A fused benzocyclooctene ring system via an aromatic cope rearrangement: Thermal reactions of trans-1-aryl-2-ethenylcyclobutanecarbonitriles

Article abstract of DOI:10.1002/hlca.201200352

The results of an aromatic Cope rearrangement of a trans-1-aryl-2- ethenylcyclobutanecarbonitrile are reported (Scheme). The use of this rearrangement for the construction of the fused benzocyclooctene ring system and a preliminary study of the electronic requirements to favor such a transformation are also described. Copyright

Full text of DOI:10.1002/hlca.201200352

Helvetica Chimica Acta – Vol. 96 (2013)
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A Fused Benzocyclooctene Ring System via an Aromatic Cope
Rearrangement: Thermal Reactions of trans-1-Aryl-2-
ethenylcyclobutanecarbonitriles
a
a
b
´
´
by Jose´ Gustavo Avila-Za´rraga* ), Adria´n Va´zquez-Sa´nchez ), and Luis Angel Maldonado )
a
´
´
) Departamento de Quꢀmica Organica, Facultad de Quꢀmica, Universidad Nacional Autonoma de
´
´
Mexico, 04510, Mexico
b
´
´
) Instituto de Quꢀmica, Universidad Nacional Autonoma de Mexico, Circuito Exterior, Ciudad
´
Universitaria, 04360, Mexico
The results of an aromatic Cope rearrangement of a trans-1-aryl-2-ethenylcyclobutanecarbonitrile
are reported (Scheme). The use of this rearrangement for the construction of the fused benzocyclooctene
ring system and a preliminary study of the electronic requirements to favor such a transformation are also
described.
Introduction. – It is well known that the Cope rearrangement is quite challenging to
achieve if one of the C¼C bonds of the hexa-1,5-diene system is part of an aromatic
ring. This is in contrast to the Claisen rearrangement, which undergoes the [3.3]-
sigmatropic rearrangement relatively easily [1]. However, there are two reports that
describe successful Cope transformations of this type [2]. For example, thermolysis of
4-phenylbut-1-ene does not lead to the Cope-rearrangement product [3][4], but on
application of very specific conditions, such transformations may be carried out [2].
Several factors limit the success of Cope rearrangements, and the strategy most
commonly used for promoting this transformation involves the insertion of a
cyclopropane ring at positions 3 and 4 of the hexa-1,5-diene moiety, leading to the
formation of a cycloheptane system [5]. This approach has been widely studied and is
commonly employed, with its success being due to the release of the cyclopropane ring
strain [6]. In a similar manner, the incorporation of a cyclobutane ring at the same
positions allows cyclooctane-ring formation, a process that was described by Vogel in
1958 [7]. These kinds of reactions are called 1,2-diethenylcyclobutane rearrangements
and have been studied by several research groups [8]. This particular sigmatropic
rearrangement may occur as undesired reactions [9] or can be implemented as a serious
strategy for the synthesis of natural products [10].
Another way to accelerate the [3.3]-sigmatropic rearrangement is the incorporation
of an O-atom into the diene moiety, enriching the electron density of the entire system.
This promotes what is known as ꢁoxy-Copeꢂ rearrangement, a name that was first
applied in 1964 by Berson [11]. Oxy-Cope rearrangements, in particular anionic oxy-
Cope rearrangements, demonstrate many advantages, including high yields, controlled
stereoselectivity, and tolerance to a wide range of functionalities, and consequently this
strategy has been studied and applied in many syntheses [12].
ꢃ 2013 Verlag Helvetica Chimica Acta AG, Zꢄrich

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The anionic oxy-Cope rearrangement, which relies on activation by an oxygen
anion, has also been used in systems that contain aromatic moieties [13][14]. However,
only one previous study has reported the incorporation of an O-atom into the aromatic
structure, activating the Cope rearrangement at distance through the benzene portion
of the hexa-1,5-diene moiety [2a], thereby making what could be termed an anionic
aromatic oxy-Cope rearrangement. The presence of other electron-donating groups at
the aromatic rings also favors a rate increase under appropriate conditions [15].
To the best of our knowledge, the extension of this methodology to the construction
of fused benzocyclooctene systems through Cope rearrangements of a 1-aryl-2-
ethenylcyclobutane precursor has not been reported, leaving some uncertainties about
its scope and generality.
As a continuation of our previous studies on aromatic Cope rearrangements in the
context of a cyclobutane ring at positions 3 and 4 of the hexa-1,5-diene moiety, we here
present the results of a study of trans-1-aryl-2-ethenylcyclobutanecarbonitriles with cis-
oriented aryl and ethenyl substituents, as precursors for systems of interest containing
the benzocyclooctene ring system [16].
Restults and Discussion. – An earlier first attempt by our group to synthesize the
benzocyclooctene natural product parvifoline by an aromatic oxy-Cope rearrangement
was unsuccessful [17], and so, in the work presented here, the conditions required to
achieve this kind of transformation were investigated.
For the construction of the starting trans-1-aryl-2-ethenylcyclobutanecarbonitrile 5,
the synthetic route shown in Scheme 1 was chosen, which involves an intramolecular
cyclization reaction of epoxynitrile 3. Thus, 3-methoxybenzeneacetonitrile (1) was
alkylated to generate 3-methoxy-a-(4-methylpent-4-en-1-yl)benzeneacetonitrile 2,
which was oxidized to give the corresponding epoxynitrile 3, see [18]. In a previous
report, we studied the cyclization reactions of a-aryl-d,e-epoxynitriles, which allowed
for the determination of the optimal conditions to modulate 4-exo and 5-endo processes
in a differentiated and stereoselective manner [18]. Following this strategy, a number of
modifications were made to the original protocol to assess the effect on the regio- and
stereoselectivity during the formation of the cyclobutanemethanol derivative (4).
Significant improvements were made with respect to the previous results, resulting in
higher yields of intermediate 4, achieved without any modification of the configuration
of the final product, which is essential for carrying out the subsequent sigmatropic shift
in such systems (Scheme 1). With adduct 4 in hand, an elimination reaction was
performed under very mild conditions, producing the Hofmann-type alkene derivative
5 in acceptable yields. Considering the structural complexity of 5, the synthesis of the
required 1-aryl-2-ethenylcyclobutane moiety exhibiting the diethenyl cis-configuration
was highly successful.
Several different conditions were tested for the subsequent Cope rearrangement, as
shown in Scheme 2. The presence of a MeO group in 5 as a potential activating group
prevented in fact the reaction from occurring, which was, however, expected because of
previous reports on such structures that revealed the importance of an oxy anion to
facilitate the rearrangement [2a]. Thus, the generation of the corresponding phenoxide
anion was investigated, and the sigmatropic rearrangement was studied in situ after
LiSEt deprotection of the MeO group. Unfortunately, during the initial experiments,

Helvetica Chimica Acta – Vol. 96 (2013)
1333
Scheme 1. Synthesis of trans-1-Aryl-2-ethenylcyclobutanecarbonitrile 5
a) 1. BuLi, THF, ꢀ 788; 2. 5-iodo-2-methylpent-2-ene; 80%. b) Acetone, Oxoneꢅ, NaHCO3, H2O; 95%.
c) BuLi, Cs2CO3, THF, reflux 64%. d) SOCl2, DABCO, benzene, 08; 49%.
high proportions of decomposition were observed. Therefore, derivative 6 was first
synthesized, followed by the ring expansion in a subsequent step. The results are shown
in the Table. The Cope rearrangement to 7 did not occur in any of the experiments in
which the solvent was THF, indicating that the energy barrier is high under these
conditions. It was deduced that the presence of the aromatic ring in the system
increased the thermal requirements for the reaction to proceed. On the other hand, the
experiments conducted in DMF demonstrated a successful rearrangement in a
regiospecific manner, with ring fusion at the ortho position. The C¼C bond present
in the cyclooctene moiety of product 7 underwent isomerization during the purification
process, thus forming the conjugated tetrahydrobenzocyclooctene 8.
Scheme 2. Different Reaction Conditions for the Aromatic Cope Rearrangement of 5 and 6
a) LiSEt, DMF or THF. b) 1. BBr3, CH2Cl2 ; 2. H2O. c) 1. BuLi, THF; 2. DMF or THF.

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Table. Results of the Attempted Aromatic Cope Rearrangement of 5 and 6
Methoda)
Solvent, temp.
Products (yield)
5
5
5
5
6
6
A
A
B
B
A
A
THF, 608
DMF, 1208
THF, 608
DMF, 1208
THF, 608
DMF, 1208
no reaction
no reaction
6 (91%)
7 (11%)b), 8 (20%)b)
No reaction
7 (17%)b), 8 (11%)b)
7 (9%)c), 8 (18%)c)
no reaction
6
6
C
C
THF, 608
DMF, 1208
7 (38%)b), 8 (27%)b)
7 (21%)c), 8 (45%)c)
a) Method A: no deprotection. Method B: i) LiSEt, THF; ii) solvent. Method C: i) BuLi, THF; ii)
b
solvent. ) Determined by 1H-NMR. c) Obtained after chromatography.
A fundamental condition for the Cope rearrangement to occur in the particular case
of diethenylcyclobutanes is the conformation of the system; i.e., the C¼C bonds that
make up the hexa-1,5-diene must be in the same plane [10][19]. There are also some
electronic features that are considered to favor the rearrangement, e.g., the presence of
an anion in the diene structure [11c]. These two conditions are important for
facilitating an effective rearrangement.
The mechanism of the Cope rearrangement has been the subject of extensive
studies and discussions. Four mechanistic alternatives, which involve the following
intermediates/transition states (TS), have been proposed: a pair of interacting allyl
radicals, an aromatic neutral TS, a cyclohexane-1,4-diyl biradical, and a dipolar
mechanism involving zwitterionic intermediates [20]. However, there is a recent
consensus that the Cope rearrangement of hexa-1,5-dienes is likely to proceed via an
aromatic chair TS. Substituents have a profound effect on the mechanism, and there is a
mechanistic continuum between the biradical and concerted pathways, with the exact
mechanism depending on the nature and position of substituents [21][22].
Theoretical studies on the possible TS structures in diethenylcyclobutane rear-
rangements favor an endo-boat-like TS because of the configuration required for the
formation of the more stable final (Z,Z)-cyclooctene ring [23]. In the present work, the
rearrangement was regio- and diastereospecific, resulting in only the (Z)-tetrahydro-
benzocyclooctene ring system 7. Based on these results, and those of previous reports,
we propose that the diene moiety that comprises the isopropenyl group and the
aromatic ring of the anion of 6 must be in the same plane. This fragment then reacts
through an endo-boat-like TS, assisted by the O-anion at the appropriate meta-position
of the aromatic ring thus inducing the anionic electronic activation of to the diene
moiety (Scheme 3).
The best results were obtained from the rearrangement of 6 under basic conditions,
thereby reinforcing the hypothesis that strong electron donation by the activating
group is required for the reaction to proceed. We propose a concerted mechanism
involving a remote oxy anion, a process analogous to that described by Marvell and Lin
[2a]. Such a concentrated mechanism is suggested by the restrictions imposed by the

Helvetica Chimica Acta – Vol. 96 (2013)
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Scheme 3. Possible Reaction Pathway Proposed for the Cope Rearrangement of the Anion of 6
final configuration of the (Z)-tetrahydrobenzocyclooctene ring system 7 [23], and
chiefly, by the experimentally observed regio- and stereospecificity of the formation of
7.
Conclusions. – These results show, for the first time, that the aromatic Cope
rearrangement of a trans-1-aryl-2-ethenylcyclobutanecarbonitrile (with cis-orientation
of the aryl and ethenyl substituents) is possible, making this approach a valuable tool
for the construction of fused benzocyclooctene ring systems. Such transformations
require relatively high temperatures (1208) and the presence of strong activating
groups, such as the electron-rich phenoxide ion, to proceed effectively.
At present, we continue our study of such systems, which contain different groups at
the aromatic ring.
This research was supported by the Facultad de Quꢀmica UNAM (PAIP 4690 – 14), and the
´
´
CONACYT through a M. S. scholarship for Adrian Vazquez (235256). We wish to thank to Marisela
´
´
Gutierrez, Margarita Guzman, and Georgina Duarte for their assistance in acquiring the spectroscopic
data.
Experimental Part
General. All chemicals and solvents were purchased from SigmaꢀAldrich. The solvents were dried
over the appropriate drying agents and distilled prior to use. Column chromatography (CC): silica gel
(SiO2; Merck 60G). IR Spectra: Perkin-Elmer-Spectrum RX I FT-IR spectrophotometer, n˜ in cmꢀ1. 1H-
and 13C-NMR Spectra: Varian-Unity-Inova-300 spectrometer; at 300 and 75 MHz, resp., in CDCl3; d in
ppm rel. to Me4Si as an internal standard, J in Hz. HR-EI-MS: Thermo-DFS spectrometer; at 70 eV; in
m/z.
trans-2-(1-Hydroxy-1-methylethyl-2-yl)-1-(3-methoxyphenyl)cyclobutanecarbonitrile (4; see [18]).
To Cs2CO3 (1 g, 0.5 equiv., 3.06 mmol), under N2, anh. THF (25 ml) was injected through a rubber

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Helvetica Chimica Acta – Vol. 96 (2013)
septum, and the mixture was stirred. Then, the flask was cooled in an ice/water bath for 10 min, before
1.6m BuLi in hexane (4.2 ml, 1.1 equiv., 6.73 mmol) was injected. The mixture was stirred for 5 min, and
then epoxide 3 (1.5 g, 1.0 equiv., 6.12 mmol) [18] in THF (5 ml) was injected and the mixture stirred for a
further 5 min. Then, the flask was removed from the ice bath, the reaction left to proceed at r.t., and then
the flask placed in a preheated oil bath at 808. Once the reflux was initiated, the mixture was stirred for
2 h and then left to cool to r.t. A 20% of NH4Cl soln. (10 ml) was added and the mixture stirred for 5 min
and then extracted with AcOEt (6 ꢁ 25 ml). The org. extracts were washed with NaHCO3 soln. (2 ꢁ
10 ml), dried (Na2SO4), and concentrated. The final crude product was purified by prep. TLC
(hexane/acetone): 960 mg (64%) of 4. Amber liquid. Spectroscopy data: see [18].
trans-1-(3-Methoxyphenyl)-2-(1-methylethyl)cyclobutanecarbonitrile (5). Under N2, anh. benzene
(5 ml) was injected through a rubber septum with stirring to alcohol 4 (500 mg, 1 equiv., 2.04 mmol). The
flask was placed in an ice/water bath for 5 min, and then SOCl2 (0.22 ml, 1.5 equiv., 3.06 mmol) was
slowly injected. The mixture was stirred for 5 min, followed by the addition of 1,4-diazabicyclo[2.2.2]oc-
tane (DABCO; 450 mg, 2 equiv., 4.08 mmol) dissolved in benzene (2 ml). The reaction was carried out
for 3.5 h, then 50% NH4Cl soln. (10 ml) was added, and the mixture stirred for 5 min, and then extracted
with AcOEt (4 ꢁ 15 ml). The org. extracts were washed with 15% HCl soln. (3 ꢁ 10 ml) and subsequently
with 15% NaHCO3 soln., dried (Na2SO4), and concentrated. The final crude product was purified by CC
(hexane/acetone): 245 mg (49%) of 5. Colorless liquid. IR (film): 3083, 2958, 2916, 2837, 2229, 1668,
1650. 1H-NMR (300 MHz, CDCl3): 1.40 – 1.41 (m, 3 H); 2.23 – 2.33 (m, 1 H); 2.36 – 2.46 (m, 1 H); 2.51 –
2.58 (m, 1 H); 2.86 (td, J ¼ 7.1, 9, 1 H); 3.74 (t, J ¼ 6.9, 1 H); 3.81 (s, 3 H); 4.65 – 4.67 (m, 1 H); 4.77 (dd,
J ¼ 0.8, 1.2, 1 H); 6.83 (ddd, J ¼ 0.9, 1.2, 7.8, 1 H); 7.0 (t, J ¼ 1.5, 1 H); 7.06 (ddd, J ¼ 0.8, 1.6, 7.5, 1 H); 7.27
(t, J ¼ 7.9, 1 H). 13C-NMR (75 MHz, CDCl3): 21.1; 21.7; 30.2; 43.2; 52.8; 55.4; 113.0; 113.2; 113.4; 119.4;
124.4; 129.5; 137.4; 142.1; 159.6. HR-EI-MS: 227.1303 (Mþ, C15H17NOþ; calc. 227.1310).
trans-1-(3-Hydroxyphenyl)-2-(1-methylethyl)cyclobutanecarbonitrile (6). To compound 5 (250 mg,
1 equiv., 1.10 mmol) under N2, anh. CH2Cl2 (5 ml) was added through a rubber septum. The mixture was
cooled to ꢀ 788 and stirred for 5 min. Then, BBr3 (0.44 ml, 1.14 g, 2.50 equiv., 2.92 mmol) was slowly
injected. The reaction was carried out for 4 h, during which time the mixture was allowed to warm to r.t.
After cooling in an ice/water bath, sat. NH4Cl soln. (10 ml) was slowly added and the mixture stirred for a
further 5 min and then extracted with CH2Cl2 (5 ꢁ 15 ml). The org. phases were washed with sat. of soln.
NaCl (3 ꢁ 10 ml), dried (Na2SO4), and concentrated. The resulting crude product was purified by CC
(hexane/acetone): 229 mg (96%) of 6. Slightly viscous colorless liquid. IR (film): 3381, 3084, 2957, 2923,
2855, 2236, 1706, 1650. 1H-NMR (300 MHz, CDCl3): 1.40 – 1.41 (m, 3 H); 2.24 – 2.33 (m, 1 H); 2.36 – 2.43
(m, 1 H); 2.51 – 2.58 (m, 1 H); 2.83 – 2.90 (m, 1 H); 3.73 (t, J ¼ 9.2, 1 H); 4.65 – 4.67 (m, 1 H); 4.77 (dd,
J ¼ 1.3, 2.7, 1 H); 5.19 (s, 1 H, exchange with D2O); 6.83 (ddd, J ¼ 0.9, 1.8, 8.1, 1 H); 7.0 (t, J ¼ 1.5, 1 H);
7.06 (ddd, J ¼ 0.9, 1.5, 7.9, 1 H); 7.27 (t, J ¼ 7.8, 1 H). 13C-NMR (75 MHz, CDCl3): 21.1; 21.7; 30.2; 43.2;
52.8; 55.4; 113.0; 113.2; 113.4; 119.4; 124.4; 129.5; 137.4; 142.1; 159.6. HR-EI-MS: 213.1150 (Mþ,
C14H15NOþ; calc. 213.1154).
(9Z)-5,6,7,10-Tetrahydro-1-hydroxy-9-methylbenzocyclooctene-5-carbonitrile (7) and (9Z)-5,6,7,8-
Tetrahydro-1-hydroxy-9-methylbenzocyclooctene-5-carbonitrile (8). To 6 (100 mg, 1 equiv., 0.469 mmol)
under N2, anh. THF (3 ml) was injected through a rubber septum. The mixture was cooled to ꢀ 788 and
stirred for 5 min. Then, 1.6m BuLi in hexane (0.32 ml, 1.1 equiv., 0.510 mmol) was injected (! immediate
formation of a white suspended solid). The solvent was removed with a stream of N2, and anh. DMF
(3 ml) was injected under vigorous stirring. The mixture was placed in an oil bath preheated to 1208 and
the reaction continued for 24 h. Then, the mixture was cooled to r.t., a sat. NH4Cl soln. (15 ml) added,
and the mixture stirred for 5 min and extracted with AcOEt (5 ꢁ 10 ml). The org. phases were washed
with sat. NaCl soln. (4 ꢁ 25 ml), dried (Na2SO4), and concentrated. The resulting crude product was
purified by prep. TLC (7 elutions with hexane/acetone): 21 mg (21%) of 7 and 45 mg (45%) of 8. Data of
7: Colorless liquid. 1H-NMR (300 MHz, CDCl3): 1.58 (s, 3 H); 1.71 – 1.76 (m, 1 H); 1.94 – 2.01 (m, 1 H);
2.13 – 2.19 (m, 2 H); 2.47 (s, 2 H); 4.10 (dd, J ¼ 3.1, 11.1, 1 H); 5.07 (s, 1 H, exchange with D2O); 5.56 (s,
1 H); 6.79 (dd, J ¼ 2.1, 8.1, 1 H); 7.01 (dd, J ¼ 1.8, 8.4, 1 H); 7.26 (t, J ¼ 7.5, 1 H). 13C-NMR (75 MHz,
CDCl3): 24.2; 28.3; 32.6; 33.2; 34.6; 113.1; 119.3; 120.4; 127.2; 132.8; 137.46; 154.7. HR-EI-MS: 213.1151
(Mþ, C14H15NOþ; calc. 213.1154).

Helvetica Chimica Acta – Vol. 96 (2013)
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Data of 8: Colorless liquid. IR (film): 3377, 3072, 3018, 1928, 2242, 1603. 1H-NMR (300 MHz,
CDCl3): 1.61 (s, 3 H); 1.72 – 1.78 (m, 2 H); 1.96 (s, 2 H); 1.98 – 2.10 (m, 2 H); 4.06 (dd, J ¼ 3.3, 11.5, 1 H);
5.08 (s, 1 H, exchange with D2O); 6.14 (s, 1 H); 6.79 (dd, J ¼0.9, 8.1, 1 H); 7.03 (dd, J ¼ 1.2, 8.4, 1 H); 7.25
(t, J ¼ 8.4, 1 H). HR-EI-MS: 213.1152 (Mþ, C14H15NOþ; calc. 213.1154).
REFERENCES
[1] A. C. Cope, L. Field, D. W. H. MacDowell, M. E. Wright, J. Am. Chem. Soc. 1956, 78, 2547.
[2] a) E. N. Marvell, C. Lin, J. Am. Chem. Soc. 1978, 100, 877; b) W. von E. Doering, R. A. Bragole,
Tetrahedron 1966, 22, 385.
[3] D. C. Hurd, H. T. Bollman, J. Am. Chem. Soc. 1933, 55, 699; D. C. Hurd, H. T. Bollman, J. Am.
Chem. Soc. 1934, 56, 447.
[4] A. C. Cope, J. E. Meili, D. W. H. MacDowell, J. Am. Chem. Soc. 1956, 78, 2551.
[5] W. von E. Doering, W. R. Roth, Angew. Chem., Int. Ed. Engl. 1963, 2, 115.
[6] E. Vogel, Angew. Chem. 1960, 72, 4; E. N. Marvell, S. W. Almond, Tetrahedron Lett. 1979, 20, 2777;
E. N. Marvell, S. W. Almond, Tetrahedron Lett. 1979, 20, 2779.
[7] E. Vogel, Justus Liebigs Ann. Chem. 1958, 615, 1.
[8] J. A. Berson, P. B. Dervan, J. Am. Chem. Soc. 1972, 94, 7597; R. C. Gadwood, R. M. Lett, J. Org.
Chem. 1982, 47, 2268; K. J. Shea, A. C. Greeley, S. Nguyen, P. D. Beauchamp, D. H. Aue, J. S.
Witzeman, J. Am. Chem. Soc. 1986, 108, 5901; P. A. Wender, C. R. D. Correia, J. Am. Chem. Soc.
1987, 109, 2523.
[9] W. E. Billups, J. H. Cross, C. V. Smith, J. Am. Chem. Soc. 1973, 95, 3438.
[10] J. Limanto, M. L. Snapper, J. Am. Chem. Soc. 2000, 122, 8071.
[11] a) J. A. Berson, M. Jones, J. Am. Chem. Soc. 1964, 86, 5017; b) J. A. Berson, M. Jones, J. Am. Chem.
Soc. 1964, 86, 5019; c) D. A. Evans , A. M. Golob, J. Am. Chem. Soc. 1975, 97, 4765; d) L. A.
Paquette, Angew. Chem., Int. Ed. Engl. 1990, 29, 609.
[12] M. Sworin, K. C. Lin, J. Am. Chem. Soc. 1989, 111, 1815; M. E. Jung, G. L. Hatfield, Tetrahedron
Lett. 1983, 24, 2931; P. Koch-Huber, R. W. Kunz, S. Bienz, Molecules Online 1999, 3, 9; L. Barriault,
I. Denissova, N. Goulet, Synthesis 2012, 44, 1833; C. F. Weise, S. Immel, F. Richter, C. Schneider, Eur.
J. Org. Chem. 2012, 1520.
[13] M. E. Jung, J. P. Hudspeth, J. Am. Chem. Soc. 1978, 100, 4309; M. E. Jung, J. P. Hudspeth, J. Am.
Chem. Soc. 1980, 102, 2463.
[14] S. S. Hussaini, A. R. N. Raj, C. A. M. A. Huq, Tetrahedron Lett. 2007, 48, 775.
[15] E. N. Marvell, T. H. C. Li, J. Am. Chem. Soc. 1978, 100, 883.
´
[16] R. Villagomez-Ibarra, C. Alvarez-Cisneros, P. Joseph-Nathan, Tetrahedron 1995, 51, 9285; J. J.
Bucchignano, W.-S. Huang, P. R. Varanasi, R. R. Yu, to IBM (US) ꢁFused Aromatic Structures and
Methods for Photolithographic Applicationsꢂ, US Pat. 2009004596 (A1), 01.01.2009.
´
´
´
´
´
[17] J. G. Avila-Zarraga, Ph. D. Thesis, Universidad Nacional Autonoma de Mexico, 2000; J. G. Avila-
Zarraga, G. Ramirez-Galicia, M. Rubio, L. A. Maldonado-Graniel, J. Mol. Struct. – THEOCHEM
2000, 531, 301.
´
´
´
´
´
[18] J. A. Lujan-Montelongo, A. Vazquez-Sanchez, J. G. Avila-Zarraga, Heterocycles 2009, 78, 1955.
[19] K. J. Shea, R. B. Phillips, J. Am. Chem. Soc. 1980, 102, 3156.
ˆ
[20] M. J. S. Dewar, L. E. Wade Jr., J. Am. Chem. Soc. 1977, 99, 4417; R. Wehrli, H. Schmid, D. Bellus,
H.-J. Hansen, Helv. Chim. Acta 1977, 60, 1325; J. J. Gajewski, N. D. Conrad, J. Am. Chem. Soc. 1978,
100, 6268; J. J. Gajewski, N. D. Conrad, J. Am. Chem. Soc. 1978, 100, 6269; D. A. Hrovat, K.
Morokuma, W. T. Borden, J. Am. Chem. Soc. 1994, 116, 1072; R. Gompper, W. R. Ulrich, Angew.
Chem., Int. Ed. Engl. 1976, 15, 299; R. Gompper, W. R. Ulrich, Angew. Chem., Int. Ed. Engl. 1976,
15, 301.
´
[21] V. N. Staroverov, E. R. Davidson, J. Am. Chem. Soc. 2000, 122, 7377; K. N. Houk, J. Gonzalez, Y. Li,
Acc. Chem. Res. 1995, 28, 81; W. von E. Doering, Y. Wang, J. Am. Chem. Soc. 1999, 121, 10967; R.
Hoffmann, W.-D. Stohrer, J. Am. Chem. Soc. 1971, 93, 6941.

1338
Helvetica Chimica Acta – Vol. 96 (2013)
[22] D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 1999,
121, 10529.
[23] M. Zora, I. ꢆzkan, J. Mol. Struct. – THEOCHEM 2003, 625, 251.
Received July 9, 2012