Diels-Alder reactions of push-pull olefins I. Reactions of dicyanovinyl acylates with cyclopentadiene, 1,3-cyclohexadiene, and anthracene a

Article abstract of DOI:10.1007/PL00013475

The reaction between 2,2-dicyanoethenyl acylates and cyclopentadiene affords 2-acyloxy-3,3-dicyano-5-norbornene derivatives with significant stereoselectivity. Reactions between the less reactive 1,3-cyclohexadiene and anthracene require elevated temperat

Full text of DOI:10.1007/PL00013475

Monatshefte fuÈr Chemie 129, 689±696 (1998)
Diels-Alder Reactions of Push-Pull Ole®ns I.
Reactions of Dicyanovinyl Acylates with
Cyclopentadiene, 1,3-Cyclohexadiene,
and Anthracenea
Ã
Martin Dees and Hans-Hartwig Otto
È
Institut fuÈr Pharmazie, Universitat Greifswald, D-17487 Greifswald, Bundesrepublik Deutschland
Summary. The reaction between 2,2-dicyanoethenyl acylates and cyclopentadiene affords 2-
acyloxy-3,3-dicyano-5-norbornene derivatives with signi®cant stereoselectivity. Reactions between
the less reactive 1,3-cyclohexadiene and anthracene require elevated temperatures and extended
reaction times. Stereoselectivity of the reactions with 1,3-cyclohexadiene is higher than of those with
cyclopentadiene.
Keywords. Diels-Alder; Push-Pull Ole®ns; Stereoselectivity.
Diels-Alder-Reaktionen von Push-Pull-Ole®nen, 1. Mitt. Reaktionen von Dicyanovinylestern
mit Cyclopentadien, 1,3-Cyclohexadien und Anthracen
È
Zusammenfassung. Reaktionen zwischen 2,2-Dicyanoethenylcarbonsaureestern und Cyclopenta-
È
dien ergeben mit signi®kanter Stereoselektivitat 2-Acyloxy-3,3-dicyan-5-norbornenderivate. Ana-
loge Reaktionen mit den weniger reaktiven Dienen 1,3-Cyclohexadien und Anthracen erfordern
È
È È
hohere Temperaturen und langere Reaktionszeiten; die Stereoselektivitat der Reaktionen mit 1,3-
È
Cyclohexadien ist deutlich groûer.
Introduction
The Diels-Alder reaction is one of the most valuable tools for the construction of
six-membered rings in synthetic chemistry [1]. Wether the reaction proceeds
normaly or with inverse electron demand depends on the electron donating or
withdrawing properties of the substituents of the dienes and dienophiles. However,
only few attention has been paid so far to the use of push-pull substituted ole®ns as
dienophiles [2]. We have investigated Diels-Alder reactions between some push-
pull ole®ns and symmetrical as well as unsymmetrical dienes. Our main goals were
to study the in¯uence of the substitution pattern on the reactivity of the
a
From the thesis of Martin Dees, University of Freiburg, 1995
Ã
Corresponding author

690
M. Dees and H.-H. Otto
dienophiles, the regioselectivity of reactions with unsymmetrical dienes, and the
stereoselectivity of these reactions.
In this paper, on some reactions between carboxylates of 2,2-dicyanoethenol
and the highly reactive cyclopentadiene, the less reactive cyclohexa-1,3-diene, and
anthracene. Furthermore, the Diels-Alder reaction between a dienophile bearing a
third electron withdrawing group and having therefore both push-pull and
captodative properties [3] and cyclopentadiene is described. A Diels-Alder reaction
between 2,2-dicyanoethenyl acetate and cyclopentadiene has been reported [4], and
some cycloadditions employing acyloxymethylenemalonates without and with
Lewis acid catalysis have been studied by Katagiri and coworkers [5]. In these
experiments, the exo-acyloxy compound is the predominating stereoisomer at
elevated temperatures, and the endo-derivative under Lewis acid catalysis; the
endo/exo ratio depends on the Lewis acid and the temperature.
Results and Discussion
2,2-Dicyanovinyl benzoate (2a), pivalate (2b), and p-nitrobenzoate (2c) were
prepared from the potassium salt of hydroxymethylidenemalononitrile (1, [6]) in
analogy to the rather crude literature procedure described for the corresponding
acetate [7] or by a more convenient pathway from hydroxymethylidenemalono-
nitrile [8]. We found tetrahydrofuran to be the solvent of choice for this reaction.
2,2-Dicyano-1-(ethoxycarbonyl)vinyl p-nitrobenzoate (6c) was prepared in a
similar manner from the potassium salt of ethyl 3,3-dicyano-2-hydroxyacrylate
[6] and p-nitrobenzoyl chloride. Especially 2a and 2b are very interesting and
Scheme

Diels-Alder Reactions of Push-Pull Ole®ns
691
useful compounds because of their ease of preparation and their stability. The
corresponding acetate decomposes within a few days, even if stored at ꢀ20ꢁC
under a nitrogen atmosphere.
At room temperature, the reactions between 2a±c and cyclopentadiene are
completed within 45 min. The yields are nearly quantitative, and all products are
isolated as crystalline compounds. The 1H NMR spectra of the crude products 3a±c
show two signals between 4.8 and 5.8 ppm with a ratio of about 2:1 correspond-
ing to 2-H of the endo- and exo-acyloxy diastereomers. The more intense
signal which is shifted down®eld has a coupling constant of J ꢂ 4 Hz, whereas
the minor signal shows a coupling constant of J ꢂ 4 Hz. According to the Karplus-
Conroy relation, the signal with J ˆ 4 Hz should correspond to the equatorial
2-H of the endo-diastereomers of 3a±c, spreading a dihedral angle of about 60ꢁ
with the bridgehead hydrogen atom 1-H. The dihedral angle of 2-H axial with 1-H
of the exo-diasteromers amounts to about 90ꢁ, resulting in a smaller coupling
constant.
The endo-diastereomers of 3a and b be separated by fractionated crystallization
in ca. 40% yield, whereas the isolation of small amounts of the better soluble exo-
isomer succeed only in the case of 3b. Compound 3c is obtained as a diastereo-
meric mixture (endo:exo ꢂ 2:1).
Compound 6c, the push-pull captodative ole®n, also reacts with cyclopenta-
diene at room temperature. The ethoxycarbonyl group, however, does not enhance
the reactivity of 6c signi®cantly compared to 2a±c. At least, no exothermic reaction
is observed as might be expected from a reaction of cyclopentadiene with a much
more reactive ole®ne. The endo- and exo-diastereomers of 7c are formed in a ratio
of about 1:1 (TLC). The diastereomer which is more soluble in apolar solvents was
separated by column chromatography and identi®ed as the endo-ethoxycarbonyl-
exo-(p-nitrobenzoyloxy) diastereomer exo-7c by an NOE experiment.
For a successful reaction of 2a or 2b with the rather unreactive cyclohexa-1,3-
diene, the reactants were re¯uxed in carbon tetrachloride for 48 h. The reactions
yield about 60±70% of the cycloadducts 4a and 4b, both isolated as diastereomeric
mixturer in a ratio of 95:5. In the crude reaction mixture, the ratio was determined
to be about 9:1. The protons at C-1 and C-2 of the bicyclo[2.2.2]oct-5-ene
derivatives 4a and 4b spread a dihedral angle of about 60ꢁ in the endo- as well as in
the exo-diastereomers. MMX calculations of the minimum energy conformations
using the program PCMODEL, Serena Software, show dihedral angles of 63.65ꢁ
for the endo- and ꢀ55.35ꢁ for the exo-isomer.
NOE experiments show a positive NO effect between 2-H and one of the
protons of the C-7/C-8 bridge (most propably 7-Hsyn) of the main diastereomers.
This indicates that in the main diastereomers of 4a and 4b the acyloxy group is
endo-oriented. Thus the endo/exo preference in these reactions between 2,2-
dicyanvinyl carboxylates and 1,3-cyclohexadiene is about 9:1, signi®cantly higher
than in the reactions with cyclopentadiene where it is about 2:1.
Re¯uxing 2a or 2b with anthracene in xylene always leads to an incomplete
formation of the reaction products 5a, b. Neither a reaction time up to 3 days nor
lowering the reaction temperature to 80ꢁC and extending at the same time the
reaction time to one week improves the yields. The addition products, however,
could easily be separated from unchanged starting material and decomposition

692
M. Dees and H.-H. Otto
products by column chromatography in about 20% yield. The presumption that in
these reactions with anthracene under the applied conditions an equilibrium
between Diels-Alder and retro-Diels-Alder reaction with an equilibrium constant of
about 0.2 is reached can be demonstrated by re¯uxing 5a in xylene, leading to the
decomposition into 2a and anthracene (TLC).
The 1H NMR spectra of 5a and 5b show two doublets (ꢀ ˆ 4.73/4.63 and 5.51/
5.27 ppm, J ˆ 3 Hz) and one singulet at ꢀ ˆ 4.90 ppm. The latter indicates the
bridgehead proton 9-H; the assignment of the doublets can only be performed by
NOE experiments, showing that the signals shifted down®eld arise from the proton
of the ethano bridge, 11-H.
In the reaction of cyclopentadiene with the dienophiles 2a±c we found a 2:1
preference for the formation of the endo-diastereomers. Assuming the reactions are
kinetically controlled, this is not to be expected. Since all isolated diastereomers
are stable compounds, at least under our reaction conditions (room temperature,
aprotic solvent), the reactions between the 2,2-dicyanovinyl carboxylates and
cyclopentadiene should be kinetically controlled. Usually, the endo-preference in
kinetically controlled Diels-Alder reactions is explained by a secondary orbital
overlap between the frontier orbitals, thereby lowering the energy of the transition
state. This type of attracting forces normally is described as an interaction between
the ꢁ-system of the diene and a group of the dienophile which is ꢁ-conjugated to its
ole®nic double bond. As the push-pull ole®ns 2 do not show this ꢁ-conjugation, we
postulate a secondary orbital overlap between the ꢁ-system of the diene and the n-
electrons of the acyloxy group of the dienophile. To the best of our knowledge, this
type of interaction has not been reported until today.
Regarding reaction temperature and time in the reactions of 1,3-cyclohexadiene
with 2a and 2b, one might come to the conclusion that the endo-products 4a and 4b
are thermodynamically more stable than the exo products. This would lead to their
preferred formation under thermodynamically controlled reaction conditions.
Force ®eld calculations (MMX), however, showed no signi®cant difference in the
stability of the endo- and exo-diastereomers. Thus, it is more likely that, as with
cyclopentadiene, the reaction of 2,2-dicyanovinyl carboxylates with 1,3-cyclohexa-
diene proceeds kinetically controlled and that the preferred formation of endo-
acyloxy products is caused by a transition state lower in energy than that leading to
the exo products.
Experimental
General
È
All con®gurations are relative; m.p.: Linstrom apparatus (uncorr.); IR spectra: Perkin-Elmer IR 841,
1
KBr if not noted otherwise; H NMR spectra: Varian T60, Bruker WP80, Bruker WH90, Varian
U300, Bruker AM400, internal TMS, ꢀ in ppm, values for 60 MHz spectra in CDCl3 if not noted
otherwise; 13C NMR spectra: Varian U300 (75.43 MHz), internal TMS, ꢀ in ppm; elemental analyses:
È
Pharmazeutisches Institut or Chemisches Laboratorium der Universitat Freiburg. Tetrahydrofuran
(THF) was dried over KOH, re¯uxed with benzophenone and sodium, and then distilled.
Dimethylformamid (DMF) was dried over P4O10 and distilled from CaH2. Other solvents were dried/
puri®ed according to literature procedures.

Diels-Alder Reactions of Push-Pull Ole®ns
693
Thin layer chromatography (TLC): DC-Alufolien silica gel 60 F254 (Merck Nr. 5554), detection
with UV light (254 nm and 366 nm) or with phosphomolybdato ammonium cerium(IV) sulfate (1.0 g
of ammonium cerium(IV) sulfate-2-hydrate and 2.5 g of phosphomolybdic acid are dissolved in 8 ml
of conc. sulfuric acid, water is added to 100 ml, and after 2 h the solution is ®ltered); column
chromatography (CC): silica gel 60 (0.063±0.200 mm, Merck Nr. 7734).
(Hydroxymethylidene) malononitrile (1): see Refs. [9, 10].
Acylation of 1, General Procedure
a) 5.10 g (50 mmol) of triethylamine is added dropwise to an ice-cooled soln. of 50 mmol of 1 and
50 mmol of the acide chloride in 200 ml of THF; the mixture is stirred for 1 h at 0ꢁC, then 15 h at
room temp. The precipitate is separated, washed twice with 50 ml of THF each, the combined org.
layers are dried over sodium sulfate, the solvent is evaporated, and the residue is puri®ed as speci®ed.
b) 20 mmol of the sodium or potassium salt of 1 is dissolved in 150 ml of THF; 20 mmol of the
acide chloride is added, and the mixture is re¯uxed as noted. Then the solvent is evaporated, 150 ml
of dichloromethane are added to the residue, and after ®ltration, the solvent is again evaporated. The
residue is puri®ed as speci®ed.
2,2-Dicyanovinyl benzoate (2a)
From 3.75 g (40 mmol) of 1, 5.65 g (40 mmol) of benzoyl chloride, and 4.05 g (40 mmol) of
triethylamine, method a; from 6.60 g (50 mmol) of 1 (potassium salt) and 7.00 g (50 mmol) of
benzoyl chloride, time 2 h, method b.
Yield: a) 6.10 g (77%), b) 8.55 g (86%); colorless crystals; m.p.: 154ꢁC (dec., n-hexane); IR:
ꢂ ˆ 3056 (CH), 2244 (CN), 1771 (CO), 1617 (C=C) cmꢀ1; 1H NMR: ꢀ ˆ 7.66±8.50 (m, 5H, aromatic
H), 8.87 (s, 1H, ole®n. H) ppm; C11H6N2O2 (198.2); calcd.: C 66.67, H 3.05, N 14.14; found: C
66.87, H 3.08, N 14.26.
2,2-Dicyanovinyl pivaloate (2b)
From 5.30 g (40 mmol) of 1 (potassium salt) and 4.85 g (40 mmol) of pivaloyl chloride, time 2 h,
method b.
Yield: 5.30 g (74%); colorless crystals; m.p.: 85ꢁC (n-hexane); IR: ꢂ ˆ 3053, 2983, 2942, 2915,
2879 (CH), 2249 (CN), 1801 (CO), 1620 (C=C) cmꢀ1; 1H NMR: ꢀ ˆ 1.37 (s, 9H, CH3), 8.48 (s, 1H,
ole®n. H) ppm; C9H10N2O2 (178.2); calcd.: C 60.67, H 5.66, N 15.72; found: C 60.58, H 5.71, N
15.59.
2,2-Dicyanovinyl p-nitrobenzoate (2c)
From 2.65 g (20 mmol) of 1 (potassium salt) and 3.7 g (20 mmol) of p-nitrobenzoyl chloride, time
2 h, method b.
Yield: 0.55 g (11%); light yellow crystals; m.p.: 166ꢁC (dec., CCl4); IR: ꢂ ˆ 3106, 3058 (CH),
2249 (CN), 1776 (CO), 1624 (C=C), 1534, 1356 (NO2) cmꢀ1; 1H NMR: ꢀ ˆ 8.47 (s, 4H, arom. H),
8.80 (s, 1H, ole®n. H) ppm; C11H5N3O4 (243.2); calcd.: C 54.33, H 2.07, N 17.28; found: C 54.46, H
2.09, N 17.22.
Synthesis of 3, General Procedure
The dienophile is dissolved in an appropriate solvent, 1.2±2 equiv. of cyclopentadiene (freshly
distilled) are added, the mixture is stirred at r.t. for 15 h, the solvent is evaporated, and the residue is
puri®ed as speci®ed.

694
M. Dees and H.-H. Otto
3,3-Dicyanobicyclo[2.2.1]hept-5-en-2-yl benzoate (3a)
From 1.00 g (5.05 mmol) of 2a and 0.60 g (9.1 mmol) of cyclopentadiene in CH2Cl2. 60 ml of n-
hexane is added to the residue.
Yield: 1.03 g (77%); mixture of isomers, endo/exo ˆ 2:1; colorless crystals.
endo-3a
From the mixture by two cristallizations from CCl4; yield: 0.49 g (37%); colorless crystals; m.p.:
130ꢁC (CCl4); IR: ꢂ ˆ 3081, 3060, 3005, 2994, 2951, 2880 (CH), 2247 (CN), 1712 (CO), 1600, 1583
(arom.), 1274, 1107 (C±O) cmꢀ1; 1H NMR (80 MHz): ꢀ ˆ 1.80±2.05 (m, 2H, 7-Hsyn, 7-Hanti), 3.38±
3.60 (m, 1H, 1-H), 3.60±3.78 (m, 1H, 4-H), 5.75 (d, J ˆ 4 Hz, 1H, 2-H), 6.40±6.63 (m, 2H, 5-H, 6-
H), 7.23±7.73, 7.93±8.10 (2 m, 5H, arom. H) ppm; C16H12N2O2 (264.3); calcd.: C 72.72, H 4.58, N
10.60; found: C 72.70, H 4.66, N 10.68.
3,3-Dicyanobicyclo[2.2.1]hept-5-en-2-yl pivalate (3b)
From 1.80 g (10.1 mmol) of 2b and 0.90 g (13.6 mmol) of cyclopentadiene in CCl4. The precipitate
(endo-3b) is separated before the solvent is evaporated.
endo-3b: Yield: 1.00 g (40%); colorless crystals; m.p.: 128ꢁC (CCl4); IR: ꢂ ˆ 3072, 2985, 2975,
1
2933, 2907, 2874 (CH), 2245 (CN), 1730 (CO), 1688 (C=C), 1146, 1132 (C±O) cmꢀ1; H NMR
(80 MHz): ꢀ ˆ 1.23 (s, 9 H, C(CH3)3), 1.75±1.98 (m, 2H, 7-Hsyn, 7-Hanti), 3.25±3.48 (m, 1H, 1-H),
3.53±3.73 (m, 1H, 4-H), 5.40 (d, J ˆ 4 Hz, 1H, 2-H), 6.30±6.53 (m, 2H, 5-H, 6-H) ppm; C14H16N2O2
(244.3); calcd.: C 68.83, H 6.60, N 1147; found: C 68.55, H 6.77, N 11.38.
exo-3b: By fractionated crystallization from n-hexane; yield 0.07 g (2.8%); colorless crystals;
m.p.: 68ꢁC (n-hexane); IR: ꢂ ˆ 3075, 2995, 2980, 2937, 2907, 2878 (CH), 2252 (CN), 1736 (CO),
1
1662 (C=C), 1130 (C±O) cmꢀ1; H NMR (80 MHz): ꢀ ˆ 1.28 (s, 9H, C(CH3)3), 2.00±2.18 (m, 2H,
7-Hsyn, 7-Hanti), 3.05±3.23 (m, 1H, 1-H), 3.55±3.70 (m, 1H, 4-H), 4.80 (d, J ˆ 2 Hz, 1H, 2-H),
6.30±6.53 (m, 2H, 5-H, 6-H).
3,3-Dicyanobicyclo[2.2.1]hept-5-en-2-yl p-nitrobenzoate (3c)
From 1.00 g (4.1 mmol) of 2c and 0.50 g (7.6 mmol) of cyclopentadiene in THF.
Yield: 1.01 g (79%); mixture of isomers, endo/exo ˆ 2:1; light yellow crystals; m.p.: 173ꢁC
(CCl4); IR: ꢂ ˆ 3110, 3078, 3055, 3001, 2949, 2881 (CH), 2249 (CN), 1722 (CO), 1529, 1350
1
(NO2), 1263, 1100 (C±O) cmꢀ1; H NMR: ꢀ ˆ 1.90±2.10 (m, 2HÂ2/3, 7-Hsyn, 7-Hanti), 2.10±2.33
(m, 2HÂ1/3, 7-Hsyn, 7-Hanti), 3.30±3.95 (m, 2H, 1-H, 4-H), 5.20 (br.s, 1HÂ1/3, 2-H), 5.83 (d,
J ˆ 4 Hz, 1HÂ2/3, 2-H), 6.43±6.73 (m, 2H, 5-H, 6-H), 8.10±8.50 (m, 4 H, arom. H) ppm;
C16H11N3O4 (309.3); calcd.: C 62.14, H 3.58, N 13.59; found: C 62.19, H 3.66, N 13.50.
Synthesis of 4, General Procedure
The dienophile and the equivalent amount of cyclohexa-1,3-diene are re¯uxed in 50 ml of CCl4 for
48 h. Then the solvent is evaporated, and the residue is redissolved in CH2Cl2 and ®ltered through
silica gel. The ®ltrate is evaporated, and the residue is crystallized as speci®ed.
endo-3,3-Dicyanobicyclo[2.2.2]oct-5-en-2-yl benzoate (4a)
From 1.0 g (5.1 mmol) of 2a and 1.0 g (12.5 mmol) of cyclohexa-1,3-diene.
Yield: 0.67 g (48%); light yellow crystals; m.p.: 167ꢁC (CCl4); IR: ꢂ ˆ 3057, 2984, 2959, 2880
1
(CH), 2248 (CN), 1720 (CO), 1283, 1266, 1097 (C±O) cmꢀ1; H NMR (300 MHz): ꢀ ˆ 1.45±1.59

Diels-Alder Reactions of Push-Pull Ole®ns
695
(m, 2H, 7-Hanti, 8-Hanti), 1.74±1.87 (m, 1H, 7-Hsyn), 2.05±2.18 (m, 1H, 8-Hsyn), 3.04±3.11 (m, 1H, 1-
H, 3.32±3.38 (m, 1H, 4-H), 5.33 (d, J ˆ 2.44 Hz, 1H, 2-H), 6.44±6.58 (m, 2H, 5-H, 6-H), 7.41±7.63,
8.03±8.09 (2m, 5H, arom. H) ppm; C17H14N2O2 (278.3); calcd.: C 73.37, H 5.07, N 10.07; found: C
73.15, H 5.04, N 10.21.
endo-3,3-Dicyanobicyclo[2.2.2]oct-5-en-2-yl pivalate (4b)
From 1.00 g (5.6 mmol) of 2b and 1.10 g (13.7 mmol) of cyclohexa-1,3-diene.
Yield: 0.98 g (68%); colorless crystals; m.p.: 114ꢁC (n-hexane); IR: ꢂ ˆ 3075, 3056, 2977, 2940,
1
2875 (CH), 2246 (CN), 1728 (CO), 1610 (C=C), 1144 (C±O) cmꢀ1; H NMR (300 MHz): ꢀ ˆ 1.22
(s, 9H, C(CH3)3), 1.36±1.52 (m, 2H, 7-Hanti, 8-Hanti), 1.65±1.78 (m, 1H, 7-Hsyn), 1.99±2.12 (m, 1H,
8-Hsyn), 2.85±2.94 (m, 1H, 1-H), 3.24±3.32 (m, 1H, 4-H), 4.97 (d, J ˆ 2.4 Hz, 1H, 2-H), 6.38±6.49
(m, 2H, 5-H, 6-H) ppm; C15H18N2O2(258.3); calcd.: C 69.74, H 7.02, N 10.84; found: C 69.50, H
6.94, N 10.99.
Synthesis of 5, General Procedure
The dienophile and the equivalent amount of anthracene are re¯uxed in 50 ml of xylene for 48 h.
Then, the solvent is evaporated, and the residue is puri®ed by CC. First, anthracene is separated by
extraction with chloroform, then the product is obtained by elution with dichloromethane (TLC
control).
12,12-Dicyano-9,10-dihydro-9,10-ethanoanthracen-11-yl benzoate (5a)
From 1.10 g (5.6 mmol) of 2a and 0.99 g (5.6 mmol) of anthracene.
Yield: 0.38 g (18%); colorless crystals; m.p.: 199ꢁC (dec.); IR: ꢂ ˆ 3069, 3045, 3029, 2986, 2957
1
(CH), 2256 (CN), 1726 (CO), 1264, 1099 (C±O) cmꢀ1; H NMR (80 MHz): ꢀ ˆ 4.73 (d, J ˆ 3 Hz,
1H, 10-H), 4.90 (s, 1H, 9-H), 5.51 (d, J ˆ 3 Hz, 1H, 11-H), 7.30±7.59 (m, 11H, 1-H-8-H, benzoyl
3-H-5-H), 7.85±7.89 (m, 2H, benzoyl 2-H, 6-H) ppm; C25H16N2O2 (376.4); calcd.: C 79.77, H 4.28,
N 7.44; found: C 79.73, H 4.37, N 7.32.
12,12-Dicyano-9,10-dihydro-9,10-ethanoathracen-11-yl pivalate (5b)
From 0.89 g (5.0 mmol) of 2b and 0.89 g (5.0 mmol) of anthracene.
Yield: 0.38 g (21%); colorless crystals; m.p.: 149ꢁC; IR: ꢂ ˆ 3073, 3048, 3030, 2972, 2931, 2906,
2873, (CH), 2247 (CN), 1730 (CO), 1140 (C±O) cmꢀ1; 1H NMR: ꢀ ˆ 1.17 (s, 9H, C(CH3)3), 4.63 (d,
J ˆ 3 Hz, 1H, 10-H), 4.90 (s, 1H, 9-H), 5.27 (d, J ˆ 3 Hz, 1H, 11-H), 7.20±7.80 (m, 8H, 1-H ± 8-H)
ppm; C23H20N2O2 (356.4); calcd.: C 77.51, H 5.66, N 7.86; found: C 77.71, H 5.72, N 7.77.
Ethyl 3,3-dicyano-2-(4-nitrobenzoyloxy)acrylate (6c)
2.05 g (10 mmol) ethyl 3,3-dicyano-2-hydroxyacrylate, potassium salt [11] and 1.90 g (10 mmol) p-
nitrobenzoyl chloride in 150 ml of THF are re¯uxed for 3.5 h. The solvent is evaporated, and the
residue is redissolved in 50 ml of benzene at room temperature and ®ltered. This procedure is
repeated once. n-Hexane (ca. 100 ml) is added to the ®ltrate until a slight opalescent solution is
formed. This solution is stored at ꢀ20ꢁC until the crystallization is completed (3±5 days). The
crystals are separated without warming.
Yield: 0.90 g (29%); light yellow crystals; m.p.: 75±76ꢁC (n-hexane); IR: ꢂ ˆ 3112, 3082, 3060,
1
2997, 2945 (CH), 2246 (CN), 1766, 1752 (CO), 1620 (C=C), 1526, 1351 (NO2) cmꢀ1; H NMR:

696
M. Dees and H.-H. Otto: Diels-Alder Reactions of Push-Pull Olefins
ꢀ ˆ 1.43 (t, J ˆ 7 Hz, 3H, CH3), 4.53 (q, J ˆ 7 Hz, 2H, CH2), 8.20±8.50 (m, 4H, arom. H) ppm;
C14H9N3O6 (315.2); calcd.: C 53.34, H 2.88, N 13.33; found: C 53.63, H 2.99, N 13.09.
Ethyl 3,3-dicyano-2-(p-nitrobenzoyloxy)bicyclo[2.2.1]hept-5-ene-2-carboxylate (7c)
From 1.50 g (4.8 mmol) of 6c and 0.50 g (7.6 mmol) of cyclopentadiene in THF as described for 2.
Yield: 1.08 g (60%); mixture of isomers, endo/exo ˆ 1:1; light yellow crystals; m.p.: 134ꢁC
(cyclohexane); IR: ꢂ ˆ 3111, 3079, 3055, 2995, 2940, 2905 (CH), 2949 (CN), 1753 (CO), 1529,
1348 (NO2), 1282, 1098 (C±O) cmꢀ1; 1H NMR (80 MHz): ꢀ ˆ 1.26 (t, J ˆ 7 Hz, 3HÂ1/2, CH3), 1.30
(t, J ˆ 7 Hz, 3HÂ1/2, CH3), 1.90±2.63 (m, 2H, 7-Hsyn, 7-Hanti), 3.63±3.88, 4.00±4.20 (2 m, 1-H, 4-
H), 4.29 (q, J ˆ 7 Hz, 2HÂ1/2, CH2), 4.38 (q, J ˆ " 7 Hz, 2HÂ1/2, CH2), 6.13±6.85 (m, 2H, 5-H, 6-
H), 8.05±8.48 (m, 4H, arom. H) ppm; C19H15N3O6 (381.4); calcd.: C 59.84, H 3.96, N 11.02; found:
C59.65, H 4.01, N 11.15.
exo-7c: Separation by CC (silica gel, diethyl ether/n-hexane ˆ 1:1; TLC control); Rf ˆ 0.49
(endo-7c: Rf ˆ 0.40); yield: 0.021 g (21%); light yellow crystals; m.p.: 131ꢁC; 1H NMR (300 MHz):
ꢀ ˆ 1.30 (t, J ˆ 7.1 Hz, 3H, CH3), 2.09 (ddd, J ˆ 11.0 Hz, 1.7 Hz, 1.7 Hz, 1H, 7-Hanti), 2.45 (dbr.s,
J ˆ 11 Hz, 1H, 7-Hsyn), 3.77 (m, 1H, 4-H), 4.13 (m, 1H, 1-H), 4.38 (q, J ˆ 7.1 Hz, 2H, CH2), 6.29
(dd, J ˆ 5.6 Hz, 3.2H2, 1H, 6-H), 6.57 (dd, J ˆ 5.6 Hz, 2.9 Hz, 1H, 5-H), 8.22±8.36 (m, 4H, arom. H).
Acknowledgements
Financial support provided by the Fonds der Chemischen Industries, Frankfurt, and Ciba, Basel, is
gratefully appreciated. We thank Volker Brecht for recording of NMR spectra and Ursula Predoiu for
experimental help.
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Received November 21, 1997. Accepted December 4, 1997