Tetraacylethenes as dienophiles and hetero dienes in two-step Diels- Alder reactions

Article abstract of DOI:10.1002/(SICI)1099-0690(199912)1999:12<3343::AID-EJOC3343>3.0.CO;2-S

Oxidation with barium manganate of the enol of tetraacetylethane (10) affords tetraacetylethylene (7a) in good yield. Treatment of the 1,3- diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling y

Full text of DOI:10.1002/(SICI)1099-0690(199912)1999:12<3343::AID-EJOC3343>3.0.CO;2-S

FULL PAPER
Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step
Diels–Alder Reactions
Helmut Quast,*[a] Maximilian Seefelder,[a] Svetlana Ivanova,[a] Markus Heubes,[a]
Eva-Maria Peters,[b] and Karl Peters[b]
Keywords: C–C coupling / Cycloadditions / Dehydrogenations / Electron deficiency / Isomerisations / Oxidations /
Zwitterionic intermediates
Oxidation with barium manganate of the enol of
tetraacetylethane (10) affords tetraacetylethylene (7a) in
good yield. Treatment of the 1,3-diketones 15a and b with
iodosobenzene in the presence of boron trifluoride does not
result in oxidative coupling yielding 10 and diacetyl-
dibenzoylethane 17, respectively, as has been reported by
Moriarty et al. Instead the known difluoroborylenolates 16
are formed. Oxidative coupling of the sodium enolate of 15b
dicyanobenzoquinone of meso-17 yields (E)-7b and an
isomer (77:23) which was assigned structure (Z)-7b on the
basis of spectroscopic evidence. – Tetraacetylethylene (7a)
reacts with 1,2-dimethylenecyclohexane to afford two
different [4+2]cycloadducts, viz. the spirocyclic dihydropyran
25 (30 %) and the tetraacetyloctalin 27. Whereas the latter is
stable, the former isomerises to the latter on heating.
Formation of both [4+2]cycloadducts and the isomerisation
with iodine affords the meso-tetraketone meso-17 and 25 Ǟ 27 are interpreted by invoking the zwitterion 26 as
diacetyldibenzoylethylene (E)-7b besides small amounts of
rac-17 and the bisenol 19. In the presence of 1,4-
diazabicyclo[2.2.2]octane, meso-17 and 19 tautomerise
yielding identical equilibrium mixtures of meso-17, rac-17,
and 19 (26:29:45). Dehydrogenation with 5,6-dichloro-2,3-
common intermediate. Monoolefines that cannot generate
stable carbenium ions don’t give [4+2]cycloadducts with 7a.
In contrast, α-methylstyrene yields the oxa Diels–Alder
product 30. The structures of (E)-7b, 19, 25, and 27 are
established by X-ray diffraction analyses.
the final goal could not be reached, the results of the pre-
sent study merit interest in their own right. We describe here
an improved, convenient preparation of tetraacetylethylene
and the first synthesis of diacetyldibenzoylethylene (7b).
Conflicting reports on the long-known oxidative coupling
of benzoylacetone (15b) are in part clarified. Eventually, 7a
turned out to be a mechanistic chameleon which gave two
different DielsϪAlder products with 1,2-dimethylenecyclo-
hexane by reacting as an electron-deficient dienophile and
as an 1-oxa-1,3-diene as well, probably via a zwitterionic in-
termediate.
Introduction
Tetracyanoethylene (1) is the archetype of strongly elec-
tron-deficient ethylenes and undergoes a host of cycload-
dition reactions.[1] Other electron-deficient ethylenes, e.g.
ethylene tetracarboxylates (4), are less prone to cycload-
ditions,[2] in part because steric strain is developed during
these reactions. Much less is known about tetraacylethyl-
enes. In fact only two compunds of this type have been de-
scribed. Tetraacetylethylene (7a) has been synthesised by
Adembri, Scotton et al.[3][4] Tetrabenzoylethylene has been
obtained in several ways from dibenzoylmethane[5] or by
oxidative cleavage of 3,4-dibenzoyl-2,5-diphenylfuran with
cerium(IV) ammonium nitrate.[6] We became interested in
[4ϩ2] cycloaddition reactions of diacetyldiacylethylenes 7,
because the intramolecular aldol reaction of the cycload-
Results and Discussion
The simple synthesis of tetraacetylethylene from the read-
ducts 8 might provide a novel access to functionalised ily available enol of tetraacetylethane (10)[11] by oxidation
[4.3.3]propellanes 9.[7] This approach appeared attractive of the thallium enolate 12 with iodine[3][4] has been repeated
since the analogous [4ϩ2]cycloadducts 2[8] and 5[9] served several times[12][13] although the reaction is rendered very
as precursors to the hetero[4.3.3]propellanes 3 and 6, inconvenient by the formation of large amounts of thallium
respectively, and many examples of intramolecular aldol re- iodide waste arising as a very fine powder that is difficult
actions that yield cyclopentenones are known.[10] Although to remove from the product and the equipment, and, of
course, to dispose of. Attempts to avoid this problem by a
[a]
Institut für Organische Chemie, Universität Würzburg
Am Hubland, D-97074 Würzburg, Germany
Fax: (internat.) ϩ 49-(0)931/888-4606
synthesis from bromoacetylacetone (13b) patterned upon
the preparation of ethylene tetracarboxylates from bromo-
malonates,[14] have met with failure.[3] Oxidation, by iodine,
of 10, previously treated with sodium ethoxide, was equally
unsuccessful.[3] Only recently, a second method has been
reported, i.e. the cerium-mediated electrochemical oxidation
E-mail: seefelde@chemie.uni-wuerzburg.de
Max-Planck-Institut für Festkörperforschung,
Heisenbergstraße 1, D-70569 Stuttgart, Germany
Fax: (internat.) ϩ 49-(0)711/689-1599
[b]
E-mail: karpet@vsibm1.mpi.stuttgart.mpg.de
Eur. J. Org. Chem. 1999, 3343Ϫ3351
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/1212Ϫ3343 $ 17.50ϩ.50/0
3343

H. Quast, M. Seefelder, S. Ivanova, M. Heubes, E.-M. Peters, K. Peters
FULL PAPER
of acetylacetone,[15] which, however, requires special equip-
ment. The difficulties encountered in the chemical oxidation
of 10 can be traced back to the sensitivity towards base of the enol derived from benzoylacetone (15b), carried out for
both the starting material 10 and the product 7a. The for- the first time by Fischer and Bülow more than a century
mer undergoes cyclisation to yield 3,4-diacetyl-2,5-dimeth- ago.[27] Coupling has been performed by oxidation of the
ylfuran (11) in the presence of lithium hydroxide, sodium sodium enolate with iodine[27Ϫ30] or anodic oxidation of the
ethoxide, or potassium carbonate,[16] the latter forms the potassium enolate,[31] anodic oxidation of 15b,[32], and oxi-
base-induced cyclic aldol adduct 14[4] and complex prod- dation of 15b with dibenzoylperoxide[33] or iodosobenzene
ucts derived thereof.[17]
in the presence of boron trifluoride.[34] The coupling prod-
Oxidative cleavage with cerium(IV) ammonium nitrate of uct was also obtained by the reaction of the sodium enolate
a highly substituted furan to yield tetrabenzoylethylene has of 15b with 2-bromo-1-phenyl-1,3-butanedione.[35] An un-
been reported by Lepage and Lepage.[6] This reagent con- usual number of different melting points have been reported
verted the furan 11 into a mixture of four products (HPLC) in the literature for the coupling product of 15b. The highest
from which 7a could be isolated in 13% yield by recrystallis- melting point (197.5Ϫ199.5°C) belongs to the tetraketone
ation. Attempts at the oxidative cleavage of 11 with 3- meso-17 whose structure has been established unequivocally
chloroperbenzoic acid or pyridinium chlorochromate[18] by an X-ray diffraction analysis.[29] The lower melting
furnished only small amounts of 7a after extended reac- points are apparently indicative of different structures.
tion periods.
A much lower melting point (155°C) was reported by
A number of reagents were tried for the direct oxidation Moriarty and coworkers for a product, claimed to be 17
of 10 to 7a. Manganese dioxide,[19] lead tetraacetate,[20] ru- (without assignment of configuration), which they had ob-
thenium tetroxide,[21] nickel peroxide,[22] potassium fer- tained from benzoylacetone by a novel method of oxidative
rate,[23]
and
5,6-dichloro-2,3-dicyanobenzoquinone[24] coupling involving treatment with iodosobenzene in the
failed to produce more than tiny amounts of 7a. By con- presence of boron trifluorideϪdiethyl ether.[34] Unfortu-
trast, promising results were obtained with a suspension of nately, their alleged coupling product turned out to be the
barium manganate in dichloromethane as solvent.[25] Moni- long-known product 16b of benzoylacetone and boron tri-
toring of the conversion with HPLC revealed the initial rise fluoride (m.p. 157°C).[36] This became now evident by a
of 7a, which slowly disappeared with the formation of the comparison of the compound that was obtained according
cyclic aldol adduct 14. This problem was avoided by the to MoriartyЈs procedure with an authentic sample of 16b.
addition of small amounts of acetic acid, which served to Likewise, acetylacetone did not undergo oxidative coupling
neutralise basic impurities in the barium manganate. Ultra- with iodosobenzene in the presence of boron trifluorideϪ
sonic irradiation of the suspension considerably accelerated diethyl ether to afford 10 as claimed[34] but yielded the
the oxidation. Thus, a simple and efficient protocol avoid- known difluoroboryloxypentenone 16a.[36]
´
ing toxic waste was developed for the direct conversion of
10 into 7a, which could be obtained in 69% yield after tetraketone presumed to be the meso diastereomer meso-17
recrystallisation from ethyl acetate. and of 144Ϫ146°C for golden-yellow needles considered as
Lacan et al. reported melting points of 178Ϫ180°C for a
Whereas the coupling product of acetylacetone (15a) ex- bis-enol 19. The structures were assigned on the basis of
clusively exists as bisenol 10 in the solid state,[26] conflicting proton, IR, and UV/Vis spectra.[31] Oxidation with iodine
reports have been published on the oxidative coupling of of the sodium enolate of benzoylacetone suspended in di-
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Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step DielsϪAlder Reactions
FULL PAPER
Figure 1. Stereographic projection of the bis-enol 19 with the num-
bering of the atoms
ethyl ether according to Fischer and Bülow[27][29] gave a
precipitate in modest yield (21%) which contained meso-17
and, surprisingly, (E)-7b (7:3 to 8:2, 1H NMR). Recrystallis-
ation from ethyl acetate furnished pure meso-17 (m.p.
196Ϫ198°C, 11%), which does not react with iron(III) chlo-
ride and does not undergo H/D exchange on treatment with
deuterium oxide.[29] Surprisingly, repeated recrystallisation
of the crude product from ethanol, as described by Fischer
and Bülow,[27] did not remove but enriched (E)-7b to yield
a colourless powder with a melting range 168Ϫ180°C that
consisted of meso-17 and (E)-7b in the ratio 3:2.
be characterised spectroscopically in this way. Its proton
and 13C spectra showed that the molecule Ϫ like meso-17 Ϫ
consisted of two structurally identical halves which closely
resembled those of meso-17. Therefore, the third tautomer
was assigned the structure of the diastereomer rac-17. Sur-
prisingly, the half-enolised tautomer 18, which must be an
intermediate in the equilibration, could not be detected by
high-field proton or 13C spectroscopy. The nature of the
various compounds that melt more or less sharply in the
temperature range between 170 and 180°C[27,28,31,33,35] re-
mains enigmatic.
Slow evaporation of the solvent from the reaction mix-
ture after removal of the solid material yielded a brown,
viscous, oily precipitate containing predominantly ben-
zoylacetone and rac-17 (3:2) besides small amounts of (E)-
7b, meso-17 and several unidentified minor products (1H
NMR, HPLC). A small amount (1%) of pale yellow, coarse
crystals formed slowly, which melted at 170Ϫ175°C. The
NMR spectra of this minor by-product indicated a struc-
ture of twofold symmetry and the characteristics of a bis-
enol. It gave an intense violet colour with iron(III) chloride.
The bis-enol structure 19 was confirmed by an X-ray dif-
fraction analysis (Figure 1). Both halves of 19 are substan-
tially planar and strongly twisted around the central car-
bonϪcarbon bond, and may be superimposed by the oper-
ation of a glide reflection plane. In the solid state, bis-enol
19 closely resembles bis-enol 10 which has been studied by
X-ray crystallography and neutron diffraction.[26]
In the absence of base, solutions of both tautomers meso-
17 and 19 in inert solvents are stable for hours. The avail-
ability and unequivocal characterisation of meso-17 and 19
offered the opportunity to investigate the base-catalysed
equilibration by approach from both sides. Toward this end,
Figure 2. Proton spectra recorded for solutions of meso-17 and 19
in [D]trichloromethane in the presence of 1,4-diazabicyclo[2.2.2]oc-
tane; the signals of the phenyl groups and of the hydroxy protons
of 19 are not shown
A number of reagents were tried for the oxidation of
high-field proton and 13C spectra were recorded for solu- tetraketone meso-17 to the tetraacylethylene 7b. Manganese
tions of meso-17 and 19 in [D]trichloromethane in the pres- dioxide and barium manganate did not react. Bromination
ence of 1,4-diazabicyclo[2.2.2]octane (Figure 2). From both with N-bromosuccinimide followed by dehydrobromination
tautomers, identical mixtures were obtained consisting of was equally unsuccessful. Eventually, oxidation was
meso-17, 19, and a third tautomer (26:45:29), which could achieved by treatment of meso-17 with 5,6-dichloro-2,3-di-
Eur. J. Org. Chem. 1999, 3343Ϫ3351
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H. Quast, M. Seefelder, S. Ivanova, M. Heubes, E.-M. Peters, K. Peters
FULL PAPER
cyanobenzoquinone for extended periods of time at elev- enes, viz. 1,3-butadiene itself, anthracene, hexachlorocyclo-
ated temperatures.[24c] Flash chromatography separated the pentadiene, and diphenylfulvene, reacted only slowly or not
resulting mixture yielding two crystalline diastereomers. at all. On the other hand, ethyl vinyl ether afforded the
The major product, obtained in 68% yield, was assigned expected dihydropyran 21.[39] Cyclopentadiene also yielded
structure (E)-7b on the basis of spectroscopic evidence. Un- a dihydropyran (cis-22). This result was interpreted in terms
equivocal proof was provided by an X-ray diffraction analy- of unfavourable interactions between the acetyl groups that
sis (Figure 3). Like tetraacetylethylene (7a),[12] the molecule would render the hypothetical alternative DielsϪAlder ad-
adopts a centrosymmetric conformation in the solid state duct tetraacetylnorbornadiene unstable.[40] Under a variety
in which steric and dipole-dipole repulsion between the car- of reaction conditions, cis-22 undergoes rearrangements to
bonyl groups are minimised. The 3-pentene-2,5-dione moi- trans-22 and the tricyclic isomer 24, presumably via the
ety is planar while the benzoyl groups are strongly twisted zwitterionic intermediate 23.[40][41]
from the central plane.
Figure 3. Stereographic projection of the tetraacylethylene (E)-7b
with the numbering of the atoms
The minor diastereomer [(Z)-7b] was isolated in 20% yield.
Its proton and 13C spectra closely resemble those of (E)-7b
and attest to the equivalence of the two acetyl and benzoyl
groups. Surprisingly however, the two carbon atoms of the
double bond give two different signals. We explain this
puzzling feature in terms of one or more asymmetrical
structures with the double bond in the (Z) configuration
and different conformations for the acetyl and benzoyl
groups, whose apparent equivalence is hence a matter of the
We employed the readily available 1,2-dimethylenecyclo-
NMR time scales. Rotation around the bond between a sin-
hexane[42] because of its permanently cisoid geometry.[43]
gle carbonyl group and a central carbon atom is probably
The reaction with tetraacetylethylene (7a) in acetonitrile
solution at 65°C was monitored by HPLC and was com-
plete in two days. Two cycloadducts were obtained as
colourless crystals, m.p. 125Ϫ127°C (50%) and m.p.
fast but exchange of the two central carbon atoms requires
geared rotation of all four groups, a process which appar-
ently occurs only slowly.
104Ϫ106°C (30%), after separation by flash chromatogra-
phy, besides small amounts (18%) of the known intramol-
ecular aldol adduct 14.[4] The structures 27 for the higher
melting cycloadduct and 25 for the lower melting one were
based on spectroscopic evidence and eventually established
by X-ray diffraction analyses (Figures 4 and 5).
The DielsϪAlder chemistry of tetraacetylethylene is far
The formation of the two cycloadducts demonstrates that
from being completely understood. Obviously, it is strongly 7a, with the same reagent and under the same conditions,
electron-deficient and hence may be anticipated to react as formally reacts as both, dienophile and hetero diene. The
1-oxa-1,3-butadiene with electron-rich alkenes in hetero Di- clue to a rationalisation of this mechanistic bifurcation was
elsϪAlder reactions with inverse electron demand[37] or as provided by thermal equilibration experiments which were
dienophile in normal DielsϪAlder reactions. The only ex- monitored by HPLC and proton spectroscopy. The octalin
ample of the latter is provided by the reaction with 2,3- 27 survived heating in [D3]acetonitrile solution for two days
dimethyl-1,3-butadiene yielding 20.[3][38] Other common di- without any change. In contrast, the spirocyclic dihydropy-
3346
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Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step DielsϪAlder Reactions
FULL PAPER
certed, however, because a concerted mechanism would
have to involve the highly strained, tricyclic boat transition
state 28. Therefore, we assume that the reaction occurs in
two steps via the zwitterionic intermediate 26. Zwitterions
of this type offer themselves as the crucial intermediates for
an explanation of the ambiguity in the DielsϪAlder reac-
tion between 7a and 1,2-dimethylenecyclohexane, and also
for the reactivity of 7a towards other dienes[3][40] and dieno-
philes.[39] Similar zwitterions have been invoked as inter-
mediates in DielsϪAlder reactions of substituted 1,3-buta-
dienes with other highly electron-deficient dienophiles, viz.
tetracyanoethylene[44] and 4-phenyl-1,2,4-triazoline-3,5-di-
one,[45] and in DielsϪAlder reactions with inverse electron
demand between 1,3-dimethyl-2-alkylidenimidazolines and
electron-deficient dienes.[46]
Figure 4. Stereographic projection of the spirocyclic dihydropyran
25 with the numbering of the atoms
We performed a few experiments, the results of which
support the mechanistic hypothesis. Obviously, the reactiv-
ity of alkenes towards 7a should be related with stability
of the cationic moiety of the zwitterions. Accordingly, the
potential dienophiles methylenecyclohexane and norbor-
nene did not react with 7a when the components were
heated in [D3]acetonitrile solution for several days at 85°C.
Only slow, undefined decomposition was observed by pro-
ton spectroscopy. In contrast, α-methylstyrene, which
should give rise to the formation of a tertiary benzyl car-
benium ion as the cationic moiety of the intermediate zwit-
terion, smoothly afforded the hetero DielsϪAlder adduct
30.
Figure 5. Stereographic projection of the tetraacetyloctalin 27 with
the numbering of the atoms
Whereas a host of examples exist for intramolecular aldol
reactions yielding cyclopentenones,[10] and a compound
similar to 27, viz. 1,1,2,2-tetraacetylcyclopropane, could be
cyclised in this way,[47] numerous attempts to achieve an
intramolecular aldol reaction with 27 under a variety of
conditions met with failure. Only a dark brown oil, which
consisted of many unidentified products, resulted.
Experimental Section
General Remarks: 1H NMR: Table 1. Ϫ 13C NMR Table 2. Ϫ Mo-
lecular formulae and masses, and elemental analyses: Table 3. Ϫ
Melting points: Kofler apparatus from Reichert, Vienna, Austria.
1
Ϫ IR: PerkinϪElmer 1420. Ϫ H and 13C NMR: Bruker AC250.
The signals of 22 were assigned by means of 1H,1H- and 13C,1H-
COSY experiments. Ϫ MS (70 eV): Finnigan MAT 8200. The exact
mass of 30 was determined by means of a Finnigan MAT 90 high-
resolution mass spectrometer and perfluorokerosene calibration. Ϫ
Flash chromatography: UV detector Knauer 87.00 (λ ϭ 254 nm)
(40 ϫ 4)cm or (35 ϫ 2.5)cm glass column packed with silica gel
32Ϫ63 µm (ICN Biomedicals), petroleum ether (boiling range
ran 25 quantitatively rearranged to 27 under these con-
ditions. The retro DielsϪAlder products 1,2-dimethyl-
enecyclohexane and 7a were not observed. This isomeri-
sation represents a [3,3]sigmatropic rearrangement, more
precisely, an oxa Cope rearrangement. It cannot be con- 50Ϫ70°C) (PE)/ethyl acetate (EA), 1.8 bar N2. Ϫ HPLC: Waters
Eur. J. Org. Chem. 1999, 3343Ϫ3351 3347

H. Quast, M. Seefelder, S. Ivanova, M. Heubes, E.-M. Peters, K. Peters
FULL PAPER
M-6000A equipped with UV detector 440 (λ ϭ 254 nm) and
differential refractometer R401, (250 ϫ 4.6)mm stainless steel col-
trohr column (Fischer, D-53340 Meckenheim)]; 1H NMR: ref.[42]
13C NMR (CDCl3): δ ϭ 26.8 (C-4, C-5), 35.3 (C-3, C-6), 107.8,
;
umn packed with silica gel LiChrosorb Si60, 5 µm (Knauer); 149.7 (H2CϭC); IR (neat liquid): ν˜ ϭ 1645 cmϪ1 (CϭC). Ϫ Iodo-
1.5 mL/min PE/EA (50:50), retention time (tR) [min] ϭ 3.0 (10),
5.0 (7a), 6.2 (27), 6.8 (1,2-dimethylenecyclohexane), 7.5 (14);
1.5 mL/min PE/EA (70:30) tR ϭ 3.6 (19), 5.0 (meso-17), 5.2 [(E)-
7b and 10], 5.4 (16b), 6.0 [(Z)-7b], 11.2 (11), 12.3 (7a). Ϫ Solvents
were distilled under purified N2 or Ar: Diethyl ether from NaH,
toluene and dimethyl sulphoxide (15 Torr) from CaH2, PE, di-
chloromethane, EA, and acetonitrile from P2O5. Dry acetonitrile
sobenzene.[48] Ϫ Barium manganate.[25]
3,4-Diacetylhex-3-ene-2,5-dione (7a): Barium manganate (20.1 g,
80 mmol) was added to a solution of 10 (3.96 g, 20 mmol) and
acetic acid (1.20 g, 20 mmol) in dichloromethane (250 mL). The
suspension was heated under ultrasonic irradiation [Sonorex
RK102 (120 W, 50 kHz), Bandelin Electronic, Berlin] and reflux for
6 h. The solid material was removed with the help of a centrifuge
to give a clear blue solution. Activated carbon (7 g) was added, and
the suspension was stirred until the solution turned pale yellow (ca.
1 h). The solid was removed by filtration. Distillation of the solvent
i. vac. and recrystallisation from EA at Ϫ20°C afforded pale yellow
needles (2.70 g, 69%), m.p. 139Ϫ140°C (ref.[3][4]: 139Ϫ140°C). Ϫ
MS: m/z (%) ϭ 196 (3) [Mϩ], 154 (13), 127 (9), 112 (26), 67 (15),
43 (100). Ϫ IR (KBr): ν˜ ϭ 1695, 1685 (CϭO), 1645 cmϪ1 (CϭC).
˚
was stored over molecular sieves (3 A). PE and EA were eventually
distilled through a 2.5-m column with stainless steel helices. Ϫ
Known starting materials, reference compounds, and reagents were
prepared according to literature procedures: 10:[11] colourless crys-
tals, m.p. 193Ϫ194°C (after repeated sublimation at 80Ϫ90°C/10Ϫ2
Torr). Ϫ 11:[23b] colourless crystals, m.p. 62Ϫ63°C (from water);
1H NMR: ref.[16c]; IR (KBr): ν˜ ϭ 1670 (CϭO), 1602 cmϪ1 (CϭC).
Ϫ 16a:[36] colourless crystals, m.p. 42Ϫ43°C (from diethyl ether);
1H NMR: ref.[36]; IR (KBr): ν˜ ϭ 1550 (CϭO), 1390 (BϪO),
1170Ϫ1020 cmϪ1 (BϪO and BϪF); MS: m/z (%) ϭ 148 (14) [Mϩ],
133 (26) [Mϩ Ϫ CH3], 129 (14) [Mϩ Ϫ F], 128 (5), 43 (100). Ϫ
Oxidative Coupling of the Sodium Salt of 15b: a) A suspension of
the sodium salt of 15b (10.1 g, 58.5 mmol) in diethyl ether (200 mL)
was heated under reflux for 2 h beneath iodine (7.62 g, 30 mmol),
which was placed into a Soxhlet thimble, so that the iodine was
leached gradually into the reaction mixture.[29] After cooling, the
precipitate was isolated by filtration and suspended in water
(100 mL). The suspension was stirred for 0.5 h. The solid material
was isolated by filtration and dried to afford a pale yellow powder
(1.15 g) consisting of (E)-7b and meso-17 (2:5, 1H NMR). Repeated
recrystallisation from ethanol afforded a colourless powder (0.45 g,
m. range 168Ϫ180°C), consisting of (E)-7b and meso-17 (2:3). Ϫ
Distillation of the solvent from the diethyl ether mother liquor gave
a dark brown oil, consisting of 15b and rac-17 (3:2), besides small
amounts of (E)-7b, meso-17, and several unidentified products.
1
16b:[36] colourless crystals, m.p. 156Ϫ158°C (from EA); H NMR:
ref.[36]; IR (KBr): ν˜ ϭ 1550 (CϭO), 1370 (BϪO), 1170Ϫ1020 cmϪ1
(BϪO and BϪF); MS: m/z (%) ϭ 210 (66) [Mϩ], 209 (86) [Mϩ
Ϫ
H], 195 (24) [Mϩ Ϫ CH3], 191 (13) [Mϩ Ϫ F], 105 (100), 77 (29),
51 (12), 43 (40). Ϫ 1,2-Dimethylenecyclohexane:[42] colourless
liquid, b.p. 70Ϫ72°C/135 Torr [distillation through a 30-cm Spal-
Table 1. Chemical shifts (δ values) in proton spectra recorded for
solutions in [D]trichloromethane
b) The pale yellow precipitate obtained according to a) was dis-
solved in a mixture of EA and water (1:1, 200 mL). The org. layer
was separated and dried with MgSO4. Distillation of the solvent i.
vac. yielded a brown solid residue (1.89 g, 21%) consisting of (E)-
1
7b and meso-17 (1:3, H NMR). Recrystallisation from EA at 4°C
afforded pale yellow crystals of meso-17 (1.04 g, 11%), m.p.
196Ϫ198°C (ref.[29]: 197.5Ϫ199.5°C). Ϫ IR (KBr): ν˜ ϭ 1710, 1665
Table 2. Chemical shifts (δ values) in 13C spectra recorded for solutions in [D]trichloromethane
3348
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Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step DielsϪAlder Reactions
cmϪ1 (CϭO). Ϫ MS: m/z (%) ϭ 322 (0.2) [Mϩ], 105 (100), 77 (37), d in a closed 100-mL Schlenk tube. The solvent was distilled i. vac.
FULL PAPER
51 (7), 43 (14). Ϫ Slow evaporation of the solvent from the diethyl
ether mother liquor obtained according to a) afforded small
amounts of pale yellow, coarse crystals and a brown oily residue.
Recrystallisation from diethyl ether afforded pale yellow crystals of
19 (0.09 g, 1%), m.p. 170Ϫ175°C. Ϫ MS: m/z (%) ϭ 322 (3) [Mϩ],
304 (44) [Mϩ Ϫ H2O], 303 (52), 105 (100), 77 (54), 51 (8), 43 (28).
to afford a dark brown oil. Flash chromatography of the residue
with PE/EA (70:30) gave three fractions:
3,3,4,4-Tetraacetylbicyclo[4.4.0]dec-1(6)-ene (27) as pale yellow
solid. Recrystallisation from PE afforded colourless crystals (0.77 g,
1
51%), m.p. 125Ϫ127°C. Ϫ H NMR (CDCl3): δ ϭ 1.55Ϫ1.65 (m,
8-H2, 9-H2), 1.85Ϫ1.95 (m, 7-H2, 10-H2), 2.25 (4 CH3), 2.39Ϫ2.47
(m, 2-H2, 5-H2). Ϫ MS: m/z (%) ϭ 304 (0.2) [Mϩ], 261 (32) [Mϩ
Ϫ CH3CO], 243 (33), 177 (15), 159 (19), 91 (11), 43 (100). Ϫ IR
(KBr): ν˜ ϭ 1710, 1690 (CϭO), 1655 cmϪ1 (CϭC).
3,4-Bis(hydroxyphenylmethylene)hexane-2,5-dione (19): Pure meso-
17 (0.54 g, 1.7 mmol) was added to a stirred solution of sodium
hydroxide (0.13 g, 3.4 mmol) in methanol (10 mL).[31] Stirring was
continued for 20 min, and the yellow solution was poured into ice-
Ϫwater (100 mL). The pH value was adjusted to 4 with aq. HCl (2
). The precipitate was isolated by filtration and dried i. vac. to
afford a colourless powder [0.47 g, 87%, purity > 95% (1H NMR)],
m. range 146Ϫ156°C. Repeated recrystallisation from ethanol af-
forded a colourless powder, m. range 150Ϫ158°C, which was pure
according to 1H and 13C NMR (ref.[31]: golden-yellow needles, m.p.
144Ϫ146°C, dec., from benzene).
4Ј,4Ј,5Ј-Triacetyl-6Ј-methyl-2-methylenespiro[cyclohexan-1,2Ј-
(3Ј,4Ј-dihydro-2H-pyran)] (25) as yellow oil, which was dissolved in
a small amount of EA. Addition of the same volume of pentane
and crystallisation from this mixture at Ϫ20°C afforded colourless
crystals (0.45 g, 30%), m.p. 104Ϫ106°C. Ϫ 1H NMR (CDCl3): δ ϭ
2
1.58Ϫ1.82 (m, 4-H2, 5-H2, 6-H2), 1.72, 3.08 (2 d, J ϭ 13.5 Hz, 3Ј-
H2), 2.00 (6Ј-CH3), 2.08Ϫ2.30 (m, 3-H2), 2.15 (CH3CO), 2.47 (2
CH3CO), 4.55, 4.75 (H2CϭC). Ϫ MS: m/z (%) ϭ 304 (0.8), 261
(0.5), 154 (22), 112 (36) 69 (7), 43 (100). Ϫ IR (KBr): ν˜ ϭ 1740,
1710, 1695 (CϭO), 1645 cmϪ1 (CϭC).
4,4,5,5-Tetraacetyl-1,2-dimethylcyclohex-1-ene (20): A solution of
7a (3.00 g, 15.1 mmol) and 2,3-dimethyl-1,3-butadiene (1.72 g,
20.4 mmol) in acetonitrile (20 mL) was stirred at 65°C for 2 d in a
closed 100-mL Schlenk tube. The solution was kept overnight at
Ϫ4°C to yield pale yellow crystals. Recrystallisation from PE af-
2,3-Diacetyl-4-hydroxy-4-methylcyclopent-2-en-1-on (14) as pale
yellow oil (0.17 g, 18%), which crystallised slowly at 4°C. Recrys-
tallisation from PE afforded colourless crystals, m.p. 52Ϫ54°C
forded colourless crystals (1.20 g, 28%), m.p. 106Ϫ108°C (ref.[3]
:
106Ϫ108°C). Ϫ 1H NMR: ref.[3] Ϫ IR (KBr): ν˜ ϭ 1700 (CϭO),
1650 cmϪ1 (CϭC). Ϫ MS: m/z (%) ϭ 278 (0.1) [Mϩ], 235 (5) [Mϩ
Ϫ CH3CO], 175 (10), 154 (17), 112 (30), 43 (100).
1
(ref.[4]: 53Ϫ55°C). Ϫ H NMR (CDCl3): ref.[4]
Rearrangement of 25 to 27: A solution of 25 (30 mg, 0.1 mmol) in
3,4-Dibenzoyl-3-hexene-2,5-dione [(E)- and (Z)-7b]: A solution of
[D3]acetonitrile (0.7 mL), contained in an evacuated, flame-sealed
meso-17 (0.50 g, 1.6 mmol) and 5,6-dichloro-2,3-dicyanobenzoqui- NMR sample tube, was heated at 85Ϫ90°C. The conversion was
1
none (0.6 g, 2.6 mmol) in toluene (50 mL) was heated at 90°C for monitored by H-NMR spectroscopy, until 25 had disappeared (6
2 d. The precipitate was isolated by filtration. The solvent was dis-
d). Thereupon the proton spectra indicated the presence of only
tilled i. vac. to afford a yellow solid. Flash chromatography with 27. Signals of the retro DielsϪAlder products 1,2-dimethyl-
PE/EA (70:30) gave as first fraction a yellow solid [(E)-7b, 0.35 g,
68%]. Recrystallisation from EA afforded rodlike yellow crystals,
m.p. 139Ϫ141°C. Ϫ MS: m/z (%) ϭ 320 (3) [Mϩ], 278 (7), 236 (9),
227 (8), 105 (100), 77 (37), 51 (9), 43 (12). Ϫ IR (KBr): ν˜ ϭ 1690,
1665 cmϪ1 (CϭO).
enecyclohexane and 7a could not be detected at any time.
4,4,5-Triacetyl-2,6-dimethyl-2-phenyl-3,4-dihydro-2H-pyran (30): a)
A solution of α-methylstyrene (177 mg, 1.5 mmol) and 7a (59 mg,
0.3 mmol) in [D3]acetonitrile (0.7 mL) was heated at 80°C. The
1
conversion was monitored by H-NMR spectroscopy, until 7a had
The second fraction yielded pale brown crystals [(Z)-7b, 0.10 g,
20%]. Recrystallisation from EA/hexane (1:1) afforded colourless
crystals, m.p. 101Ϫ102°C. Ϫ MS: m/z (%) ϭ 320 (2) [Mϩ], 278 (9),
236 (1), 227 (4), 105 (100), 77 (37), 51 (8), 43 (12). Ϫ IR (KBr):
ν˜ ϭ 1670, 1650 (CϭO), 1635 cmϪ1 (sh., CϭC).
disappeared (4 d). Thereupon the proton spectrum indicated the
presence of 30 besides small amounts of unidentified products.
b) A solution of of α-methylstyrene (1.77 g, 15 mmol) and 7a
(0.59 g, 3 mmol) in dry acetonitrile (10 mL) was heated under re-
flux for 4 d. Flash chromatography of the reaction mixture with
PE/EA (1:1) and distillation of the solvent i. vac. yielded a dark
yellow oil (0.63 g, 67%), which was purified by repeated flash chro-
matography with PE/EA (7:3) to afford a pale yellow oil (0.37 g,
39%). Ϫ 1H NMR (CDCl3): δ ϭ 1.51, 1.54, 2.21, 2.46, 2.52 (5
Table 3. Molecular formulae and masses, and elemental analyses
2
CH3), 2.41 (d, 1 H), 2.74 (d, 1 H, J ϭ 14.1 Hz), 7.1Ϫ7.4 (Ph). Ϫ
MS: m/z (%) ϭ 314 (1) [Mϩ], 255 (6), 254 (12), 169 (6), 121 (13),
105 (16).
X-ray Diffraction Analyses were performed on transparent, yellow
[(E)-7b], pale yellow (19), and colourless crystals (25, 27). The cell
parameters were determined on the basis of 70 reflections. The
number of reflections reported in Table 4 were obtained with Mo-
Kα radiation and 2 Θmax ϭ 55° (graphite monochromator, ω-scan).
Measurements were carried out with a system Siemens P4. The
programme SHELXTL PLUS[49] was employed. The structure was
solved by direct methods and refined anisotropically by the least-
squares method. The weighting scheme for Rw is 1/σ2. The posi-
tions of the hydrogen atoms were calculated by the riding model
and included with isotropic descriptions.[50]
[4؉2]Cycloadditions of 7a and 1,2-Dimethylenecyclohexane: A solu-
tion of 7a (0.98 g, 5.0 mmol) and 1,2-dimethylenecyclohexane
(1.08 g, 10 mmol) in acetonitrile (14 mL) was heated at 65°C for 2
Eur. J. Org. Chem. 1999, 3343Ϫ3351
3349

H. Quast, M. Seefelder, S. Ivanova, M. Heubes, E.-M. Peters, K. Peters
Table 4. Experimental details and results of the X-ray diffraction analyses
FULL PAPER
[a]
[b]
Maximum and minimum of the remaining electron density in the final differential Fourier synthesis.
[10b]
Ϫ For more recent examples see:
D. Pauley, F. Anderson,
[10c]
Acknowledgments
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H.
We thank Dr. G. Lange and Mr. F. Dadrich for recording the mass
spectra. Financial support of this work by the Fonds der Chemi-
schen Industrie, Frankfurt am Main, is gratefully acknowlegded.
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3350
Eur. J. Org. Chem. 1999, 3343Ϫ3351

Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step DielsϪAlder Reactions
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Crystallographic data (excluding structure factors) for the struc-
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Received June 21, 1999
[O99368]
Eur. J. Org. Chem. 1999, 3343Ϫ3351
3351