Thermal electrocyclization reactions II: Benzooctatetraenes and benzodecapentaenes

Article abstract of DOI:10.1016/j.tet.2013.12.027

Thermal electrocyclization reactions of benzooctatetraenes and benzodecapentaenes substituted with R=H, Cl, and methyl were studied experimentally and computationally. Methyl and unsubstituted benzooctatetraenes and benzodecapentaenes give the [4.2.0]bicyclooctadiene products by 8π,6π-electrocyclization. Chlorine substitution led to thermal rearrangement of the initially formed 8π,6π-electrocyclization intermediates to give unprecedented products.

Full text of DOI:10.1016/j.tet.2013.12.027

Tetrahedron 70 (2014) 886e891
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Tetrahedron
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Thermal electrocyclization reactions II: benzooctatetraenes and
benzodecapentaenes
a
c
,
,
ꢁ a
*
ꢁ b
ꢀ
ꢀ
ꢁ d
ꢀ
ꢀ
*
Dragana Vuk , Zeljko Marinic , Kresimir Molcanov , Davor Margetic , Irena Skoric
a Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, 10000 Zagreb, Croatia
b
ꢀ
ꢁ
ꢀ
NMR Centre, RuCer Boskovic Institute, Bijenicka cesta 54, 10001 Zagreb, Croatia
c
ꢀ
ꢁ
ꢀ
Division of Physical Chemistry, RuCer Boskovic Institute, Bijenicka cesta 54, 10001 Zagreb, Croatia
Laboratory for Physical-Organic Chemistry, Division of Organic Chemistry and Biochemistry, RuCer Boskovic Institute, Bijenicka cesta 54, 10001
d
ꢀ
ꢁ
ꢀ
Zagreb, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2013
Received in revised form 29 November 2013
Accepted 10 December 2013
Available online 18 December 2013
Thermal electrocyclization reactions of benzooctatetraenes and benzodecapentaenes substituted with
R¼H, Cl, and methyl were studied experimentally and computationally. Methyl and unsubstituted
benzooctatetraenes and benzodecapentaenes give the [4.2.0]bicyclooctadiene products by 8
trocyclization. Chlorine substitution led to thermal rearrangement of the initially formed 8
trocyclization intermediates to give unprecedented products.
p
p
,6
,6
p
p
-elec-
-elec-
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Polyenes
Electrocyclizations
Reaction mechanism
Rearrangements
DFT calculations
1. Introduction
of electrocyclization reactions in nature. This tandem reaction se-
quence involves a cascade of thermally induced 8p, followed by 6p
Dienes and polyenes are compounds, which intervene in a large
number of organic reactions, and are constituents of a large number
of natural and human-made compounds, including terpenes, cho-
lesterol, and vitamin A.1 Characteristic type of reactions for conju-
gated dienes and polyenes are electrocyclizations, which are
synthetically important and believed to be involved in biosynthesis
electrocyclizations leading directly to compounds that contain the
bicyclo[4.2.0]octa-2,4-diene ring system.5 1,3,5,7-Octatetraenes in
which the two internal olefins have the Z-configuration are ther-
mally unstable with respect to the 8p,6p electrocyclization cascade,
which explains their tendency to undergo further pericyclic re-
actions and scarce occurrence of this motif in natural products.
of many natural compounds.2 Among them, thermal 6
p
electro-
Natural products that arise from 8p,6p electrocyclizations rep-
cyclizations of conjugated trienes are the most common of all
electrocyclizations employed in the synthesis. In such process, all-
carbon, as well as heterocyclic products could be formed.3,4 Such
reactions lead to conjugated cyclohexadiene units and, although
thermodynamically favorable, usually require activation by rela-
tively high temperatures. The cyclohexadiene products can partic-
ipate in further reactions, such as DielseAlder cycloadditions, and
such reaction cascades form extremely complex molecular
architectures.
resented by SNF4435 C, ocellapyrone A, and elysiapyrone A, have
sparked significant synthetic activity, which has led to a better
understanding of electrocyclization cascades and polyene chemis-
try in general and preparation of natural products, such as shima-
lactones,6 and pre-kingianin A.7
As a part of our ongoing interest in reaction mechanisms8 and
intramolecular photocycloaddition reactions of substituted hexa-
trienes with one double bond incorporated into the benzene ring9
we recently reported synthesis and photocycloadditions of novel o-
butadienylstyrenes.10 In continuation of this work we here present
results of thermal rearrangements of benzooctatetraenes (1e2) and
benzodecapentaenes (6e8) possessing substituents at 2-
position(s) of polyene moiety.
The 8p,6p electrocyclic cascade (sometimes referred to as
Black’s cascade) is arguably the most elegant display of the power
The aim of this account is to study the influence of substitution
patterns on the electrocyclization processes of conjugated
* Corresponding authors. Tel.: þ385 1 456 1008; fax: þ385 1 468 0195; e-mail
ꢀ
ꢁ
ꢁ
addresses: margetid@irb.hr (D. Margetic), iskoric@fkit.hr (I. Skoric).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.12.027

D. Vuk et al. / Tetrahedron 70 (2014) 886e891
887
tetraenes, in which substituents are positioned at the 2-position(s)
of conjugated polyenes. In our previous paper, influence of terminal
substituents on the product endo/exo selectivity was investigated.11
additionally confirmed for 5 by X-ray crystallography (Fig. 2). Ste-
reochemistry of the endo-4 isomer was deduced from 2D COSY,
NOESY, and HSQC NMR experiments, the most indicative being
NOESY correlations (Supplementary data).
2. Results and discussion
2.1. Experimental study
The starting compounds 1, 2, and 6e8 (Schemes 1 and 2) were
prepared employing one-pot synthetic approach based on double
Wittig reaction, starting from an aromatic diphosphonium salt and
two different aldehydes as mixtures of two (compounds 1 and 2) or
three geometric isomers (compounds 6e8) according to the pro-
cedures described in previous papers for heteroarylstilbene de-
rivatives.12 Using commercially available (E) isomers of the
chloro-cinnamaldehyde and -methyl-cinnamaldehyde, the sec-
a-
Fig. 2. ORTEP-314 drawing of thermal product 5. Displacement ellipsoids are drawn for
the probability of 30% and hydrogen atoms are shown as spheres of arbitrary radii.
Only R enantiomer is shown for 5.
a
ond double bond (looking from the ortho substituted central ben-
zene ring) was introduced in the starting compounds 1, 2, and 6e8
retaining the (E) configuration. Hence, the number of the geometric
isomers was reduced; the reaction mixtures contain cis- and trans-
isomers of 1 and 2, and cis,cis-, cis,trans-, and trans,trans-isomers of
6e8 (only the first double bond connected to the ortho substituted
central benzene ring is designated). By proper addition strategy of
these aldehydes, formaldehyde and base the starting compounds 1,
2, and 6e8 were obtained in very good yields (51e91%).13
Replacement of the a-methyl group in 1 with chlorine in sub-
strate 2 led to an unusual reaction and only one stereomeric ther-
mal product 5 was obtained (toluene, 16 h, in 90% yield, Scheme 1).
On reflux in benzene (80 ꢀC, 16 h) the cis-1 isomer from mixture of
isomers was converted also to the endo-4, but in 65% yield and the
cis-2 from mixture of isomers was converted also to 5 but in 80%
yield, indicating that a higher temperature is required for the trans/
cis isomerization of trans-1. It was found that the cis-1 reacts
thermally even in the photochemical reaction11 giving endo-4 as
the minor product, aside of major photochemical product. Reflux-
ing of the mixture of isomers of 1 at higher temperature in xylene
(144 ꢀC, 16 h) resulted in complete disappearance of the reactant,
with only traces of endo-4 present, presumably due to ring cleavage
to methyl-naphthalene and styrene. This was corroborated by
analysis of 1H NMR spectra of the thermolysis product of 1 in xy-
lene, which showed only presence of characteristic signals for
methyl-naphthalene and styrene and the unreacted trans-1. Fur-
thermore, GC/MS analysis of a sample after the thermal reaction of
mixture of isomers of 1 showed exclusive presence of trans-1 iso-
mer, along with the new signals (Mþ 104 and 142), which are as-
sociated to styrene and 2-methyl-naphthalene. GC/MS analysis of
a sample obtained after the thermolysis of the chloro-derivative 2
did not show any MS signal, which can be attributed to naphtha-
lene, chloro-naphthalene or styrene (Mþ 128, 162, and 104). This
additional information highlights the difference in the thermolysis
reaction mechanism of 2 in comparison with the methyl derivative
1.
HA
R
HB
CH3
HD
Cl
HA
HB
HC
HE
toluene
reflux
HC
HF
endo-4 (90%)
1: R = CH3
2: R = Cl
3: R = H
5 (90%)
Scheme 1. Thermal reactions of benzooctatetraenes 1 and 2.
Scheme 2. Thermal reaction of benzodecapentaenes 6e8.
Thermal reactions of benzooctatetraenes 1 and 2 are shown in
Scheme 1. Upon refluxing 1 or 2 in toluene (111 ꢀC, 20 h), products
endo-4 (90%) and 5 (90%) were obtained. Obtained results for a-
methyl substitution at the butadiene unit are expected (product
endo-4), while chloro-derivative 2 differs from those obtained in
our previous paper for unsubstituted 3. The structures of products
endo-4 and 5 were elucidated using NMR spectroscopy (Fig. 1) and
The compound 5 possesses one stereogenic center, and crys-
tallizes as a racemate (in the centrosymmetric space group P 21/c).
However, enantiomeric disorder is present in the crystals, with
molecules of both enantiomers occupying the same crystallo-
graphic position (with occupancies 0.68 and 0.32, respectively).
Since the geometries of both enantiomers are similar, the disorder
can be described as two different positions of the stereogenic
Cl1eC7eH7 group: the component designated as
a (i.e.,
Cl1aeC7aeH7a) has R configuration (Fig. 2), while the component
b (i.e., Cl1beC7beH7b) has S configuration. Crystal packing is
achieved only through CeH/p interactions (Table 1).
Further extension of conjugated system to benzodecapentaenes
6e8 shows similar thermal reactivity as observed for 1 and 2
(Scheme 2) in toluene. In the case of thermolysis of mixtures of
cis,cis-6e8 and cis,trans-6e8, unsubstituted and methyl substituted
benzodecapentaenes 6 and 7 produced related 8p,6p-electro-
cyclization products endo-9 and endo-10. Again, polyene sub-
stitution with chlorine affected the reactivity leading to the
isolation of an entirely different product 11, whose structure was
PPM
4.8
4.4
4.0
3.6
3.2
2.8
2.4
Fig. 1. 1H NMR spectra of thermal products endo-4 and 5 (aliphatic region).

888
D. Vuk et al. / Tetrahedron 70 (2014) 886e891
Table 1
Differences in the chemical behavior of 1, 2, and 3 can be
explained by the thermal instability of the initial product of 8 ,6p-
Geometric parameters of the CeH/p and interactions in crystal structures of 5 and
11
p
electrocyclization endo-12 (Scheme 3). Homolytic cleavage of
cyclobutane ring gives biradical intermediate 13, which undergoes
1,2-alkyl shift to form biradical 14. Finally, 1,2-migration of chlorine
followed by aromatization gives 5. Alternatively, rearrangement of
endo-12 could take place in a single step via cyclic six electron shift.
Homolytic cleavage of cyclobutane ring has its precedence in the
elimination of styrene and formation of naphthalene described in
one of our previous papers,11 but was not detected in the case of 2.
X/Cg
/A
g
CeX/Cg C/Cg
Symm.
op. on Cg
a
ꢀ
ꢀ
ꢂ
ꢂ
/
/
/A
Compound 5
C2eH2/C1/C6
C5eH5/C11/C16
2.93
2.97
10.82 134
16.04 129
15.46 130
3.630(6) ꢁx, 1/2þy, ꢁ1/2ꢁz
3.627(5) x, 1/2ꢁy, ꢁ1/2þz
3.602(5) 1ꢁx, ꢁ1/2þy, 1/2ꢁz
C17eH17/C11/C1 2.94
Compound 11
C3eH3/C7/C12
2.84
17.17 164
3.61 154
10.56 154
3.788(3) 1ꢁx, ꢁy, ꢁz
3.733(4) ꢁx, 1ꢁy, ꢁz
3.684(4) 1ꢁx, 1ꢁy, ꢁz
C11eH11/C15/C20 2.87
C25eH25/C4/C13 2.83
a
g
¼angle defined by a line connecting center of gravity of the aromatic ring with
H atom and the normal to the aromatic ring.
determined by NMR spectroscopy (Fig. 3, as well as of products
endo-9 and endo-10) and proven by X-ray crystallography (Fig. 4).
Stereochemistry of the endo-8 and endo-10 isomers was deduced
from 2D COSY, NOESY, and HSQC NMR experiments, the most in-
dicative being NOESY correlations (see Supplementary data). Fur-
thermore, GC/MS analysis of a sample after the thermal reaction of
mixture of isomers of 6 showed presence of only trans-6 isomer,
along with the new signals (Mþ 128 and 206), which are associated
to naphthalene and 1,4-diphenyl-1,3-butadiene, respectively.
Refluxing of the mixture of isomers of 6e8 in benzene did not show
any reaction after 8 h, and in xylene (144 ꢀC, 8 h) resulted in
cleavage and disappearance of the products endo-9, endo-10 and 11,
respectively (same as in toluene).
Scheme 3. Reaction mechanism for thermal reaction of benzooctatetraene 2.
The formation of product 11 could be envisaged by several re-
action pathways shown in Scheme 4. The heating of 8 triggers the
10
p-electrocyclization and formation of a cyclic intermediate 15
(Path A). Concominant 6
p
-electrocyclization leads to 16, which
after elimination of hydrochloric acid aromatizes to 1,2-
dihydrophenanthrene 11. However, quantum-chemical calcula-
tions (B3LYP/6-31G*) for parent octatetraene indicated significantly
lower activation barrier for 8
cyclization process (by 16.6 and 15.6 kJ molꢁ1, respectively), sug-
gesting that 8 -electrocyclization is the preferred pathway (Path
B). In 8 ,6 -electrocyclization route product 17 is obtained, from
p- than for 6p- or 10p-electro-
p
p
p
which two pathways lead to final product 11, depending on the
timing of HCl elimination. If cyclobutene bond cleavage of 17 takes
place first (Route A), biradical 18 is formed, which cyclizes to 17. In
the second route (B) HCl elimination and aromatization takes place
first, followed by recombination to 11. Chemical intuition favors the
last reaction route, since aromatization of 17 gives a thermody-
namically more stable product 20, and also introduces additional
ring strain in cyclobutene system, which promotes the ring opening
process.
Fig. 3. 1H NMR spectra of thermal products endo-9, endo-10, and 11 (aliphatic region).
Fig. 4. ORTEP-314 drawing of thermal product 11. Displacement ellipsoids are drawn
for the probability of 30% and hydrogen atoms are shown as spheres of arbitrary radii.
The molecular structure of 11 is shown in Fig. 4. The conformation
of its centralring, C1/C14 isbetween anenvelopeand screw-boat, as
indicated by CremerePople15 parameters: ring puckering amplitude
¼114.5(4)ꢀ and 4¼268.2(4)ꢀ. The 3D packing is ach-
ꢂ
is 0.419(3) A,
q
Scheme 4. Reaction mechanism for thermal reaction of benzodecapentaene 8.
ieved exclusively through CeH/p interactions (Table 1).

D. Vuk et al. / Tetrahedron 70 (2014) 886e891
889
2.2. Quantum-chemical study
The obtained energy profile is similar to the one found in
a previous study on structurally related benzooctateraenes.11 It
shows a relatively small energy barrier is necessary for the con-
formational change 24/27 compared to electrocylization pro-
cesses. According to the CurtineHammet principle, the endo/exo
product ratio is dictated by the relative heights of the energy bar-
rier rather than the stabilities of the two intermediates. Sub-
sequently the endo-4 product is readily obtained via transition state
25-TS, since the energy barrier for this reaction is lower than the
barrier for the formation of exo-4 products (via transition state 28-
TS) regardless of substituent (by 10.5, 4.6, and 15.0 kJ molꢁ1, for 1, 2,
and 3, respectively). These results are in good accord with the ex-
perimentally observed exclusive formation of the endo-4 product.
Benzodecapentaene system 6 with additional olefinic bond be-
haves in similar manner as 3, and endo-9 is product preferred.
The origins of the experimentally observed endo-stereochemical
outcomes of the thermal reactions of 1, 2, 6, and 7 and additional
support for structural assignments were sought computationally.
Transition state calculations, using density functional theory (DFT)
were employed using recently developed DFT functional M062X,
which is specifically designed for obtaining better reaction ener-
getics. A summary of the computational results is given in Fig. 5 and
Table 2.
3. Conclusions
The thermal reactions of benzooctatetraenes and benzodeca-
pentaenes substituted with R¼H, Cl, and Me give different products
depending on their substitution. In the case of H and Me sub-
stitution, the [4.2.0]bicyclooctadiene products of 8p,6p-electro-
cyclization are obtained. Chlorine substituted benzooctatetraene
and benzodecapentaene gave unprecedented products, which are
most likely formed by the thermal rearrangement of chlorine
substituted products initially formed by 8
4. Experimental section
4.1. General
p,6p-electrocyclization.
The 1H spectra were recorded on a spectrometer at 600 MHz.
The 13C NMR spectra were registered at 150 MHz, respectively. All
NMR spectra were measured in CDCl3 using tetramethylsilane as
reference. The assignment of the signals is based on 2D-CH corre-
lation and 2D-HH-COSY experiments. UV spectra were measured
on a UV/VIS Cary 50 spectrophotometer. IR spectra were recorded
on a FTIR-ATR (film). Irradiation experiments were performed in
a quartz vessel in toluene solution in a photochemical reactor
Fig. 5. Reaction energy profile for thermal electrocyclization of 1e3 calculated at the
M06/6-311þG(d,p)//M06/6-31G(d)þZPVE level of theory (kJ/mol).
ꢂ
equipped with 3000 A lamps. All irradiation experiments were
carried out in de-aerated solutions by passing a stream of argon
prior to irradiation. Melting points were obtained using microscope
equipped apparatus and are uncorrected. HRMS analysis were
carried out on a mass spectrometer (MALDI TOF/TOF analyzer),
equipped with Nd:YAG laser operating at 355 nm with firing rate
200 Hz in the positive (Hþ) or negative (Hꢁ) ion reflector mode.
Silica gel (0.063e0.2 mm) was used for chromatographic purifica-
tions. Thin-layer chromatography (TLC) was performed silica gel 60
Table 2
Relative energies (kJ/mol) calculated at the M06/6-311þG(d,p)//M06/6-31G(d)þ
ZPVE level of theory of 1e3 derivatives, with zero point energy corrections included
Molecule
1 (R¼Me)
0.0
109.2
26.3
103.2
80.5
39.3
113.7
ꢁ88.6
ꢁ83.8
2 (R¼Cl)
0.0
110.9
24.4
109.4
76.3
39.7
113.0
ꢁ105.6
ꢁ94.4
3 (R¼H)
0.0
128.2
37.3
111.7
93.3
52.9
126.7
ꢁ73.6
ꢁ71.1
1e3
24-TS
25
26-TS
27-TS
28
29-TS
endo-4
exo-4
F
254 plates. Solvents were purified by distillation.
Cinnamaldehyde, -methyl-cinnamaldehyde and
namaldehyde were obtained from a commercial source,
a
a
-chloro-cin-
-o-
b,b
xylyl(ditriphenylphosphonium)dibromide was prepared from o-
xylyldibromide and triphenylphosphine in dimethylformamide.
4.2. (1E,3E)-2-Methyl-1-phenyl-4-(o-styrenyl)-1,3-butadiene
The proposed reaction mechanism for transformation of 1e3 to
endo/exo-4 involves an 8 ,6 -electrocyclization cascade, (Fig. 5). In
the first step (8 electrocyclization) conrotatory ring closure via
transition state 23-TS takes place, leading to an intermediate pen-
taene 24. The aromaticity of benzene moiety is restored in the
(1); (1Z,3E)-2-chloro-1-phenyl-4-(o-styrenyl)-1,3-butadiene
(2)
p
p
p
Starting compounds 1 and 2 were prepared by Wittig reaction13
from o-xylylenebis(triphenylphosphonium bromide) and the cor-
second reaction step, (disrotatory 6
p
-electrocyclization), via 25-TS,
responding aldehydes, a-chloro-cinnamaldehyde and a-methyl-
to form endo-4. To obtain the second possible isomeric product,
exo-4, a conformational interconversion (ring flipping of the CH2
group) of the intermediate 24 takes place via transition state 26-TS
cinnamaldehyde. To a stirred solution of the triphenylphospho-
nium salt (0.001 mol) and the corresponding aldehyde (0.011 mol)
in absolute ethanol (200 mL), a solution of sodium ethoxide
(0.253 g, 0.011 mol in 15 mL of absolute ethanol) was added
dropwise. Stirring was continued under a stream of nitrogen for
to the boat conformer 27. The disrotatory 6p-electrocyclization of
27 via transition state 28-TS, leads to the product exo-4.

890
D. Vuk et al. / Tetrahedron 70 (2014) 886e891
one hour at rt. Under the stream of dry nitrogen gaseous formal-
dehyde (obtained by decomposition of paraformaldehyde taken in
excess, 1.5 g) was introduced and the next quantity of sodium
ethoxide (0.253 g, 0.011 mol) in absolute ethanol (15 mL) was
added dropwise. The reaction was completed within 3e4 h (usually
was left to stand overnight). After removal of the solvent, the res-
idue was worked up with water and toluene. The toluene extracts
were dried (anhydrous MgSO4) and concentrated. The crude re-
action mixture was purified and the isomers of products 1 and 2
were separated by repeated column chromatography on silica gel
using petroleum ether as the eluent. The first fractions yielded cis-
and the last fractions trans-isomers.
4.5. General procedure for thermal reactions of 6e8
mixture of cis,cis-6e8 and cis,trans-6e8 (0.05 g; 6:
0.00015 mol, 7: 0.00012 mol, 8: 0.00015 mol) was dissolved in
toluene (25 mL) and refluxed for 8 h. Solvent was removed in vacuo
and the residue was chromatographed on a silica gel column using
petroleum ether as the eluent. In the first fractions the thermal
products 9 (0.040 g, 20%),10 (0.020 g, 40%) or 11 (0.030 g, 60%) were
isolated.
A
4.5.1. 3-[(E)-2-Phenylethenyl]-4-phenyltricyclo[6.4.0.02,5
]
dodeca-
1(8),6,9,11-tetraene (endo-9). Rf (30% CH2Cl2/petroleum ether)
0.56; colorless crystals; mp 142e145 ꢀC; UV (EtOH) lmax (log ε) 263
(4.01); dH (600 MHz, CDCl3) 7.55 (d, J¼7.6 Hz, 1H), 7.32 (dt, J¼7.6,
0.9 Hz, 1H), 7.24e7.28 (m, 5H), 7.13 (d, J¼7.6 Hz, 1H), 7.10 (t,
J¼7.6 Hz,1H), 7.05 (d, J¼7.6 Hz,1H), 6.99 (t, J¼7.6 Hz, 2H), 6.65e6.70
(m, 3H), 6.44 (dd, J¼9.6, 2.6 Hz, 1H, H-A), 6.04e6.08 (m, 1H), 5.72
(dd, J¼9.6, 1.7 Hz, 1H, H-B), 3.74 (t, J¼5.1 Hz, 1H, H-C/H-D), 3.54 (d,
J¼14.3 Hz, 1H, H-C/H-D), 3.44 (dd, J¼12.5, 5.9 Hz, 1H, H-E/H-F), 2.81
(ddd, J¼14.3, 12.5, 4.1 Hz, 1H, H-E/H-F); dC (150 MHz, CDCl3) 140.1
(s), 139.0 (s), 137.4 (s), 134.4 (s), 132.7 (d), 130.0 (2d), 130.0 (d), 127.6
(d), 126.7 (3d), 125.9 (d), 125.8 (d), 125.7 (d), 125.6 (d), 125.5 (d),
122.3 (d), 49.1 (d), 48.5 (d), 41.7 (d), 33.4 (d); nmax (evaporated film
from CHCl3) 2923, 1677, 1493, 693 cmꢁ1; HRMS MHþ, found
335.1759. C26H22 requires 335.1749.
4.3. General procedure for thermal reactions of 1 and 2
A solution of cis-1 (0.05 g, 0.00019 mol) or cis-2 (0.05 g,
0.00020 mol) was dissolved in toluene (25 mL) and refluxed for
16 h. The solvent was removed in vacuo and the residue was
chromatographed on a silica gel column using petroleum ether as
the eluent. In the first fractions the thermal products endo-4
(45 mg, 90%) or 5 (45 mg, 90%) were isolated.
4.3.1. 5-Methyl-4-phenyltricyclo[6.4.0.02,5]dodeca-1(8),6,9,11-
tetraene (endo-4). Rf (petroleum ether) 0.36; colorless oil; UV
(EtOH) lmax (log ε) 322 (3.22, sh), 277 (3.90, sh), 270 (3.92); dH
(600 MHz, CDCl3)
d
7.30 (t, J¼7.7 Hz, 2H, H-ar), 7.21 (t, J¼7.7 Hz, 1H,
H-ar), 7.21 (d, J¼7.7 Hz, 2H, H-ar), 7.12 (d, J¼8.6 Hz, 2H, H-ar),
7.00e7.02 (m, 2H), 6.32 (d, J¼9.9 Hz, 1H, H-A), 5.11 (d, J¼9.9 Hz, 1H,
H-B), 3.61 (dd, J¼7.7, 11.2 Hz, 1H, H-C), 3.25 (t, J¼9.1 Hz, 1H, H-D),
2.50e2.57 (m, 1H, H-E), 2.28e2.40 (m, 1H, H-F), 1.32 (s, 3H, eCH3);
dC (150 MHz, CDCl3) 138.7 (s), 135.1 (s), 131.6 (2d), 127.6 (2d), 127.1
(2d), 126.6 (d), 126.5 (d), 126.3 (d), 125.8 (d), 125.8 (d), 53.2 (d), 41.0
(d), 30.5 (t), 27.5 (q); nmax (evaporated film from CHCl3) 2943, 1452,
698 cmꢁ1; HRMS MHþ, found 245.1346. C19H18 requires 245.1336.
4.5.2. 3-[(E)-1-Methyl-2-phenylethenyl]-5-methyl-4-phenyl tricyclo
[6.4.0.02,5]dodeca-1(8),6,9,11-tetraene (endo-10). Rf (30% CH2Cl2/
petroleum ether) 0.56; colorless oil; UV (EtOH) lmax (log ε) 323
(3.54), 256 (4.24); dH (600 MHz, CDCl3) 7.31e7.35 (m, 3H),
7.26e7.30 (m, 4H), 7.22e7.25 (m, 1H), 7.20 (d, J¼7.3 Hz, 2H),
7.13e7.17 (m, 2H), 7.07 (dd, J¼7.1, 1.6 Hz, 1H), 7.05 (dd, J¼7.1, 1.6 Hz,
1H), 6.36 (d, J¼10.0 Hz, H-A), 6.34 (s, 1H), 5.20 (d, J¼10.0 Hz, 1H, H-
B), 3.60 (d, J¼9.0 Hz, 1H, H-C/H-E), 3.54 (t, J¼9.0 Hz, 1H, H-F), 3.23
0
(d, J¼9.0 Hz, 1H, H-C/H-E), 1.81 (d, J¼0.6 Hz, 3H, ꢁCH3 ), 1.35 (s,
4.3.2. 1-Chloro-2-(2-naphthyl)-1-phenylethane (5). Rf (petroleum
ether) 0.15; colorless crystals; mp 57e59 ꢀC; UV (EtOH) lmax (log ε)
332 (3.36), 318 (3.48), 306 (3.41), 288 (3.69, sh), 278 (3.85), 270
(3.84), 261 (3.73, sh); dH (600 MHz, CDCl3) 7.81 (dd, J¼7.7,1.7 Hz,1H,
H-ar), 7.76 (t, J¼8.6 Hz, 2H, H-ar), 7.60 (s, 1H), 7.44e7.49 (m, 2H),
7.40 (d, J¼7.7 Hz, 2H), 7.30e7.37 (m, 3H), 7.25 (dd, J¼8.6, 1.2 Hz, 1H,
H-ar), 5.15 (t, J¼7.4 Hz, 1H, H-A), 3.55 (dd, J¼7.4, 14.0 Hz, 1H, H-B/H-
C), 3.50 (dd, J¼7.4, 14.0 Hz, 1H, H-B/H-C); dC (150 MHz, CDCl3) 140.6
(s), 134.5 (s), 132.9 (s), 131.9 (s), 128.1 (2d), 127.9 (d), 127.7 (d), 127.4
(d), 127.1 (d), 127.1 (d), 127.0 (d), 126.7 (d), 125.5 (d), 125.1 (2d), 63.5
(d), 46.2 (t); nmax (evaporated film from CHCl3) 2924, 1725, 1453,
697 cmꢁ1; HRMS Mþ, found 266.0853. C18H15Cl requires 266.0857.
ꢁCH3); dC (150 MHz, CDCl3) 138.5 (s), 138.0 (s), 137.7 (s), 134.1 (s),
132.1 (d), 131.8 (s), 131.7 (s), 128.5 (d), 127.8 (d, d), 127.7 (d), 127.5
(2d), 127.3 (2d), 127.0 (d), 126.6 (d), 126.5 (d), 126.0 (d), 125.8 (d),
125.5 (d), 123.9 (d), 57.4 (d), 50.8 (d), 47.1 (d), 27.3 (q), 16.1(q); nmax
(evaporated film from CHCl3) 3023, 1600, 1493, 700 cmꢁ1; HRMS
Mþ, found 362.2035. C28H26 requires 362.2029.
4.5.3. 3-Chloro-1,2-diphenyl-1,2-dihydrophenanthrene (11). Rf (30%
CH2Cl2/petroleum ether) 0.56; colorless crystals; mp 100e102 ꢀC;
UV (EtOH) lmax (log ε) 342 (3.98), 326 (4.07), 312 (3.94), 245 (4.74);
dH (600 MHz, CDCl3) 8.24 (d, J¼7.9 Hz, 1H), 7.82 (d, J¼7.9 Hz, 1H),
7.74 (s, 1H), 7.57e7.62 (m, 2H), 7.50 (t, J¼7.9 Hz, 1H), 7.21 (t,
J¼7.3 Hz, 1H), 7.11e7.16 (m, 3H), 6.99e7.04 (m, 3H), 6.71e6.78 (m,
2H), 5.02 (d, J¼8.0 Hz, H1, 1H), 3.88 (d, J¼8.0 Hz, H2, 1H); dC
(150 MHz, CDCl3) 137.9 (s), 136.0 (s), 135.3 (s), 135.1 (s), 133.1 (s),
132.3 (s), 129.7 (s), 128.5 (s), 128.3 (s), 130.2 (2d), 128.9 (d), 128.1 (d),
127.4 (2d), 127.3 (d), 127.2 (d), 127.0 (d), 126.5 (d), 126.0 (d), 125.5
(d), 125.1 (d), 121.9 (d), 121.4 (d), 54.4 (d), 53.0 (d); nmax (evaporated
film from CHCl3) 2923, 1738, 1452, 698 cmꢁ1; HRMS Mþ, found
365.1095. C26H19Cl requires 365.1091.
4.4. (1E,3E)-1-{o-[(1E,3E)-4-Phenyl-1,3-butadienyl]phenyl}-4-
phenyl-1,3-butadiene (6); (1E,3E)-4-{o-[(1E,3E)-3-Methyl-4-
phenyl-1,3-butadienyl]phenyl}-2-methyl-1-phenyl-1,3-
butadiene (7); (1Z,3E)-4-{o-[(1E,3Z)-3-chloro-4-phenyl-1,3-
butadienyl]phenyl}-2-chloro-1-phenyl-1,3-butadiene (8)13
To
a stirred solution of the triphenylphosphonium salt
(0.005 mol) and the corresponding aldehyde (0.011 mol) in abso-
lute ethanol (100 mL) a solution of sodium ethoxide (0.253 g,
0.011 mol in 15 mL of absolute ethanol) was added dropwise. The
reaction was completed within 3e4 h (usually was left to stand
overnight). After removal of the solvent, the residue was worked up
with water and toluene. The toluene extracts were dried (anhy-
drous MgSO4) and concentrated. The crude reaction mixture was
purified and the isomers of products 6, 7, and 8 were separated by
repeated column chromatography on silica gel using petroleum
ether as the eluent. The first fractions yielded cis,cis-, cis,trans- and
the last fractions trans,trans-isomers.
Single crystal measurements were performed on an Oxford
Diffraction Xcalibur Nova R (microfocus Cu tube) at room temper-
ature [293(2) K]. Program package CrysAlis PRO16 was used for data
reduction. The structure was solved using SHELXS97 and refined
with SHELXL97.17 The model was refined using the full-matrix least
squares refinement; all non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were modeled as riding entities using
the AFIX command. Due to the disorder (see Section 2.1) and the
small size of the crystal (Fig. SI, Table 1) data for 5 were of some-
what inferior quality. Molecular geometry calculations were per-
formed by PLATON,18 and molecular graphics were prepared using

D. Vuk et al. / Tetrahedron 70 (2014) 886e891
891
ORTEP-3,14 and CCDC-Mercury.19 Crystallographic and refinement
data for the structures reported in this paper are shown in
Supplementary data, Table 1.
3. Oxaelectrocyclization: Fotiadou, A. D.; Zografos, A. L. Org. Lett. 2012, 16,
5664e5667.
4. Azaelectrocyclization: Cheng, X.; Waters, S. P. Org. Lett. 2013, 15, 4226e4229.
5. Kim, K.; Lauher, J. W.; Parker, K. A. Org. Lett. 2012, 14, 138e141.
6. Sofiyev, V.; Navarro, G.; Trauner, D. Org. Lett. 2008, 19, 149e152.
7. Sharma, P.; Ritson, D. J.; Burnley, J.; Moses, J. E. Chem. Commun. 2011,
10605e10607.
4.5.3.1. Computational details. All computational studies re-
ported in this work were performed using the Gaussian09 pro-
gram,20 implemented on dual core Opteron 240 personal computer
under Linux operating system and computer cluster Isabella at the
Computing center of the University of Zagreb within the density
functional theory (DFT) framework. Both B3LYP and M06-2X hybrid
functionals were used along with the 6-31G(d) and 6-311þG(d,p)
basis sets. All the stationary points were characterized by harmonic
analysis and activation energies were computed including zero-
point vibrational energy corrections.
ꢁ
8. Shang, M.; Butler, D. N.; Warrener, R. N.; Margetic, D. Tetrahedron Lett. 2013, 54,
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5335e5337; Margetic, D.; Warrener, R. N.; Butler, D. N.; Jin, C.-M. Tetrahedron
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2012, 68, 3306e3318; Margetic, D.; Butler, D. N.; Warrener, R. N.; Murata, Y.
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Tetrahedron 2011, 67, 1580e1588; Margetic, D.; Eckert-Maksic, M.; Troselj, P.;
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Marinic, Z. J. Flour. Chem. 2010, 131, 408e416; Margetic, D.; Warrener, R. N.;
Butler, D. N. Aust. J. Chem. 2000, 53, 959e963.
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9. Kikas, I.; Skoric, I.; Marinic, Z.; Sindler-Kulyk, M. Tetrahedron 2010, 66,
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9405e9414; Vidakovic, D.; Skoric, I.; Horvat, M.; Marinic, Z.; Sindler-Kulyk, M.
Tetrahedron 2008, 64, 3928e3934; Basaric, N.; Marinic, Z.; Sindler-Kulyk, M. J.
Org. Chem. 2006, 71, 9382e9392; Skoric, I.; Flegar, I.; Marinic, Z.; Sindler-Kulyk,
M. Tetrahedron 2006, 62, 7396e7407; Skoric, I.; Basaric, N.; Marinic, Z.; Visn-
jevac, A.; Kojic-Prodic, B.; Sindler-Kulyk, M. Chem.dEur. J. 2005, 11, 543e551;
Butkovic, K.; Basaric, N.; Lovrekovic, K.; Marinic, Z.; Visnjevac, A.; Kojic-Prodic,
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Acknowledgements
B.; Sindler-Kulyk, M. Tetrahedron Lett. 2004, 45, 9057e9060.
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M. J. Photochem. Photobiol. A: Chem. 2009, 207, 190e196.
This research was funded by grants from the Croatian Ministry
of Science, Education and Sports (125-0982933-2926, 098-
0982929-2917, 098-1191344-2943 and 098-0982933-3218). The
computing center of the Zagreb University is thanked for allocating
the time at the computer cluster Isabella.
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11. Skoric, I.; Pavosevic, F.; Marinic, Z.; Sindler, M.; Eckert-Maksic, M.; Vazdar, M.;
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Margetic, D. Org. Biomol. Chem. 2011, 9, 6771e6778.
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cycles 1991, 32, 2357e2363.
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13. Skoric, I.; Kikas, I.; Kovacs, M.; Fodor, L.; Marinic, Z.; Molcanov, K.; Kojic-Prodic,
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B.; Horvath, O. J. Org. Chem. 2011, 21, 8641e8657; Kikas, I.; Horvath, O.; Skoric, I.
J. Mol. Struct. 2013, 1034, 62e68.
14. Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565e566.
15. Cremer, D.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 1354e1358.
Supplementary data
16. CrysAlis PRO; Oxford Diffraction Ltd: U.K., 2007.
17. Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112e122.
18. PLATON: Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7e13.
Supplementary crystallographic data for this paper can be ob-
tained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223 336033; or
deposit@ccdc.cam.ac.uk). CCDC-963633 and 963634 contain the
supplementary crystallographic data for this paper. Supplementary
data related to this article can be found at http://dx.doi.org/10.1016/
j.tet.2013.12.027.
19. Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor, R.;
Towler, M.; ven de Streek, J. J. Appl. Crystallogr. 2006, 39, 453e457.
20. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Na-
katsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng,
G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery,
J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K.
N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.;
Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.;
Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
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