Synthesis, structural characterization and photoisomerization of cyclic stilbenes

Article abstract of DOI:10.1016/j.tet.2012.03.038

Six cyclic stilbene derivatives with hindered free rotation around the C(vinyl)-C(phenyl) single bond were synthesized by McMurry coupling. The torsion angles around the double and the single bond, and the CC bond length were obtained for many of the compounds from their solid-state structures. The photochemical isomerization was subsequently investigated for all derivatives under various conditions. The parent 1-(1-tetralinylidene)tetralin underwent efficient oxidative electrocyclization. The 2,2,2′,2′- tetramethylated analogue was resistant towards photooxidation, however, its cis-isomer thermally reisomerized to the more stable trans-isomer.

Full text of DOI:10.1016/j.tet.2012.03.038

Tetrahedron 68 (2012) 4048e4056
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis, structural characterization and photoisomerization of cyclic stilbenes
a
,
b
c
b
d
e
Michael Oelgem o€ ller , Rudolf Frank , Peter Lemmen , Dieter Lenoir , Johann Lex , Yoshihisa Inoue
*
a
James Cook University, School of Pharmacy and Molecular Sciences, Townsville QLD 4811, Australia
b
c
€
Institut f u€ r Okologische Chemie, Helmholtz Zentrum M u€ nchen, D-85758 Oberschleißheim, Germany
Institut f u€ r Organische Chemie und Biochemie, Technische Universit a€ t M u€ nchen, Lichtenbergstr. 4, D-85747 Garching, Germany
Institut f u€ r Organische Chemie, Universit a€ t zu K o€ ln, Greinstr. 4, D-50939 K o€ ln, Germany
d
e
Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Six cyclic stilbene derivatives with hindered free rotation around the C(vinyl)eC(phenyl) single bond
were synthesized by McMurry coupling. The torsion angles around the double and the single bond, and
the C]C bond length were obtained for many of the compounds from their solid-state structures. The
photochemical isomerization was subsequently investigated for all derivatives under various conditions.
The parent 1-(1-tetralinylidene)tetralin underwent efficient oxidative electrocyclization. The 2,2,2 ,2 -
tetramethylated analogue was resistant towards photooxidation, however, its cis-isomer thermally re-
isomerized to the more stable trans-isomer.
Received 30 November 2011
Received in revised form 21 February 2012
Accepted 12 March 2012
Available online 20 March 2012
0
0
Keywords:
McMurry coupling
Stilbene
Ó 2012 Elsevier Ltd. All rights reserved.
Photochemistry
Photoisomerization
Dihydrophenanthrene
1. Introduction
All derivatives were prepared in moderate to good combined
yields of 22e73% by reductive coupling of the corresponding ke-
7
,8
The photochemistry of stilbene and its derivatives has been
tones with low valent titanium (Scheme 1; Table 1). In almost all
cases, mixtures of both geometric isomers were obtained. Analysis
1,2
intensively studied over the last decades. In a number of reports,
four- and five-membered cyclic stilbene derivatives have been
employed as sterically restricted substrates since the torsion
1
of the crude reaction product of 1 by H NMR spectroscopy revealed
that, besides the desired stilbene 1, the corresponding 1,2-diol (not
shown) was formed. Subsequent column chromatography gave
a 1:1 mixture of both isomers of 1. Schroeder and co-workers have
recently described the separation of these isomers via their picric
3
around the single bond is hindered in these systems. However,
4
despite recent applications as light-driven molecular rotors, mo-
lecular force probes5 or model compounds for enantiodifferenti-
6
9
ating photoisomerizations with chiral sensitizers,
no
acid complexes, but since these authors did not obtain pure cis-1,
comprehensive study on the photoinduced isomerization of cyclic
stilbenes has been reported so far. Isolated examples have been
described in the literature but no experimental details were given.
This study aims to fill this gap by describing the synthesis and
structural characterization of several cyclic stilbene derivatives as
well as the results of photoisomerization experiments.
preparative HPLC was applied for their separation instead. The cis-
2
2
. Results and discussions
1
2
3
.1. Synthesis of cyclic stilbenes
Six cyclic compounds were chosen for the present study (Fig. 1)
varying in the size of the attached ring (1e3 and 6) and/or the
substitution pattern (3e5).
4
5
6
*
Corresponding author. Tel.: þ61 7 4781 4543; fax: þ61 7 4781 6078; e-mail
address: michael.oelgemoeller@jcu.edu.au (M. Oelgem o€ ller).
Fig. 1. Selected cyclic stilbene derivatives (only trans-isomers shown).
0
040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.03.038

M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
4049
isomer of 2 was clearly detected in an amount of ca. 5% by TLC and
GCeMS analysis of the crude mixture but is known to be unstable
around the double (
Q
) and the single bond (
4
), and the C]C bond
13
length (r), respectively. The values obtained experimentally from
X-ray crystallographic analyses are summarized in Table 2 and are
compared to those of parent stilbene. Noteworthy, different crystal
modifications have been reported for trans-2 and trans-3.
10
towards ambient light. Consequently, solely the trans-isomer of 2
could be isolated in pure form. GCeMS analysis also revealed the
presence of dimeric products and higher oligomers. Repeated
crystallization of 3 gave two crops of pure trans-3 and a third
fraction of both isomers (ca. 2:1 in favour of cis-3). The cis-isomer of
3
was subsequently isolated from this mixture by preparative HPLC.
Θ
11
Due to the known thermal instability of its cis-isomer, the
0
0
r
2
,2,2 ,2 -tetramethyl substituted substrate 5 gave exclusively its
φ
trans-isomer. The conversion of the McMurry coupling was low
with approximately 50% in this case.
Scheme 2. Structural parameters (shown for trans-isomers).
Table 2
(
)
Experimental torsion angles
Q
and
4
, bond length r of stilbene and 1e6
n
O
R
TiCl4, Zn
ꢀ
ꢀ
ꢀ
Compound
Q
( )
4
( )
r (A)
Ref.
R
(
)
pyridine, THF
a
n
R
trans-Stilbene
cis-Stilbeneb
trans-1
1
d
5
w43
0
4
1, 0
1
1.324 (3)
14a
(
)
n
1.33 (2)
15
1
1.327 (2)
12
n = 1-4; R = H, CH3
cis/trans-1-6
cis-1
3
1, 1
2
0
4, 9
6
12
22
37
0
1.330 (2)
This work
c,d
Scheme 1. Synthesis of 1e6 via McMurry coupling.
trans-2
1.335 (4), 1.349 (4)
1.351 (1)
16
17
trans-2d
d
trans-2
2
1.351 (2)
3e
c
cis-2
21, 20
46
41
40
45
70
65
1.343 (2), 1.340 (2)
1.348 (3)
1.345 (3)
1.359 (4)
1.364 (3)
18
Table 1
Product yields and selectivity for 1e6
trans-3d
cis-3
This work, 19
This work
This work
11
e
Compound
n
R
Yield (%)
Trans/cis ratioa
trans-4
trans-5
trans-6
cis-6
f
1
2
3
4
5
6
1
2
3
3
3
4
H
H
H
22
70
52
73
30
51
50:50
100:0b
1.347 (2)
12
0
1.339 (2)
This work
80:20
52:48
100:0b
70:30
0 0
(5,7,5 ,7 )
(2,2,2 ,2 )
a
CH
CH
H
3
3
At 300 K for the non-disordered molecule.
Gas electron diffraction.
Two crystallographically independent molecules.
Different modifications.
Cis-isomer not obtained in crystalline form.
Cis-isomer thermally unstable and reisomerizes to trans-isomer.
0
0
b
c
d
e
f
a
Determined by 1H NMR spectroscopic analysis of the crude product.
No cis-isomer detected.
b
Only the trans-isomers incorporating the four- and five-
2
.2. Experimental ground-state geometries
membered rings (trans-1 and 2) are planar or almost planar with
respect to their torsion angles and . While the cis-isomer of 1 is
Q
4
For almost all derivatives, recrystallization from n-hexane or n-
fairly planar, the corresponding cis-2 shows considerable torsion
pentane gave suitable crystals for X-ray structure analysis. Solely
cis-4 gave a micro-crystalline, cotton like material. The structures of
cis-1, cis- and trans-3, trans-4 and cis-6 are shown in Fig. 2. The
structures of trans-1 and trans-6 were reported by us earlier.
Stilbene derivatives are commonly characterized using three
important structural parameters (Scheme 2): the torsion angles
ꢀ
ꢀ
ꢀ
around the double (
Q
¼4 and 9 ) and the single bond (
4
¼21 and
ꢀ
2
0 ), respectively. All derivatives carrying six- and seven-
membered aliphatic rings (3e6) exhibit a pronounced torsion
around the single bond, whereas the double bond twist is, however,
significant only for the six-membered cyclic compounds 3e5. The
increased flexibility of the aliphatic rings in compounds 3e6 allows
12
2
0
for the population of different conformations. This conforma-
tional flexibility is particularly noticeable in the solid-state struc-
tures of both isomers of 3. While one of the aliphatic six-membered
rings in cis-3 adopts a twist-boat-like conformation, the second one
accepts a boat-like geometry. Both aliphatic rings additionally show
large degrees of disorder in the crystal. Notably, both of the known
solid-state structures of the trans-isomer of 3 are similarly dis-
19
torted due to packing differences in the crystal. The conforma-
tional flexibility in solution is largest for the seven-membered ring
containing stilbenes 7 as noticeable by their broad and unresolved
1
H NMR spectra.
2.3. Photochemical isomerizations
2.3.1. UV/vis spectra of 1e6. The UV/vis spectra of 1e6 in n-hexane
are shown in Fig. 3. With increasing ring size of the stilbenes, the
absorption bands of both cis- and trans-isomers of 2, 3 and 6
steadily shifted to shorter wavelengths due to the decrease in
conjugation between the p-electronic systems of the phenyl rings
and that of the double bond. Interestingly, the incorporation of
Fig. 2. Molecular structures of cis-1, cis-3, trans-3, trans-4 and cis-6.
a four-membered ring drove the main bands of cis- and trans-1 to

4050
M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
(a) 50
conditions (Table 3). Solutions of trans-2 in n-hexane were irradiated
at 300 or 350 nm (ꢁ20 nm) in either Quartz or Duran vessels. Irra-
diations were monitored by GC until photostationary states were
apparently reached. The highest amount of cis-2 of 46.5% was ach-
ieved with UVB light in a Quartz vessel. Subsequent irradiation on
a large preparative scale for 8 h furnished a cis-2 content of 45.6% (by
GC). Careful isolation and purification under exclusion of light gave
two batches of cis-2 in high purities of >98% (by GC).
trans-1
cis-1
trans-2
cis-2
4
3
2
1
0
0
0
0
0
Table 3
Experimental irradiation conditions
Glass
Wavelength (nm)
Time (h)
cis-2 (%)a
Quartz
Quartz
Quartz
Quartz
Duran
350 ꢁ 20
350 ꢁ 20
300 ꢁ 20
300 ꢁ 20
300 ꢁ 20
1
2
1
2
1
3.8
4.0
46.0
46.5
41.3
225
225
225
250
250
250
275
275
275
300
325
350
350
350
375
400
400
400
Wavelength [nm]
(b)
3
2
1
0
0
0
a
trans-3
cis-3
trans-4
cis-4
Determined by GC analysis.
2.3.3. Photoisomerizations of 1e6. Photoinduced isomerizations
were studied for all trans-isomers in n-hexane (Scheme 3). With the
exception of compound 6, irradiations were performed using 254
and 300 nm light. Selected irradiations were additionally conducted
with the corresponding cis-isomers. The experiments were moni-
tored by UV/vis or HPLC analysis, respectively. Selected reactions
were furthermore performed in cyclohexane-d12 in a quartz NMR-
1
tube to allow for monitoring by H NMR spectroscopy.
0
0
300
325
375
( )n
( )n
n(
)
R
Wavelength [nm]
hν
(c)
3
2
1
n-hexane
R
trans-5
trans-6
cis-6
( )n
R
R
trans-1-6
n = 1-4; R = H, CH3
cis-1-6
0
0
0
Scheme 3. Photoisomerization of trans-1e6 in n-hexane.
On irradiation with 254 nm light from a 60 W low-pressure
mercury lamp under nitrogen, the four-membered ring analogue
trans-1 underwent efficient trans/cis isomerization. The major
peaks of trans-1 between 260 and 330 nm steadily decreased and
isosbestic points were observed near 250 and 330 nm (Fig. 4), re-
spectively. Similar results were achieved using a high-pressure
mercury lamp equipped with a 300 nm filter (not shown).
300
325
375
An additional irradiation experiment at 254 nm was performed
with a more concentrated and degassed solution of trans-1 in cy-
clohexane-d12 in a quartz NMR-tube. The progress of the reaction
Wavelength [nm]
Fig. 3. (aec) UV/vis spectra of 1e6 (in n-hexane).
1
was subsequently monitored by H NMR spectroscopy every
3
0 min. The amount of trans-1 dropped gradually until a photosta-
tionary state of 55:45 (trans/cis) was reached after 2.5 h of irradi-
ation. The final reaction mixture showed strong yellow
shorter wavelengths relative to the higher homologue 2, probably
due to the deformation of the central double bond and/or the
hindered conjugation with the phenyl rings. In line with literature
reports, solely the four- and five-membered stilbene analogues 1
and 2 demonstrate vibrational fine structures, which are broadened
a
1
fluorescence and H NMR spectroscopic analysis revealed the
presence of several degradation products (Fig. 5). No attempt was
made to isolate or characterize these products. Since neither isomer
3
21
for cis-2. Remarkably, both cis-isomers of 1 and 2 underwent rapid
of 1 can undergo oxidative electrocyclization, the degradation
photoisomerization during UV/vis measurement. For example,
virtually pure cis-2 was converted to over 90% (by GC) to trans-2
after one scan. Due to their structural freedom and large torsion
products most likely arose from intermolecular cycloaddition re-
actions at this high concentration. Intramolecular H-shifts have
alternatively been reported for the related five-membered ana-
2
2
around the single bond (
absorption bands without any vibrational structure.
4
), the larger derivatives 3e6 gave broad
logue 2.
12
The photoisomerization of trans-2 was likewise investigated in n-
hexane. Using 254 nm light (not shown), the main peaks in the UV-
spectra dropped marginally until a photostationary state was appar-
ently reached after approximately 5 min. An isosbestic point was ob-
served at 342 nm and HPLC analysis confirming that clean
2.3.2. Synthesis of cis-2 through preparative photoisomerization. Since
the cis-isomer of 2 could not be obtained via McMurry coupling, it
was synthesized by preparative photoisomerization instead. Opti-
mization studies were initially conducted on a small scale in
a Rayonet chamber reactor to find the most suitable reaction
3a
photoisomerization had occurred. To prevent undesirable cis-to-trans
reisomerization, an additional reaction was monitored by HPLC. To

M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
4051
1.0
0.8
0.6
0.4
0.2
0.0
(a)
1
1
0
0
.5
.0
.5
.0
0 min
5 min
0
min
1
3
0 min
225
250
275
300
325
350
375
400
2
25
250
275
300
325
350
375
400
Wavelength [nm]
(b)
Wavelength [nm]
1.0
0.8
15
0
min
min
Fig. 4. UV/vis spectral changes of trans-1 on irradiation with 254 nm light in n-hexane
10 min intervals).
Filter
(
1
1
5
0
0
0
0
0
0
.6
.4
.2
.0
225
250
275
300
325
350
375
400
Wavelength [nm]
Fig. 6. UV/vis spectral changes of (a) cis-2 on irradiation with 254 nm light (1 min
intervals) and (b) trans-2 on irradiation with 300 nm light (10 s intervals) in n-hexane.
ring-opening regenerated cis-3 whereas oxidation furnished 7
(Scheme 4). Even under degassed conditions, the formation of 7
could not be suppressed. A similar behaviour has been reported, for
0
0
23
example, for 3,3 ,5,5 -tetramethoxystilbene.
Upon irradiation at 254 nm under degassed conditions, the main
absorption band of trans-3 at 286 nm decreased steadily and
a bathochromic shift was noticeable (Fig. 7a). The characteristically
structured absorption of the cyclization product 7 emerged during
the course of the reaction. Irradiation of trans-3 was additionally
conducted on a preparative scale and the progress was monitored by
HPLC. After approximately 2 h, a trans/cis ratio of 56:44 was reached.
Despite exhaustive purging with argon prior to exposure to light, the
Fig. 5. 1H NMR (400 MHz; cyclohexane-d12) of trans-1 prior to (bottom) and after (top)
irradiation at 254 nm.
allow for sufficient amounts of samples for the monitoring process, the
experiment was conducted on preparative scale. After approximately
2
5 min of irradiation, a constant trans/cis ratio of 77:23 was achieved.
hν
hν
1
The same ratio was determined by H NMR spectroscopy when
a degassed solution of trans-2 was irradiated in cyclohexane-d12 in
a quartz NMR-tube. No other products could be detected by HPLC or H
1
NMR analysis. The cis-isomer of 2 behaved similarly when irradiated at
trans-3
cis-3
hν
254 nm in n-hexane (Fig. 6a). Upon irradiation at 300 nm, the UV-
spectral changes of trans-2 were more pronounced (Fig. 6b). The pho-
tostationary state was reached after 6 min and the trans/cis ratio was
determined by HPLC to be 37:63. An isosbestic point was again ob-
served at 342 nm indicating clean photoisomerization. No degradation
products could be detected by HPLC analysis.
oxidation
The parent six-membered stilbene analogue 3 showed a more
complex photoreactivity. Upon irradiation, the corresponding
hexahydroperylene derivative 7 was obtained under all conditions
examined. The cis-isomer of 3 obviously has a favourable geometry
for electrocyclic ring-closure to the octahydroperylene in-
H
H
7
8
21
termediate 8. The formation of 8 was noticeable from its distinct
Scheme 4. Photoisomerization and oxidative electrocyclization of trans-3.
yellow colour, which gradually faded on standing. Electrocyclic

4052
M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
(
a) 1.0
0
min
0.8
0.6
0.4
0.2
0.0
1
0 min
225
250
275
300
325
350
375
400
Wavelength [nm]
(b)
2.5
2.0
1.5
1.0
0.5
0.0
0 min 15
5 min
1
Filter
1
5
0
0
Wavelength [nm]
Fig. 8. 1H NMR (400 MHz; cyclohexane-d12) of trans-3 prior to (bottom) and after (top)
irradiation at 254 nm.
0
min
were found at 262 and 327 nm, respectively. When a preparative-
scale reaction was monitored by HPLC, a constant trans/cis ratio
of 64:36 was reached after 2 h. The same ratio was determined by
1
0 min
1
H NMR spectroscopic analysis of the reaction mixture. Upon ir-
225
250
275
300
325
350
375
400
radiation at 300 nm, cis-4 showed a similar behaviour. The
Wavelength [nm]
Fig. 7. UV/vis spectral changes of (a) trans-3 on irradiation with 254 nm light (de-
gassed) and (b) trans-3 on irradiation with 300 nm light (aerated) in n-hexane (1 min
intervals).
(a)1.0
0
min
0
0
0
.8
.6
.4
1
10 min
cyclization product 7 was still formed. H NMR spectroscopic anal-
ysis of the crude reaction mixture revealed a composition of 55:41:4
trans-3/cis-3/7). As would be expected, the formation of 7 was more
(
pronounced when cis-3 was used as starting material (not shown).
When trans-3 was irradiated at 300 nm under aerated conditions,
the absorption bands of 7 readily appeared and dominated the UV/
vis spectra (Fig. 7b). The sample was kept in the dark for 15 min prior
to recording to allow for a disappearance of the octahydroperylene
intermediate 8. The hexahydroperylene derivative 7 was sub-
0.2
0.0
24
sequently isolated from this reaction mixture by preparative HPLC
225
250
275
300
325
350
375
400
When the UV/vis was recorded immediately after irradiation, the
characteristic absorption band of 8 could be observed at 454 nm. Its
disappearance was followed by repeated UV/vis scans over a period
of 15 min (shown as inset in Fig. 7b). Oxidative electrocyclization
was again more prominent for the cis-isomer of 3 (not shown).
A non-degassed solution of trans-3 in cyclohexane-d12 was
furthermore irradiated at 254 nm in a quartz NMR-tube and the
Wavelength [nm]
(b)
2.0
1.5
15
0
min
min
Filter
4
1
5
0
0
1
reaction was monitored in 30 min intervals by H NMR spectro-
scopic analysis (Fig. 8). The signals corresponding to trans-3 drop-
ped gradually whereas those of cis-3 increased initially before
reaching a plateau after approximately 5 h. The amount of the ox-
idation product 7 increased almost linearly. After 11 h of irradiation,
a mixture of trans-3/cis-3/7 in a ratio of 63:23:14 was obtained.
Introduction of methyl substituents as sterical blockers, either at
the aromatic rings as in 4 or the aliphatic rings as in 5, completely
suppressed electrocyclization. For example, irradiation of the
1.0
0.5
0.0
225
250
275
300
325
350
375
400
0 0
5
,5 ,7,7 -tetramethyl substituted cis-4 at 254 nm gave selective
Wavelength [nm]
photoisomerization. The absorption band at 293 nm dropped
continuously and shifted to longer wavelengths until a photosta-
tionary state was reached after 10 min (Fig. 9a). Isosbestic points
Fig. 9. UV/vis spectral changes of (a) trans-4 on irradiation with 254 nm light and (b)
cis-4 on irradiation with 300 nm light in n-hexane (1 min intervals).

M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
4053
photostationary state was readily reached after just 3 min of irra-
diation (Fig. 9b). An attempt was made to detect the corresponding
octahydroperylene analogue; however, its characteristic absorption
between 350 and 550 nm could not be detected.
When irradiated at 254 nm in cyclohexane-d12 in a quartz NMR-
tube, trans-4 was efficiently converted into its cis-isomer. After
(a)
1
0
0
0
0
.0
.8
.6
.4
.2
0
min
6
0 min
Wavelength [nm]
7
.5 h, the trans/cis ratio remained constant at 68:32 (Fig. 10).
1
Careful inspection of the H NMR spectrum revealed the presence
of a minor secondary photoproduct. Its structure remained un-
known; however, it may have been formed via H-shift or cycload-
dition reaction.
0
min
0
0
The photoisomerization of the 2,2,2 ,2 -tetramethylated stilbene
6 min
ꢀ
analogue trans-5 with 254 nm light in THF and at ꢂ78 C has been
described earlier.11 The corresponding cis-5 was found unstable at
room temperature but could be detected by low temperature NMR
analysis (Scheme 5).
0.0
225
250
275
300
325
350
375
400
Wavelength [nm]
(b)
hν1
hν2
1
0
0
.0
.9
time1 time2
.8
0
1
2
3
4
5
6
7
8
Cycles
Fig. 11. (a) UV/vis spectral changes of trans-5 on irradiation with 254 nm light (de-
gassed; 1 min intervals) and (b) photochemicalethermal switching of trans-5 on ir-
radiation with 300 nm light (degassed) followed by standing for 1 h in n-hexane.
Due to its weak absorption around 300 nm, the photo-
isomerization of the seven-membered cyclic stilbene 6 was in-
vestigated solely with 254 nm light. When trans-6 was used, the
UV/vis spectra showed only marginal changes (not shown). An
additional, preparative-scale experiment was subsequently moni-
Fig. 10. 1H NMR (400 MHz; cyclohexane-d12) of trans-4 prior to (bottom) and after
(
top) irradiation at 254 nm.
tored by HPLC. After approximately 1 h, a constant trans/cis ratio of
1
77:23 was reached. H NMR spectroscopic analysis of the reaction
mixture confirmed this ratio. Likewise, cis-6 was irradiated at
54 nm in n-hexane. The absorption maximum at 247 nm gradually
2
increased and shifted to shorter wavelengths (Fig. 12). Isosbestic
points were found at 250 and 303 nm, respectively. After 10 min,
the photostationary state was reached.
hν
Δ
0
min
trans-5
cis-5
1.0
0.5
0.0
Scheme 5. Photochemical and thermal isomerization of trans-5.
1
0 min
When irradiated at 254 nm in n-hexane, the absorption maxi-
mum at 308 nm decreased in intensity and shifted to shorter
wavelengths. Isosbestic points were detected at 247 and 352 nm,
respectively, suggesting clean isomerization. After approximately
6
min, a seemingly constant absorption was obtained (Fig. 11a). The
thermal reisomerization was subsequently monitored and was
found completed after 1 h (shown as inset in Fig. 11a). An analogous
irradiation experiment at 300 nm gave similar results. The photo-
ethermal isomerization cycle was repeated seven times with only
marginal changes in the absorption of the UV/vis (Fig. 11b). Com-
225
250
275
300
325
350
375
400
Wavelength [nm]
2
5
pound trans-5 thus functions as a photo-thermalchemical switch.
Subsequent HPLC analysis suggested the formation of a secondary
Fig. 12. UV/vis spectral changes of trans-6 on irradiation with 254 nm light in n-
side-product in a small amount (<5%).
hexane (1 min intervals).

4054
M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
3
. Conclusion
d
¼3.71 (s, 4H, CH
2
), 7.17e7.34 (2m, 8H, Harom.). 13C NMR (100 MHz;
), 119.2 (2d, CH), 122.7 (2d, CH), 127.4 (2s,
CDCl ):
3
d
¼36.6 (2t, CH
2
The syntheses and photoisomerizations of six cyclic stilbene
Cq), 127.5 (2d, CH), 127.9 (2d, CH), 144.6 (2s, Cq), 144.8 (2s, Cq). MS
þ
analogues have been realized. The four-, five- and seven-membered
derivatives underwent efficient photoisomerization reactions. The
photochemistry of the six-membered ring compounds with and
without methyl substituents was more diverse. While the parent 1-
(EI, 70 eV): m/z (%)¼204 (M , 71), 203 (100), 202 (90), 201 (18), 101
ꢀ
ꢂ1
(10). IR (gas, 200 C):
n
max (cm )¼3081, 2937, 1599, 1452, 743.
Calcd for C16H12: C 94.07, H 5.92; found: C 93.75, H 6.23%. Crystal
data (from n-hexane): monoclinic, space group P2
1
/c, a¼5.320(1),
ꢀ
ꢀ
ꢀ
3
(
1-tetralinylidene)tetralin underwent rapid oxidative electro-
b¼9.566(1), c¼20.937(1) A,
radiation, 2317 reflections measured, 2018 reflections with
(I), R
b
¼90.48(1) , V¼1065.5(2) A ; Z¼4; Mo
cyclization under all conditions examined, substitution at the aro-
matic or aliphatic ring prevented electrocyclization and restored
clean photoisomerization. The cis-2,2,2 ,2 -tetramethylated ana-
Ka
I>2
s
1
¼0.036, wR ¼0.089. The crystallographic data have been
2
0
0
deposited with Cambridge Crystallographic Data Center as sup-
plementary publication no. CCDC-852805.
logue showed rapid thermal reisomerization.
4
4
. Experimental section
4.2.2. 1-(1-Indanylidene)indane (2).7a McMurry coupling (using
17.5 mmol 1-indanone) and subsequent recrystallization from
.1. General
methanol gave 1.44 g (70%) of trans-2 as yellowish plates. Although
cis-2 could be clearly detected by TLC and GCeMS analysis in
amounts of ca. 5%, all attempts to isolate it in pure form failed.
Column chromatography was carried out by using Merck or Wako
ꢀ
1
Silica gel 60 (0.015e0.040 mm). Analytical HPLC analysis was per-
formed on a Hitachi LaChrom or JASCO Gulliver instrument and
preparative HPLC on a JAI LC-908, respectively. Melting points were
determined on a B u€ chi SMP-20 melting point apparatus and are
trans-2: yellowish plates. Mp 138e140 C. H NMR (400 MHz;
CDCl ): ), 7.10e7.27 (br m, 6H, Harom.), 7.55 (d,
¼3.09 (m, 8H, CH
2H, Harom.). C NMR (100 MHz; DMSO-d ): ), 30.8
¼30.0 (2t, CH
(2t, CH ), 123.6 (2d, CH), 124.2 (2d, CH), 125.7 (2d, CH), 126.2 (2d,
3
d
2
13
6
d
2
2
uncorrected. NMR spectrawere recorded ona BrukerAC 400 ora JEOL
EX 400 (400 and 100 MHz), respectively, using the solvent residual
CH), 134.3 (2s, Cq), 142.0 (2s, Cq), 146.0 (2s, CH). MS (EI, 70 eV): m/z
þ
(%)¼232 (M , 100), 217 (68), 202 (23), 117 (56). IR (KBr):
n
max
ꢂ1
peakasinternal standard. The chemical shifts (d) are given inparts per
(cm )¼3100, 3060, 3020, 2920, 1600, 1475, 770, 760, 720. Calcd for
million; coupling constants (J) in hertz. MS spectra were recorded on
a HewlettPackard HP5889A MSEngine(EI, 70 eV) and UV/visspectra
on a Shimadzu UV-3101PC spectrophotometer using n-hexane
18
C H16: C 93.06, H 6.94: found: C 92.83, H 7.09%.
4.2.3. 1-(1-Tetralinylidene)tetralin (3).7b McMurry coupling (using
24 mmol 1-tetralone) and subsequent column chromatography (elu-
ent: n-hexane/1% ethyl acetate) gave 1.05 g (46%) of trans-3 and 0.1 g
(6%) of cis-3. The trans/cis ratio of the crude product was determined
(
Kanto Chemical) as solvent. IR spectra were measured on a JASCO FT/
IR-230 or Perkin Elmer 1600 Series FT/IR spectrometer. Elemental
analysis was performed by Fa. Pascher, Andernach, Germany. X-ray
crystallographic analyses were performed on a Rigaku AFC7R or
a Nonius-Kappa-CCD diffractometer. Solvents and reagents were
commercially available and were used without further purification.
1
by H NMR spectroscopic analysis as 80:20. trans-3: colourless solid.
ꢀ
1
Mp (from n-hexane) 143e145 C. H NMR (400 MHz; acetone-d
¼1.84 (q, J¼6.8, 4H, CH ), 2.77 (t, J¼6.8, 4H, CH ), 2.83 (t, J¼6.8, 4H,
CH ), 7.28 (m, 6H, Harom.), 7.47 (m, 2H, Harom.). C NMR (100 MHz;
CDCl ): ), 29.4 (2t, CH ), 29.7 (2t, CH ), 124.7 (2d, CH),
¼24.3 (2t, CH
26.6 (2d, CH), 127.7 (2d, CH), 130.0 (2d, CH), 132.9 (2s, Cq), 138.1 (2s,
6
):
d
2
2
1
3
2
4
.2. Synthesis of stiff stilbenes via McMurry coupling
3
d
2
2
2
1
þ
General procedure: TiCl
4
(0.77 ml, 7 mmol) was added carefully to
Cq),139.7 (2s, Cq). MS (EI, 70 eV): m/z (%)¼260 (M ,100), 232 (20), 231
ꢀ
anhydrous THF (75 ml) at 0 C under nitrogen. The yellow reaction
mixture was stirred for 10 min before zinc powder (1.33 g, 4 mmol)
was added in one portion. To the stirred mixture was added drop-
wise a solution of the corresponding ketone (6 mmol) in THF (25 ml)
and the resulting black slurry was refluxed for 20 h. After cooling
down to room temperature, the reaction mixture was treated with
(23), 217 (21), 215 (19), 202 (13), 131 (15), 130 (41), 129 (33), 128 (25),
117 (11), 116 (13), 115 (25), 91 (14), 77 (6). Calcd for C20H20: C 92.26, H
7.74; found: C 92.05, H 7.85%. Crystal data (from n-pentane): mono-
ꢀ
clinic, space group P2
1
/n, a¼13.101(2), b¼8.291(2), c¼13.488(1) A,
ꢀ
ꢀ
3
b
¼96.233(8) , V¼1456.3(3) A ;Z¼4;MoK
aradiation, 3584 reflections
measured, 2140 reflections with I>3 (I), R
s
1
¼0.055, wR ¼0.041. The
2
7
5 ml of 10% aqueous K
CO
2 3
solution. The black mixture was filtered
crystallographic data have been deposited with Cambridge Crystal-
lographic Data Center as supplementary publication no. CCDC-168613.
and the filtrate was extracted with diethylether (2ꢃ50 ml) and n-
ꢀ
1
hexane (2ꢃ50 ml). The combined organic layers were washed with
cis-3: colourless solid. Mp (from methanol) 113e115 C. H NMR
(400 MHz; CDCl ): ), 2.55 (t, J¼6.8, 4H, CH ),
¼1.84 (q, J¼6.8, 4H, CH
2.71 (t, J¼6.8, 4H, CH ), 6.77 (t, J¼7.6, 2H, Harom.), 6.89 (d, J¼7.6, 2H,
arom.), 7.02 (t, J¼7.6, 2H, Harom.), 7.10 (d, J¼7.6, 2H, Harom.). C NMR
100 MHz; CDCl ): ), 28.3 (2t, CH ), 30.2 (2t, CH ),124.6
4
water and brine, dried over MgSO , and the solvent was evaporated
3
d
2
2
in vacuo. The crude products were purified in small portions by
column chromatography or recrystallization, respectively.
2
13
H
(
3
d¼23.2 (2t, CH
2
2
2
4
.2.1. 1-(1-Benzocyclobutenylidene)benzocyclobutene
(2d, Cq), 126.2 (2d, CH), 127.3 (2d, CH), 130.5 (2d, CH), 131.6 (2s, Cq),
9
26
þ
(1). McMurry coupling (using 6 mmol benzocyclobuten-1-one
)
137.7(2s,Cq),140.3 (2s, Cq). MS(EI, 70 eV):m/z(%)¼260 (M ,100), 232
and subsequent column chromatography (eluent: n-hexane/10%
acetone) gave a ca. 1:1 mixture of trans-1 and cis-1 (0.45 g, 22%) as
a colourless solid. This isomeric mixture was separated by pre-
(21), 231 (23), 217 (24), 215 (18), 202 (12), 131 (17), 130 (44), 129 (32),
128 (25), 117 (10), 116 (14), 115 (26), 91 (13), 77 (5). Calcd for C20
92.26, H, 7.74; found: C 92.01, H 7.70%. Crystal data (from n-hexane):
monoclinic, space group P2
/c, a¼13.507(1), b¼5.529(1),
H20: C
parative HPLC (Chiralcel OD-R column; 20ꢃ250 mm; particle size
1
ꢀ
ꢀ
ꢀ
ꢀ
3
1
0
m
m; eluent: acetonitrile/water 2:1; flow: 9.5 ml/min; 25 C).
c¼19.902(1) A,
3192 reflections measured, 2141 reflections with I>2
wR
b
¼97.37(1) , V¼1474.0(3) A ; Z¼4; Mo K
a
(I), R
radiation,
¼0.069,
ꢀ
1
trans-1: colourless solid. Mp 148e150 C. H NMR (400 MHz;
s
1
13
CDCl
100 MHz; CDCl
27.4 (2s, Cq), 127.5 (2d, CH), 128.1 (2d, CH), 144.9 (2s, Cq), 145.0 (2s,
3
):
d
¼3.80 (s, 4H, CH
2
), 7.11e7.33 (2m, 8H, Harom.). C NMR
2
¼0.208. The crystallographic data have been deposited with
(
1
3
):
d
¼37.6 (2t, CH
2
), 118.8 (2d, CH), 122.7 (2d, CH),
Cambridge Crystallographic Data Center as supplementary publica-
tion no. CCDC-852806.
þ
Cq). MS (EI, 70 eV): m/z (%)¼204 (M , 74), 203 (100), 202 (97), 201
ꢀ
ꢂ1
0
0
(
7
16), 101 (11). IR (gas, 200 C):
51. Calcd for C16
n
max (cm )¼3080, 2937, 1598, 1453,
4.2.4. 5,5 ,7,7 -Tetramethyl-1-(1-tetralinylidene)tetralin (4). McMurry
coupling (using 17.5 mmol 5,7-dimethyl-1-tetralone) and sub-
sequent column chromatography (eluent: n-hexane) gave 1.1 g (40%)
H
12: C 94.07, H 5.92; found: C 93.69, H 6.30%. cis-1:
ꢀ
1
3
colourless solid. Mp 123e125 C. H NMR (400 MHz; CDCl ):

M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
4055
of trans-4 and 0.92 g (33%) of cis-4. The trans/cis ratio of the crude
(12), 216 (7), 215 (9), 202 (8),145 (52),144 (13),143 (19),142 (7),141
(6), 129 (18), 128 (14), 117 (11), 115 (14), 91 (10). Calcd for C22
91.61, H 8.39; found: C 91.83, H 8.17%. Crystal data (from n-hexane):
monoclinic, space group P2
/c, a¼8.429(1), b¼19.465(1),
1
product was determined by H NMR spectroscopic analysis as 55:45.
H24: C
ꢀ
1
trans-4: colourless solid. Mp (from n-hexane) 191e193 C. H NMR
(
(
2
(
400 MHz; CDCl
s, 6H, CH
), 2.53 (t, J¼6.0, 4H, CH
H, Harom.), 6.94 (s, 2H, Harom.). C NMR (100 MHz; CDCl
2q, CH ), 20.7 (2q, CH ), 23.9 (2t, CH ), 26.3 (2t, CH ), 28.6 (2t, CH
3
):
d
¼1.73 (q, J¼6.0, 4H, CH
2
), 2.15 (s, 6H, CH
), 6.84 (s,
):
¼19.7
),
3
), 2.23
1
ꢀ
ꢀ
ꢀ
3
3
2
), 2.65 (t, J¼6.0, 4H, CH
2
c¼10.900(1) A,
b
¼112.27(1) , V¼1655.0(3) A ; Z¼4; Mo K
a
radia-
(I),
13
3
d
tion, 3576 reflections measured, 3046 reflections with I>2s
3
3
2
2
2
R
1
¼0.039, wR ¼0.093. The crystallographic data have been de-
2
1
28.6 (2d, CH), 129.3 (2d, CH), 131.7 (2s, Cq), 132.8 (2s, Cq), 134.4 (2s,
posited with Cambridge Crystallographic Data Center as supple-
mentary publication no. CCDC-852807.
ꢂ1
Cq), 134.5 (2s, Cq), 137.6 (2s, Cq). IR (NaCl):
2
8
n
max (cm )¼2982, 2922,
858, 2820, 1470, 1439, 1190, 874, 852. Calcd for C24H28: C 91.08, H
.92; found: C 90.91, H 9.02%. Crystal data (from n-hexane): mono-
4.3. Synthesis of cis-2 by preparative irradiation
ꢀ
clinic, space group C2/c, a¼18.768(1), b¼4.976(1), c¼20.051(1) A,
ꢀ
ꢀ
3
b
¼91.91(1) , V¼1871.5(4) A ; Z¼4; Mo K
a
radiation, 2033 reflections
4.3.1. Small scale irradiation. Solutions of trans-2 (10 mg in 10 ml n-
hexane) were degassed with nitrogen, and irradiated in Pyrex or
Quartz vessels using a Rayonet Photochemical Chamber Reactors
(Model RPR-100) and various wavelengths. The progress of each
isomerization was monitored by GC analysis.
measured, 1095 reflections with I>2 (I), R
s
1
¼0.065, wR ¼0.121. The
2
crystallographic data have been deposited with Cambridge Crystal-
lographic Data Center as supplementary publication no. CCDC-
ꢀ
8
52808. cis-4: colourless solid. Mp (from n-hexane) 190e192 C.
1
H NMR (400 MHz; CDCl
CH
CH
CDCl
3
):
d
¼1.89 (q, J¼6.4, 4H, CH
2
), 1.93 (s, 6H,
3
), 2.23 (s, 6H, CH
), 6.52 (s, 2H, Harom.), 6.73 (s, 2H, Harom.). C NMR (100 MHz;
): ), 20.7 (2q, CH ), 23.9 (2t, CH ), 26.3 (2t, CH ),
¼19.7 (2q, CH
8.6 (2t, CH ), 128.6 (2d, CH), 129.3 (2d, CH), 131.7 (2s, Cq), 132.8 (2s,
3
), 2.50 (t, J¼6.4, 4H, CH
2
), 2.66 (t, J¼6.4, 4H,
4.3.2. Preparative irradiation. A solution of trans-2 (1.0 g,
4.3 mmol) in n-hexane (1000 ml) was carefully degassed with ni-
trogen and irradiated in a Rayonet Photochemical Chamber Reactor
(Model RPR-100, 300ꢁ20 nm) for 8 h. The content of cis-2 was
determined by GC analysis as 45.6%. The solution was slowly con-
centrated in the dark, and crystalline fractions of pure trans-2 were
repeatedly removed by decantation. After concentration to about
13
2
3
d
3
3
2
2
2
2
Cq), 134.4 (2s, Cq), 134.5 (2s, Cq), 137.6 (2s, Cq). IR (NaCl):
n
max
ꢂ1
(
cm )¼2925, 2857, 1470, 1441, 850. Calcd for C24H28: C 91.08, H
8
.92; found: C 90.92, H 9.04%.
1
00 ml, the mother liquor contained 73% of cis-2 (determined by GC
0
0
4
(
.2.5. 2,2,2 ,2 -Tetramethyl-1-(1-tetralinylidene)tetralin
analysis). After further evaporation to dryness, the solid residue
was purified by flash column chromatography (eluent: n-hexane)
in the dark. cis- and trans-2 were eluted as overlapping zones.
Fractions of 60 mg (99.6% cis-2 by GC), 85 mg (97.6%), 60 mg (86.5%)
and 40 mg (67.7%) were collected. Recrystallization of the first two
fractions from methanol yielded colourless plates in GC-purities of
11
5). McMurry coupling (using 17.5 mmol 2,2-dimethyl-1-
27
tetralone ) and subsequent recrystallization from methanol gave
.83 g (30%) of trans-5 as a colourless solid. cis-2 could not be
detected by TLC or GCeMS analysis, respectively. trans-5: colourless
0
ꢀ
1
solid. Mp 195 C. H NMR (400 MHz; CDCl
s, 6H, CH ), 1.53 (m, 2H, CH ), 1.80 (m, 2H, CH
.10e7.25 (br m, 8H, Harom.). C NMR (100 MHz; CDCl
CH ), 27.4 (2q, CH ), 31.8 (2q, CH ), 39.4 (2s, Cq), 41.8 (2t, CH
3
):
d
¼0.68 (s, 6H, CH
), 2.86 (m, 4H, CH
):
¼26.7 (2t,
),124.2
3
),1.10
ꢀ
(
7
3
2
2
2
),
99.7% and 98.7%, respectively. cis-2: colourless plates. Mp 69.5 C.
1
3
1
3
d
H NMR (500 MHz; CDCl
3
):
d
2 2
¼2.81 (m, 4H, CH ), 3.00 (m, 4H, CH ),
2
3
3
2
7.15 (m, 4H, Harom.), 7.29 (d, J¼6.9, 2H, Harom.), 8.07 (d, J¼6.9, 2H,
13
(
1
3
(
2d, CH), 126.9 (2d, CH), 127.2 (2d, CH), 133.0 (2d, CH), 137.4 (2s, Cq),
3 2 2
Harom.). C NMR (90 MHz; CDCl ): 30.7 (2t, CH ), 34.8 (2t, CH ),
þ
41.0 (2s, Cq), 143.8 (2s, Cq). MS (EI, 70 eV): m/z (%)¼316 (M , 100),
123.3 (2d, CH), 125.2 (2d, CH), 125.5 (2d, CH), 127.2 (2d, CH), 135.1
01 (21), 260 (24), 246 (13), 245 (54), 232 (21), 231 (16), 225 (42), 217
(2s, Cq), 140.6 (2s, Cq), 148.3 (2s, CH). MS (EI, 70 eV): m/z (%)¼232
þ
21), 202 (30),158 (42),143 (75),131 (42),105 (32), 91 (22). IR (KBr):
(M , 100), 217 (68), 215 (42), 202 (33), 117 (80), 115 (66). Calcd for
ꢂ1
n
max (cm )¼3060, 3000, 2840, 1595, 1570, 1490, 1430, 1390, 1370,
C
18
H16: C 93.06, H 6.94: found: C 92.95, H 7.01%.
7
60. Calcd for C24
H28: C 91.08, H 8.92; found: C 89.90, H 8.92%.
4.4. Isolation of 7 by preparative irradiation
4
.2.6. 1-(1-Benzocycloheptenylidene)benzocycloheptene
12
(6). McMurry coupling (using 24 mmol 1-benzosuberone) and
A solution of trans-3 (42.8 mg, 0.16 mmol) in n-hexane (20 ml)
subsequent column chromatography (eluent: n-hexane/5% ace-
tone) gave 0.92 g (27%) of trans-6 and 0.83 g (24%) of cis-6. The
trans/cis ratio of the crude product was determined by H NMR
was irradiated in a quartz vessel with a high-pressure mercury
lamp EHB-300 (Eikosha) for 20 h while exposed to air. The solution
was concentrated to dryness and the residue was purified by pre-
parative HPLC (Sumichiral OA-3300 column; 20ꢃ250 mm; particle
1
spectroscopic analysis as 70:30. trans-6: colourless solid. Mp
ꢀ
1
1
79e180 C. H NMR (400 MHz; THF-d
8
; 253 K):
), 1.85e1.95 (br m, 4H, CH
), 3.0 (br dt, 2H, CH ), 7.10e7.25 (m, 6H,
arom.), 7.38 (d, 2H, Harom.). C NMR (100 MHz; D CNO ; 363 K):
), 30.4 (2t, CH ), 32.8 (2t, CH ), 36.0 (2t, CH ), 125.8
d
¼1.35 (br quintet,
size 5
mm; eluent: n-hexane/i-PrOH 200:1; flow: 5.0/9.0 ml/min;
ꢀ
4
H, CH
2
), 1.60e1.78 (br m, 2H, CH
.65e2.69 (m, 4H, CH
2
2
),
25 C; detection: UV 254 nm). Drying in vacuo gave 2 mg (5%) of
2
H
d
2
2
1,2,3,10,11,12-hexahydroperylene 7 as a slightly yellowish solid.
13
24
ꢀ
1
3
2
Compound 7: mp 190e192 C. H NMR (400 MHz; CDCl
(m, 4H, CH ), 3.09 (m, 8H, CH
), 7.31 (d, J¼8.4, 2H, Harom), 7.44 (dd,
J¼8.4, 2H, Harom), 8.51 (d, J¼8.4, 2H, Harom.). UV/vis (n-hexane):
max¼256, 264, 285, 297, 310 nm.
3
):
d
¼2.09
¼27.9 (2t, CH
2
2
2
2
2
2
(
2d, CH), 126.5 (2d, CH), 128.9 (2d, CH), 128.6 (2d, CH), 138.6 (2s,
þ
Cq), 141.1 (2s, Cq), 143.7 (2s, Cq). MS (EI, 70 eV): m/z (%)¼288 (M ,
l
1
1
(
8
00), 245 (7), 231 (10), 217 (10), 216 (7), 215 (10), 202 (7), 145 (54),
44 (14), 143 (19), 142 (7), 141 (7), 129 (17), 128 (14), 117 (10), 115
14), 91 (10). Calcd for C22 24: C 91.61, H 8.39; found: C 91.84, H
.15%. cis-6: colourless solid. Mp 149e151 C. H NMR (400 MHz;
THF-d ; 268 K): ), 1.80 (br q, 2H, CH ), 1.98 (br
¼1.50 (br q, 2H, CH
m, 6H, CH ), 2.72 (br dd, 2H, CH ), 3.00 (br t, 4H, CH ), 6.40 (d, 2H,
arom.), 6.64 (d, 2H, Harom.), 6.85 (dt, 2H, Harom.), 6.99 (d, 2H, Harom.).
4.5. Photoisomerizations
H
ꢀ
1
ꢀ
Irradiations were conducted in n-hexane at ca. 25 C using
a ultra high-pressure mercury lamp HX-500 (Wacom) equipped
with a bandpass filter 300 FS 10-50 (Andover) and a water filter
(5 cm quartz cell), or a low-pressure mercury lamp EB-60 (Eikosha).
Unless otherwise noted in the main text, the solutions were care-
fully deaerated with argon prior to irradiation. Quartz cuvettes
(typically 0.05 mmol/l) or tubes (typically 1 mmol/l) were applied
as reaction vessels. The course of the irradiation was followed by
8
d
2
2
2
2
2
H
1
3
C NMR (100 MHz; D
CH ), 31.5 (2t, CH ), 36.0 (2t, CH
3
CNO
2
; 345 K):
d¼28.0 (2t, CH
2
), 29.7 (2t,
2
2
2
), 125.4 (2d, CH), 125.8 (2d, CH),
1
28.1 (2d, CH),128.6 (2d, CH), 137.9 (2s, Cq), 140.4 (2s, Cq), 144.4 (2s,
þ
Cq). MS (EI, 70 eV): m/z (%)¼288 (M , 100), 245 (8), 231 (12), 217

4056
M. Oelgem o€ ller et al. / Tetrahedron 68 (2012) 4048e4056
UV/vis spectroscopy, HPLC analysis or NMR spectroscopy, re-
spectively. In the last case, quartz NMR-tubes and solutions in cy-
clohexane-d12 (typically 50 mmol/l) were used. Where necessary,
UV/vis spectra were recorded after the absorption of the dihy-
drophenanthrene intermediate has disappeared (ca. 15 min after
each irradiation step). For HPLC analysis, the conditions were as
4. (a) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa, B. L.
Nature 1999, 401, 152; (b) Harada, N.; Koumura, N.; Feringa, B. L. J. Am. Chem.
Soc. 1999, 119, 7256.
5
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follows: compound 2: Chiralcel OD column; (4.6ꢃ250 mm); parti-
6. (a) Yoshimura, Y.; Tsujimoto, K. Book of Abstracts, Annual Meeting on Photo-
chemistry, Kyoto, 2002; p 189; (b) Yoshimura, Y. Tsujimoto, K. Book of Abstracts,
ꢀ
cle size 10
m
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79 th National Meeting of the Chemical Society of Japan, Kobe, 2001; p 727.
tection: UV 342 nm. Compound 3: Chiralcel OJ (4.6ꢃ250 mm);
7. (a) Lenoir, D.; Lemmen, P. Chem. Ber. 1980, 113, 3112; (b) Lemmen, P.; Lenoir, D.
ꢀ
particle size 10
m
m; eluent: 100% n-hexane; flow: 1 ml/min; 25 C;
Chem. Ber. 1984, 117, 2300.
8. For reviews, see: (a) Lenoir, D. Synthesis 1989, 883; (b) McMurry, J. Chem. Rev.
detection: UV 303 nm. Compound 4: Chiralcel OD (4.6ꢃ250 mm);
1989, 89, 1513.
particle size 10
m
m; eluent: 100% n-hexane; flow: 0.5 ml/min;
9
. Ernst, D.; Rupp, L.; Frank, R.; K u€ hnle, W.; Lemmen, P.; Lenoir, D.; Schroeder, J.;
Grimm, C.; Steinel, T. Z. Phys. Chem. 2002, 216, 555.
ꢀ
3
0 C; detection: UV 263 nm. Compound 6: Chiralcel OD
1
0. Spiteller, P.; Jovanovic, J.; Spiteller, M. Magn. Reson. Chem. 2003, 41, 475.
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Origane, Y.; Lemmen, P.; Lex, J.; Inoue, Y. J. Chem. Soc., Perkin Trans. 2 2002, 1760.
(
0
4.6ꢃ250 mm); particle size 10
mm; eluent: 100% n-hexane, flow:
.3 ml/min; 30 C; detection: UV 249 nm.
ꢀ
1
1
3. (a) M u€ ller, M.; Hohlneicher, G. J. Am. Chem. Soc. 1990, 112, 1273; (b) Hohlneicher,
G.; M u€ ller, M.; Demmer, M.; Lex, J.; Penn, J. H.; Gan, L.-X.; Loesel, P. D. J. Am.
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The authors wish to thank Yumi Origane, Fumiko Aoki and
Dusan Hesek for technical assistance (HPLC, NMR, X-ray), Prof. J.
Jovanovic and Prof. A. A. Pinkerton for help in the preparation of
this manuscript and Prof. P. Schiess for the donation of a sample of
benzocyclobutenone. Y.I. thanks the Japan Science and Technology
Agency (ERATO) and the Japan Society for the Promotion of Science
1
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(
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