were obtained from sulfolene 1 and
p-benzoquinone (69.0 mg, 2 mol equiv.) at 120 ЊC for 3 h by
the general method.
cisoid-4a,5-cis-4a,8a-5,8-Epoxy-6,7-dimethylene-4a,5,6,7,8,
8a-hexahydro-1,4-naphthoquinone cis-exo-5e, δH 3.13 (2 H, s),
5.15 (2 H, s, bridgehead H), 5.20 (2 H, s), 5.35 (2 H, s) and 6.81
(2 H, s); δC 53.40 (d), 86.26 (d), 103.48 (t), 142.20 (t), 145.29 (s)
and 196.01 (s); m/z 202 (M+) (Found: M+, 202.0634. Calc. for
C12H10O3: M, 202.0630).
Tetramethyl
1,4-epoxy-1,2,3,4,5,6,7,8-octahydronaphtha-
lene-2-endo,3-endo,6,7-tetracarboxylate cis-endo-7a, δH 2.49
(2 H, m), 2.82 (2 H, m), 3.06 (2 H, m), 3.47 (2 H, d, J 1.53), 3.62
(6 H, s), 3.67 (6 H, s) and 4.89 (2 H, d, J 1.53, bridgehead H); δC
24.52 (t), 40.15 (d), 47.90 (d), 51.94 [q, the signals of the methyl
groups of the methoxycarbonyl groups on C-2(3) and C-7(6)
overlappped], 81.98 (d), 140.04 (s), 170.74 (s) and 173.45 (s);
m/z 382 (M+, 4.2%), 351 (M+ Ϫ OCH3, 14.4) and 238
(M+ Ϫ C6H8O4, base) (Found: M+, 382.1267. Calc. for
C18H22O9: M, 382.1263).
Tetramethyl
1,4-epoxy-1,2,3,4,5,6,7,8-octahydronaphtha-
lene-2-exo,3-exo, 6,7-tetracarboxylate cis-exo-7a, δH 2.36 (2 H,
dd, J 6.10 and 15.60), 2.57 (2 H, dd, J 5.00 and 15.60), 2.83 (2 H,
s), 3.11 (2 H, m), 3.68 (6 H, s), 3.70 (6 H, s) and 5.06 (2 H, s,
bridgehead H); δC 22.76 (d), 40.40 (d), 47.84 (d), 52.06 (q),
52.19 (q), 82.55 (d), 140.31 (s), 171.99 (s) and 172.94 (s); m/z 382
(M+, 0.9%), 351 (M+ Ϫ OCH3, 4.8), 238 (M+ Ϫ C6H8O4, 27.9)
and 178 (M+ Ϫ CO2CH3 × 3, base) (Found: M+, 382.1269).
Retro-Diels–Alder reaction of cis-endo-7a. A solution of
adduct cis-endo-7a (21.8 mg, 0.056 mmol) in benzene (1 cm3)
was heated at 150 ЊC for 1 h in a sealed tube. After removal of
the solvent, the residue was purified by column chroma-
tography (silica gel; hexane–AcOEt 19:1) to give compound 9a
(11.3 mg, 84%).
Diels–Alder reaction of sulfolene 1 with fumaronitrile at
150 ЊC (entry 14 of Table 1). After column chromatography
(hexane–AcOEt 4:1), compounds trans-5b (20.1 mg, 36%) and
9b (21.2 mg, 38%) were both obtained as oils from sulfolene 1
and fumaronitrile (74.0 mg) at 150 ЊC for 3 h by the general
method.
5,6-Dimethylene-7-oxabicyclo[2.2.1]heptane-trans-2,3-
dicarbonitrile trans-5b, δH (90 MHz) 3.09 (1 H, m), 3.38 (1 H,
m), 5.26 (3 H, m), 5.34 (1 H, s), 5.43 (1 H, s) and 5.61 ( 1 H, s);
δC 37.61 (d), 38.59 (d), 82.12 (d), 84.40 (d), 106.06 (t), 108.34
(t), 116.39 (d), 118.01 (s), 140.13 (s) and 142.67 (t); m/z 172
(M+, 4.2%) and 146 (M+ Ϫ CN, base) (Found: M+, 172.0639.
C10H8N2O requires M, 172.0637).
4,5,6,7-Tetrahydrobenzo[c]furan-trans-5,6-dicarbonitrile
9b, δH (90 MHz) 3.01 (2 H, m), 3.20–3.28 (4 H, m) and 7.37 (2 H,
m); δC(22.5 MHz) 22.28 (t), 28.57 (d), 114.93 (s), 118.20 (s) and
138.88 (d); m/z 172 (M+, 4.8%) and 94 (M+ Ϫ C4H2N2, base)
(Found: M+, 172.0642. C10H8N2O requires M, 172.0637).
Diels–Alder reaction of sulfolene 1 with N-phenylmaleimide at
120 ЊC (entry 15 of Table 1). After column chromatography,
adduct cis-exo-7c (115.9 mg, 82%) was obtained as an oil from
sulfolene 1 and N-phenylmaleimide (166.0 mg) at 120 ЊC for 1 h
by the general method.
Diels–Alder reaction of sulfolene 1 with 1,4-naphthoquinone at
150 ЊC (entry 18 of Table 1). After column chromatography,
adducts cis-exo-5f (46.7 mg, 58%) and 9f (3.4 mg, 4%) were
obtained from sulfolene 1 and 1,4-naphthoquinone (100.0 mg,
2 mol equiv.) at 150 ЊC for 4 h by the general method.
cisoid-4,4a-cis-4a,9a-1,4-Epoxy-2,3-dimethylene-1,2,3,4,4a,
9a-hexahydroanthraquinone cis-exo-5f, δH 3.34 (2 H, s), 5.24
(2 H, s, bridgehead H), 5.28 (2 H, s), 5.37 (2 H, s), 7.76 (2 H, s)
and 8.12 (2 H, s); δC 54.46 (d), 86.85 (d), 103.30 (t), 127.20 (d),
134.45 (d), 138.67 (s), 145.70 (s) and 194.66 (s); m/z 252 (M+)
(Found: M+, 252.0781. Calc. for C16H12O3: M, 252.0786).
4,11-Dihydroanthro[2,3-c]furan-5,10-dione 9f, δH 2.78 (2 H,
dd, J 5.80 and 16.20), 3.10 (2 H, dd, J 5.80 and 16.20), 3.51 (2
H, m), 7.36 (2 H, s), 7.75 (2 H, m) and 8.10 (2 H, m); δC 19.76
(t), 47.43 (d), 117.00 (s), 126.45 (d), 131.95 (s), 133.95 (d),
138.70 (d) and 185.05 (s); m/z 252 (M+) (Found: M+, 252.0785.
Calc. for C16H12O3: M, 252.0786).
Diels–Alder reaction of sulfolene 1 with 5-hydroxy-1,4-
naphthoquinone (juglone) at 150 ЊC (entry 20 of Table 1). After
column chromatography, adduct cis-exo-5g (51.5 mg, 58%) as a
yellow oil and unchanged substrate 1 (6.2 mg, 13% recovery)
were obtained from sulfolene 1 and 5-hydroxy-1,4-naphtho-
quinone (juglone) (111.0 mg, 2 mol equiv.) at 120 ЊC for 4 h by
the general method.
cisoid-4,4a-cis-4a,9a-1,4-Epoxy-5-hydroxy-2,3-dimethylene-
1,2,3,4,4a,9a-hexahydroanthraquinone cis-exo-5g, δH 3.30 (1
H, d, J 1.37), 3.35 (1 H, d, J 1.37), 5.26 (2 H, s, bridgehead H),
5.27 (2 H, s), 5.37 (2 H, s), 6.12 (1 H, br s), 7.27 (1 H, m) and
7.66 (2 H, m); δC 53.96 (d), 54.43 (d), 87.15 (d), 87.20 (d), 103.52
(t, the signals of the two methylene groups for C-2 and C-3
overlapped), 118.22 (s), 118.78 (d), 123.93 (d), 135.19 (s), 137.50
(d), 145.39 (s), 145.50 (s), 162.18 (s), 194.02 (s) and 201.24 (s);
m/z 268 (M+) (Found: M+, 268.0729. Calc. for C16H12O4: M,
268.0735).
Diels–Alder reaction of sulfolene 1 with dimethyl maleate under
high-pressure conditions (entry 1 of Table 2)
N,N Ј-Diphenyl-1,4-epoxy-1,2,3,4,5,6,7,8-octahydronaphtha-
lene-2-exo,3-exo,6,7-tetracarboximide cis-exo-7c, δH 2.49 (2 H,
m), 3.01 (2 H, s), 3.03 (2 H, d, J 14.8), 3.38 (2 H, m), 5.25 (2 H,
s, bridgehead H) and 7.25–7.50 (10 H, m, ArH); δC 20.50 (t),
37.51 (d), 48.32 (d), 83.62 (d), 126.32 (d), 126.55 (d), 128.78 (d),
128.92 (d), 129.19 (d), 131.59 (s), 131.72 (s), 140.76 (s), 174.98
(s) and 177.87 (s); m/z 267 (M+ Ϫ C10H7NO2 base) (Found:
M Ϫ C10H7NO2, 267.0909. Calc. for C16H13NO3: m/z 267.0894).
Diels–Alder reaction of sulfolene 1 with maleic anhydride at
room temperature (entry 16 of Table 1). A solution of sulfolene
1 (50.0 mg, 0.32 mmol), 4-methoxyphenol (10 mg) and maleic
A typical procedure was as follows: furansulfolene 1 (50.0 mg,
0.32 mmol), 4-methoxyphenol (10 mg) and dimethyl maleate
(0.12 cm3, 3 mol equiv.) were dissolved in CH2Cl2 (1.8 cm3) and
the solution was placed in a PTFE cylinder and allowed to react
at the temperature, the pressure and for the time indicated in
Table 2. After evaporation of the mixture, the crude product
was purified by column chromatography (silica gel; hexane–
AcOEt 4:1) to afford adduct cis-endo-3a (78.3 mg, 81%).
Dimethyl 4,7-epoxy-2,2-dioxo-1,3,4,5,6,7-hexahydrobenzo-
[c]thiophene-5-endo,6-endo-dicarboxylate cis-endo-3a, νmax
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J. Chem. Soc., Perkin Trans. 1, 1997
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(CHCl3)/cmϪ1 1744, 1324 and 1175; δH 3.61 (1 H, d, J 2.45), 3.62
(1 H, d, J 2.45), 3.63 (6 H, s), 3.99 (2 H, s), 4.00 (2 H, s) and 5.18
(2 H, d, J 2.45, bridgehead H); δC 48.40 (d), 52.24 (q), 56.58 (t),
79.97 (d), 141.03 (s) and 169.93 (s); m/z 238 (M+ Ϫ SO2, 3.7%)
and 94 (M+ Ϫ SO2 Ϫ C6H8O4, base) (Found: M+, 238.0844.
Calc. for C12H14O5: M, 238.0841).
Other products were obtained in an analogous way. The
adduct trans-3a from dimethyl fumarate was a thermally
unstable powder.
Dimethyl 4,7-epoxy-2,2-dioxo-1,3,4,5,6,7-hexahydrobenzo-
[c]thiophene-trans-5,6-dicarboxylate trans-3a, νmax(CHCl3)/
cmϪ1 1735, 1319 and 1181; δH 2.91 (1 H, d, J 4.0), 3.67 (3 H, s),
3.76 (1 H, s), 3.80 (3 H, s), 3.99 (2 H, d, J 15.2), 4.02 (2 H, d,
J 15.2), 5.30 (1 H, s, bridgehead H) and 5.35 (1 H, d, J 4.00,
bridgehead H); m/z 243 (M+ Ϫ CO2CH3), 238 (M+ Ϫ SO2,
0.4), 178 (M+ Ϫ SO2 Ϫ CO2CH3, 26.5) and 94 (M+ Ϫ SO2 Ϫ
C6H8O4, base) (Found: M+, 238.0842).
was heated at 150 ЊC for 3 h in a sealed tube. After concentra-
tion, the residue was purified by column chromatography (silica
gel; hexane–AcOEt 9:1) to give compound 8a (20.5 mg, 84%).
Diels–Alder reaction of compound cis-exo-5a with DMAD under
thermal conditions
A solution of compound cis-exo-5a (24.3 mg, 0.103 mmol) and
DMAD (0.014 cm3, 1.1 mol equiv.) in benzene (0.5 cm3) was
heated at 150 ЊC for 3 h in a sealed tube. After concentration,
the residue was purified by column chromatography (silica gel;
hexane–AcOEt 9:1) to give adduct cis-exo-12a (4.0 mg, 10%) as
an oil, compound 8a (9.0 mg, 37%) and the recovery of sub-
strate cis-exo-5a (2.2 mg, 9%).
Tetramethyl 1,4-epoxy-1,2,3,4,5,8-hexahydronaphthalene-2-
exo,3-exo,6,7-tetracarboxylate cis-exo-12a, δH 3.19 (4 H, m),
3.42 (2 H, m), 3.68 (6 H, s), 3.79 (6 H, s) and 5.12 (2 H,
s, bridgehead H); m/z 349 (M+ Ϫ OCH3, 0.9%), 268 (M+ Ϫ
C6H8O2), 204 (M+ Ϫ CO2CH3 × 3, 35.5) and 94 (M+ Ϫ
C6H6O4 Ϫ C6H8O6, base) (Found: M Ϫ OCH3, 349.0928. Calc.
for C17H17O8: m/z 349.0922).
Diels–Alder reaction of adduct cis-endo-5a with DMAD under
high-pressure conditions
The type A adduct cis-endo-5a (50.0 mg, 0.21 mmol), 4-
methoxyphenol (10 mg) and DMAD (0.028 cm3, 1.1 mol equiv.)
were dissolved in CH2Cl2 (1.8 cm3) and the solution was placed
in a PTFE cylinder and allowed to react at 12 kbar for 48 h
(28 ЊC). After removal of the solvent, the residue was purified
by column chromatography (silica gel; hexane–AcOEt 4:1) to
give cis-endo-12a (79 mg, quant.) as a pale yellow oil.
Tetramethyl 1,4-epoxy-1,2,3,4,5,8-hexahydronaphthalene-
2-endo,3-endo,6,7-tetracarboxylate, cis-endo-12a, δH 3.25 (4 H,
m), 3.53 (2 H, d, J 2.44), 3.63 (6 H, s), 3.78 (6 H, s) and 4.96 (2
H, d, J 2.44, bridgehead H); δC 27.15 (q), 47.89 (d), 51.84 (q),
52.28 (q), 81.59 (d), 132.69 (s), 138.21 (s), 168.28 (d) and 170.38
(s); m/z 349 (M+ Ϫ OCH3, 1.1%), 268 (M+ Ϫ C6H8O2), 204
(M+ Ϫ CO2CH3 × 3, 49.7) and 94 (M+ Ϫ C6H6O4 Ϫ C6H8O6,
base) (Found: M Ϫ OCH3, 349.0924. Calc. for C17H17O8: m/z
349.0922).
Diels–Alder reaction of compound cis-endo-5a with 5-hydroxy-
1,4-naphthoquinone (juglone)
A solution of compound cis-endo-5a (50.0 mg, 0.21 mmol) and
5-hydroxy-1,4-naphthoquinone (juglone) (40.1 mg, 1.1 mol
equiv.) in CH2Cl2 (2 cm3) was heated at 80 ЊC for 10 h in a sealed
tube. After concentration, the residue was purified by column
chromatography (benzene–CH2Cl2 9:1) to give compound 11
(50.5 mg, 58%) as a yellow oil.
1,4-Epoxy-7-hydroxy-6,11-dioxo-1,2,3,4,5,5a,6,11,11a,12-
decahydronaphthacene-2-endo,3-endo-dicarboxylate 11, δH (90
MHz) 2.52 (2 H, m), 2.78 (2 H, m), 3.10 (2 H, m), 3.51 (2 H, d,
J 1.51), 3.61 (3 H, s), 3.65 (3 H, s), 4.89 (2 H, d, J 1.51), 7.32
(1 H, m) and 7.64 (2 H, m); m/z 412 (M+) (Found: M+,
412.1152. Calc. for C22H20O8: M, 412.1157).
References
Retro-Diels–Alder reaction of adduct cis-endo-12a
1 S. Yamada and H. Takayama, Chem. Lett., 1979, 583; S. Yamada,
T. Suzuki and H. Takayama, Tetrahedron Lett., 1981, 22, 2591;
S. Yamada, H. Ohsawa, T. Suzuki and H. Takayama, Chem. Lett.,
1983, 1003; S. Yamada, T. Suzuki, H. Takayama, K. Miyamoto,
L. Matsunaga and Y. Nawata, J. Org. Chem., 1983, 48, 3483;
S. Yamada, H. Ohsawa, T. Suzuki and H. Takayama, J. Org. Chem.,
1986, 51, 4934; H. Takayama and T. Suzuki, J. Chem. Soc., Chem.
Commun., 1988, 1044.
2 T. Suzuki, K. Kubomura, H. Fuchii and H. Takayama, J. Chem. Soc.,
Chem. Commun., 1990, 1687; T. Suzuki, K. Kubomura and
H. Takayama, Chem. Pharm. Bull., 1991, 39, 2164; K. Ando,
N. Akadegawa and H. Takayama, J. Chem. Soc., Chem. Commun.,
1991, 1765; K. Ando, C. Hatano, N. Akadegawa, A. Shigihara and
H. Takayama, J. Chem. Soc., Chem. Commun., 1992, 870; T. Suzuki,
H. Fuchii and H. Takayama, Heterocycles, 1993, 35, 57; K. Ando,
N. Akadegawa and H. Takayama, J. Chem. Soc., Perkin Trans. 1,
1993, 2263; T. Hayashi, Y. Kawakami, K. Konno and H. Takayama,
J. Chem. Soc., Perkin Trans. 1, 1993, 2387; T. Suzuki, K. Kubomura
and H. Takayama, Heterocycles, 1994, 38, 961; K. Konno, S. Maki,
S. Sagara and H. Takayama, Tetrahedron Lett., 1995, 36, 1865.
3 J. Jolivet, Ann. Chem., 1960, (5), 1165; T. A. Eggelte, H. De Konig
and H. O. Huisman, Tetrahedron, 1973, 29, 2491.
4 W. G. Dauben and H. O. Krabbenhoht, J. Am. Chem. Soc., 1976, 98,
1992; H. Kotsuki, H. Nishizawa, M. Ochi and K. Matsuoka, Bull.
Chem. Soc. Jpn., 1982, 55, 496.
5 J. Jurcak, T. Kozluk, S. Filipek and C. H. Eugster, Helv. Chim. Acta,
1983, 66, 222.
6 W. H. Watson, J. Galloy, P. D. Bartlett and A. A. M. Roof, J. Am.
Chem. Soc., 1981, 103, 2022; O. Pilet and P. Vogel, Helv. Chim. Acta,
1981, 64, 2563; P. Viioget, M. Bonivento, R. Roulet and P. Vogel,
Helv. Chim. Acta, 1984, 67, 1630.
A solution of compound cis-endo-12a (10.1 mg, 0.03 mmol) in
benzene (0.56 cm3) was heated at 150 ЊC for 3 h in a sealed tube.
After removal of the solvent, the residue was purified by
column chromatography (silica gel; hexane–AcOEt 4:1) to
give compound 8a (7.0 mg, quant.) as an oil.
Dimethyl 4,7-dihydrobenzo[c]furan-5,6-dicarboxylate 8a, δH
3.54 (4 H, s), 3.82 (6 H, s) and 7.29 (2 H, s); δC 22.57 (t), 52.37
(q), 116.68 (s), 133.02 (s), 137.57 (d) and 168.49 (s); m/z 236
(M+, 27.1%), 221 (M+ Ϫ CH3, 8.5) and 118 (M+ Ϫ C4H6O4,
base) (Found: M+, 236.0685. Calc. for C12H12O5: M, 236.0684).
Diels–Alder reaction of compound trans-5a with DMAD under
thermal conditions
A solution of compound trans-5a (24.4 mg, 0.103 mmol) and
DMAD (0.014 cm3, 1.1 mol equiv.) in benzene (0.5 cm3) was
heated at 150 ЊC for 3 h in a sealed tube. After concentration,
the residue was purified by column chromatography (silica gel;
hexane–AcOEt 9:1) to give adduct trans-12a (21.9 mg, 55%)
as an oil and substrate 8a (9.5 mg, 39% recovery).
Tetramethyl 1,4-epoxy-1,2,3,4,5,8-hexahydronaphthalene-
trans-2,3,6,7-tetracarboxylate, trans-12a, δH 2.91 (1 H, m), 3.37
(4 H, m), 3.69 (3 H, s), 3.71 (1 H, s), 3.77 (6 H, s), 3.83 (3 H, s),
5.07 (1 H, m, bridgehead H) and 5.51 (1 H, s, bridgehead H);
m/z 349 (M+ Ϫ OCH3, 2.3%) and 94 (M+ Ϫ C6H6O4 Ϫ C6H8O6,
base) (Found: M+ Ϫ OCH3, 349.0925. C17H17O8 requires
349.0922).
Diels–Alder reaction of adduct cis-endo-5a with DMAD under
thermal conditions
Paper 6/04600B
Received 2nd July 1996
Accepted 9th September 1996
A solution of compound cis-endo-5a (24.2 mg, 0.103 mmol)
and DMAD (0.014 cm3, 1.1 mol equiv.) in benzene (0.5 cm3)
256
J. Chem. Soc., Perkin Trans. 1, 1997