The exo-outstretching effect governing the exclusive stereoselectivity of the intramolecular [2 + 2] photocycloaddition of vinylarenes

Article abstract of DOI:10.1002/hlca.200590103

Intramolecular [2 + 2] photocycloaddition of a dimeric vinylpyridine 1 through a singlet-excited species efficiently formed the corresponding syn- and anti-pyridinophanes 2 and 3. The syn-isomers were elucidated spectroscopically and by X-ray crystallography as exo,syn-configured. The high selectivity under formation of exo,syn-2 was thoroughly investigated. Consequently, an exo-outstretching effect, which is observed around the periphery of a face-to-face-oriented system between two aromatic nuclei as a transition state, on cyclobutane ring formation was discovered for the first time.

Full text of DOI:10.1002/hlca.200590103

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The exo-Outstretching Effect Governing the Exclusive Stereoselectivity of the
Intramolecular [2 ‡ 2] Photocycloaddition of Vinylarenes
by Jun Nishimura*, Takashi Funaki, Noriaki Saito, Seiichi Inokuma, Yosuke Nakamura, So Tajima,
Toshitada Yoshihara, and Seiji Tobita
Department of Nano-Material Systems, Graduate School of Engineering, and Department of Chemistry,
Faculty of Engineering, Gunma University, Kiryu 376-8515, Japan
(phone: ‡ 81(277)301310; fax: ‡ 81(277)301314; e-mail: nisimura@chem.gunma-u.ac.jp)
Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday
Intramolecular [2 ‡ 2] photocycloaddition of a dimeric vinylpyridine 1 through a singlet-excited species
efficiently formed the corresponding syn- and anti-pyridinophanes 2 and 3. The syn-isomers were elucidated
spectroscopically and by X-ray crystallography as exo,syn-configured. The high selectivity under formation of
exo,syn-2 was thoroughly investigated. Consequently, an exo-outstretching effect, which is observed around the
periphery of a face-to-face-oriented system between two aromatic nuclei as a transition state, on cyclobutane
ring formation was discovered for the first time.
Introduction. ± Rolf Huisgen is one of the pioneers working on reaction
mechanisms with the most-sophisticated methods and spectroscopic techniques
available in the middle of the last century. His interest has covered reactions
proceeding under non-catalytic, catalytic, thermal or photochemical conditions etc.1).
His careful and clear formulation of scientific arguments and statements have attracted
many scientists through the generations. Moreover, in carrying out kinetics and
mechanistic works, his scrutiny in the analysis of results is always understood by his co-
workers, by suggesting to them not to ignore any discrepancy, tiny though it may be,
which is believed to lead to some new significant discovery. In this paper, we are
pleased to present some mechanistic aspects of photochemical reactions arising from a
discrepancy observed at the preparative level.
Vinylarenes such as styrene have attracted much interest from photochemical and
photophysical points of view [2]. Since our discovery of the facile intramolecular [2 ‡ 2]
photocycloaddition of vinylarenes towards cyclophanes in 1986, some stereoselective
product formation was often observed on the cyclobutane orientation, although almost
all cases show the general selectivity such as exclusive cis-cyclobutane ring formation
and selective syn-arrangement of the aromatic rings. In fact, several substrates like 1-
vinylnaphthalene derivatives formed an exo-directed cyclobutane ring exclusively,
because they have vinyl groups that are entirely exo-directed due to the severe steric
interaction between peri-H-atoms and vinyl CH2 H-atoms [3]. The selectivity can be
rationalized by the so-called NEER2) principle [4].
1
)
See [1]. Many examples can be found in the more than 580 papers published, a number still continuously
growing.
2
)
Principle of ꢁnone equilibration of excited rotamersꢂ (NEER).
ꢀ 2005 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta ± Vol. 88 (2005)
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On the other hand, substrates like 2-vinylthiophene derivatives also show high
stereoselectivities, affording only the exo,syn-isomer [5]. They have almost no
preference in the vinyl group conformation at the ground state, being different from
the above cases. Since the yields of ꢁthiophenophanesꢂ were not high enough, no effort
has been made to investigate this transformation from a mechanistic point of view.
Recently, however, we found another example of a highly stereoselective formation
of cyclophane adducts from flexible vinylarene derivatives in nearly quantitative yield
[6]. This result prompted us to examine the underlying stereochemical effect. In this
paper, we report and discuss the stereoselectivities observed in intramolecular
photoreactions of compounds having 3-vinylpyridine, styrene, and 3-methylstyrene
moieties.
Results. ± The dimeric 3-vinylpyridine derivative 1, having a flexible ether linkage,
was prepared from commercially available 2,5-dibromopyridine by a sequence of
nucleophilic aromatic substitution and Stille coupling reactions, as summarized in
Scheme 1. By irradiation under preparative conditions (see Exper. Part), it gave the cis-
isomers [6] exo,syn-2 and anti-3 in 84% yield. Note that in isomer 2, the N-atoms of the
pyridine rings are exo-oriented relative to the cyclobutane ring. No trace of endo,syn-4
was detected in the reaction mixture by analytical HPLC and NMR experiments. The
result is summarized in Scheme 2.
Compound 5 was prepared from 4-bromophenol and 2,5-dibromopyridine
(Scheme 1). By irradiation, it gave only isomer 6 in 78% yield (Scheme 3). Again, as
in 2, the N-atoms of the pyridine rings are in an exo-orientation relative to the formed
cyclobutane ring, and the endo-adduct 7 was not observed.
The dimeric 3-methylstyrene derivative 8 was prepared from commercially
available 4-hydroxy-3-methylphenyl methyl ketone by the sequence shown in Scheme 1
[7] in an overall yield of 66%. By irradiation, it gave also cis-isomers, but the major syn-
adduct was the endo-isomer 9, and not the exo-isomer 10 (side product together with
11; Scheme 4). (Note that the relation of the substituents in 9 and the cyclobutane ring
is entirely different from that of the pyridine N-atoms in 2.)
The total yield of cyclophanes 9 ± 11 was 53%, high enough for mechanistic
considerations under exclusion of artifacts. In contrast, compound 8 gave cyclophanes
in relatively low yields. This can be rationalized by its tendency to polymerize: styrenes
are easily polymerized under irradiation, but vinylpyridines are not.
The structures of compounds 2, 3, 6, and 9 ± 11 were determined thoroughly by
NMR experiments. Products 2, 9, and 10 have Cs symmetry, and, thus, gave rise to
simple spectra. Resolution and CD spectroscopy were applied to determine the chiral
nature of 3 (see Exper. Part). X-Ray crystallography gave definite structural evidence
for the most-important and major products 2, 6, and 9, as shown in Fig. 1 [6]. Between 2
and 3, and among 9, 10, and 11, no isomerization was observed over several months, as
well as for 6 and 7 (not obtained). Therefore, their ratios are not those attained after
any ground-state equilibrium or isomerization. Moreover, prolonged irradiation of the
photo-adducts obtained caused their decomposition, but the isomer ratios remained
constant.
The steric energy dependence on the dihedral angle CArÀCArÀOÀCH2 of 2-
methoxy-5-vinylpyridine (12) and 1-methoxy-2-methyl-4-vinylbenzene (13) as models

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Scheme 1
a) HO(CH2)2OH, NaH, THF/DMF. b) (H2CˆCH)SnBu3, [Pd(PPh3)4], 2,6-di(tert-butyl)-4-methylphenol,
toluene. c) 1. NaOH, THF/H2O; 2. ClCH2CH2OH, THF/H2O. d) 1. NaH, THF; 2. 2,5-dibromopyridine, THF/
‡
DMF. e) TsOCH2CH2OTs, K2CO3, DMF. f) NaBH4, EtOH. g) TsOÀHPy , benzene.
of 1 and 8, respectively, was calculated by the MM2 method. The data are shown in
Fig. 2. According to these data, lone-pairs of the O- and N-atoms in 12, and the Me and
CH2 groups in 13, interact strongly repulsively in a certain dihedral-angle range w, while
they hardly perturb each other around less than 908 (minima). Hence, the aromatic N-
atom and the Me group do determine considerably the conformations in the ground
state, which are naturally reflected in the arrangements of the two aromatic nuclei that
make up the excimer (face-to-face transition state) of the intramolecular reaction. In
Fig. 3, the most-stable conformers are depicted, showing two major factors, w and steric

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Scheme 2
Scheme 3

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Scheme 4
Fig. 1. X-Ray crystal structures (ORTEP) of compounds 2, 6, and 9
interaction between the edges of molecules, which determine the structures of the face-
to-face transition state.
As shown in Table 1, O2 and the triplet quencher 1,3-pentadiene (mixture of cis and
trans) [8] hardly affected the photoreaction of 1, especially the isomer ratio. The latter

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Fig. 2. Dependence of the steric energy on the dihedral angle w (CArÀCArÀOÀC) for a) 2-methoxy-5-
vinylpyridine (12) and b) 1-methoxy-2-methyl-4-vinylbenzene (13) as models for compounds 1 and 8, resp.
would be influenced largely if long-lived triplet-excited species played an important
role in the photoprocess.
It was found that the absorption, fluorescence, and excitation spectra of 1 (Fig. 4)
resemble much those of its half model 12. Yet, the fluorescence lifetime of the former

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Fig. 3. Representations of the more-stable conformers of 12 and 13 as models for the face-to-face transition state
(drawn as half components in syn-arrangement). The exo-outstretching towards the vinyl group is indicated.
Table 1. Yields and Isomers Ratios in the Photoreaction of 1. Conditions: Substrate, 2 mm; solvent, MeCN;
bubbling N2 or O2 for 30 min before reaction; irradiation by a 400-W Hg lamp (Pyrex filter) for 5 min from a
distance of 5 cm.
Condition
TQa)
Yield [%]b)
Ratio 2/3
1
2
3
Total
O2-free
O2-sat.
O2-free
O2-free
O2-free
±
±
59.2
59.1
55.4
55.8
58.8
24.1
24.9
26.3
29.1
25.0
14.2
16.0
16.5
13.6
16.2
97.5
100.0
98.2
98.5
100.0
1 : 0.59
1 : 0.64
1 : 0.63
1 : 0.47
1 : 0.65
1 equiv.
3 equiv.
30 equiv.
a) Triplet quencher (mixture of cis- and trans-1,3-pentadiene). b) Exper. error<Æ 0.6%.
(2.47 ns at 350 nm) was distinctly shorter than for the latter (3.41 ns at 350 nm) in
cyclohexane, when excited at 300 nm. This 28% reduction in lifetime is probably due to
the formation of an intermediate biradical with a singlet-excited vinylpyridine moiety
encountering a ground-state one intramolecularly [9].
Discussion. ± An asynchronous, nonconcerted mechanism is generally accepted for
many vinylarene [2 ‡ 2] photocycloaddition reactions when carried out in nonpolar
media without sensitizers or additives [2f]. Such processes follow a stepwise
mechanism after the formation of a singlet biradical (Scheme 5), instead of a concerted
mechanism along the conservation of orbital symmetry (Woodward ± Hoffmann rule
[10]).
The reactions of 1, 5, and 8 are expected to proceed through a short-lived singlet
biradical (Table 1), formed by the reactions of singlet-excited moieties. Therefore, the

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Fig. 4. Absorption (Abs.), fluorescence (Fluo.), and fluorescence excitation (Exc.) spectra of 1 in MeCN vs.
cyclohexane at 295 K. 2-Methoxy-5-vinylpyridine showed the same spectra (data not shown).
high stereoselectivity must arise from the preferable conformer at the ground state, as
schematically shown in Fig. 5. As mentioned in the Introduction, some nonsymmetric
vinylarenes have preferable conformers with respect to their vinyl groups. Therefore, it
was examined whether 3-vinylpyridine, which is the key structure of 1, has any
conformational preference, together with 2-vinylthiophene. As shown in Table 2, their
conformers have almost the same heat of formation, indicating that these molecules are
not in a preferable conformation at the ground state. In other words, the rotation about
the s-bond between the vinyl and pyridyl groups is not responsible for the stereo-
selectivity observed in this work.
Table 2. PM3-Calculated Heats of Formation (DH0f ) for Conformers of 2-Vinylthiophene and 3-Vinylpyridine
Conformer
DH0f [kcal/mol]
DDH0f [kcal/mol]
endo-2-Vinylthiophene
exo-2-Vinylthiophene
endo-3-Vinylpyridine
exo-3-Vinylpyridine
48.45
47.71
46.53
46.45
0.74
0.08

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Scheme 5. Currently Accepted [2 ‡ 2] Photocycloaddition Mechanism for Styrenes
To the best of our knowledge, excimers of vinylarenes have been observed neither
by fluorescence nor absorption spectroscopy. It is considered that excimer-type
combination of two vinylarene molecules is fast, giving rise to a singlet 1,4-biradical,
even though it has never been detected spectroscopically. Therefore, it is reasonably
assumed that the face-to-face combination of two vinylarene molecules produces a
singlet 1,4-biradical, as long as the stereoelectronic conditions are satisfied. Otherwise,
it cleaves the weak bond and returns to the starting locally excited state or ground state.
Also, once a singlet 1,4-biradical is formed, it ends up close to the cyclobutane ring very
rapidly. In other words, the rate-determining step is the bond-formation between the b-
C-atoms of two vinyl groups, which must be sufficiently close together at the very
moment, i.e., between 1.6 Š (slightly longer than the CÀC single-bond length [11]) and
3.7 Š (the longest distance for the successful [2 ‡ 2] photocycloaddition of cinnamic
acid derivatives in crystalline states [12]). Eventually, the moieties involved in the
intramolecular bond-formation of the short-lived species, like singlet-excited styrenes,
must be well arranged or organized in terms of angle, distance, orientation, etc.
According to the conformational dependence of 1 (see Fig. 2), the aromatic 3-CH
group (opposite to the N-atom) and the CH2 group of the tether restrict the
conformation about the CArÀO bond. The preferable rotamer has a certain range of w
angles, as depicted for 12 and 13 in Fig. 6. In other words, the major initial cofacial syn-
combination has the exo-outstretched structure 12 (or 13), through which 1, 5, and 8
gave exo-2, exo-6, and endo-9 as major products, respectively. Since the isomer 10 was
also obtained as a minor product, the exo-outstretching effect due to the Me ´´´ Me
steric repulsion, as well as the most-favorable conformer with w ˆ 0, is also considered
to occur to a relatively small extent in this system (Scheme 6).
The experimental results obtained can be also rationalized through the concerted
mechanism of the Woodward ± Hoffmann rules, if a small time-lag or a tilted interaction

Helvetica Chimica Acta ± Vol. 88 (2005)
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Fig. 5. Rationalization of the selectivity of photoreactions by means of the NEER2) principle
Fig. 6. Conformational analysis of moieties 12 and 13 upon photocyclization
of two vinyl groups is allowed for their a- and b-positions to contact each other,
respectively, before making the CÀC bonds. Since the electron density is usually higher
in b-position than in a-position of vinylarenes, the exo-outstretched transition state is
also favorable to initiate the reaction in this alternative mechanism.
Conclusions. ± The [2 ‡ 2] photocycloaddition of the dimeric 3-vinylpyridine
derivative 1 gave rise to syn- and anti-pyridinophanes 2 and 3. The major syn-adducts

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Scheme 6. Possible Intramolecular Arrangements for two Vinylarene Moieties
were elucidated as exo-isomers by spectroscopic methods and X-ray crystallography
[6]. The unusually high selectivity was thoroughly secured. An exo-outstretching effect
governs the stereoselectivity of the intramolecular photocycloaddition of vinylarenes.
The effect was confirmed by several other examples, including products from the
analogues 5 and 8.
Experimental Part
1. General. Toluene and THF were purified by distillation over Na after prolonged reflux under N2
atmosphere. Reagent-grade DMF was dried over 4-Š molecular sieves. Reagent-grade MeCN was purified by
distillation after drying over 4-Š molecular sieves. AcOEt, cyclohexane, and penta-1,3-diene (cis/trans mixture)
were purified by distillation. HPLC Analysis was carried out on an ODS column (MeOH). CD Spectra were
recorded in MeCN at r.t. Transient spectra were taken in MeCN and cyclohexane. For laser-flash-photolysis
experiments, the 308-nm pulse of a XeCl excimer laser was used as an excitation source. Fluorescence lifetimes
were determined with a time-correlated single-photon counting fluorometer. NMR Spectra were recorded on a
500-MHz FT-NMR spectrometer; chemical shifts d in ppm rel. to Me4Si as internal standard, coupling constants
J in Hz. Mass spectroscopy: in m/z. Elemental analyses were carried out at the Technical Research Center for
Instrumental Analysis, Gunma University. MM2 and PM3 Calculations were performed with MOPAC.

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2. Preparation of Starting Materials. 2.1. 2,2'-[1,2-Ethanediylbis(oxy)]bis[5-ethenylpyridine] (1) [6]. General
Procedure (GP 1). A soln. of 2,5-Dibromopyridine and ethane-1,2-diol in THF/DMF was allowed to react with
NaH to afford 2,2'-[1,2-ethanediylbis(oxy)]bis[5-bromopyridine] in 94% yield (Scheme 1). The dibromide was
subjected to Pd0-catalyzed coupling with tributyl(ethenyl)tin to afford the desired dimer 1 in 63% yield [6].
Colorless solid. M.p. 64.8 ± 65.68. 1H-NMR (CDCl3): 8.10 (d, J ˆ 2.5, 2 H); 7.70 (dd, J ˆ 8.5, 2.5, 2 H); 6.78 (d,
J ˆ 8.5, 2 H); 6.66 (dd, J ˆ 17.7, 11.0, 2 H); 5.65 (d, J ˆ 17.7, 2 H); 5.22 (d, J ˆ 11.0, 2 H); 4.67 (s, 4 H).
2.2. 5-Ethenyl-2-[2-(4-ethenylphenoxy)ethoxy]pyridine (5). Prepared in analogy to GP 1. Colorless solid.
M.p. 103.4 ± 104.28. 1H-NMR (CDCl3): 8.11 (d, J ˆ 2.1, 1 H); 7.71 (dd, J ˆ 8.9, 2.1, 1 H); 7.37 (d, J ˆ 8.9, 1 H);
7.34 (d, J ˆ 8.6, 1 H); 6.91 (d, J ˆ 8.9, 1 H); 6.83 (d, J ˆ 8.9, 1 H); 6.78 (d, J ˆ 8.6, 1 H); 6.67 ± 6.62 (m, 2 H);
5.65 (d, J ˆ 17.7, 1 H); 5.62 (dd, J ˆ 17.7, 0.9, 1 H); 5.23 (d, J ˆ 11.0, 1 H); 5.13 (dd, J ˆ 11.0, 0.9, 1 H); 4.69 ± 4.67
‡
‡
(m, 2 H); 4.34 (m, 2 H). HR-EI-MS: 267.1255 (M , C17H17NO2 ; calc. 267.1259).
2.3. 1,1'-[1,2-Ethanediylbis(oxy)]bis[4-ethenyl-2-methylbenzene] (8) [7]. Prepared in analogy to GP 1.
Colorless solid. M.p. 118.0 ± 118.98. 1H-NMR (CDCl3): 7.23 (s, 2 H); 7.20 (d, J ˆ 8.3, 2 H); 6.84 (d, J ˆ 8.3, 2 H);
6.64 (dd, J ˆ 17.7, 11.0, 2 H); 5.61 (d, J ˆ 17.7, 2 H); 5.11 (d, J ˆ 11.0, 2 H); 4.34 (s, 4 H); 2.22 (s, 6 H).
3. Photochemical Cyclization Reactions. 3.1. 10,13-Dioxa-8,15-diazatetracyclo[12.2.2.26,9.02,5]eicosa-
6,8,14,16,17,19-hexaene (2 and 3). General Procedure (GP 2). A 1000-ml flask with a magnetic stirring bar
was loaded with a soln. of 1 (400 mg, 1.5 mmol) in MeCN (750 ml), and through this soln. N2 gas was bubbled for
20 min. Then, the soln. was irradiated with a 400-W high-pressure Hg lamp using a Pyrex filter. The reaction was
monitored by HPLC and TLC. After the disappearance of 1 (ca. 2 h), the mixture was evaporated, and the
crude product was purified by column chromatography (CC) (SiO2, toluene/AcOEt) to afford the syn- and anti-
isomers 2 and 3, resp. Racemic (Æ)-3 was resolved by means of chiral HPLC (Chiralcel OD; i-PrOH) into its
antipodes.
Data of 2 (syn-isomer). Isolated yield: 42%. Colorless solid. M.p. 119.4 ± 120.18. 1H-NMR (CDCl3): 7.28 (d,
J ˆ 2.5, 2 H); 6.94 (dd, J ˆ 8.3, 2.5, 2 H); 6.33 (d, J ˆ 8.3, 2 H); 5.17, 5.14 (q-like AB system, J ˆ 6.3 each, 2 H);
4.42, 4,39 (q-like AB system, J ˆ 6.3 each, 2 H); 4.12 ± 4.07 (m, 2 H); 2.54 ± 2.48 (m, 2 H); 2.42 ± 2.36 (m, 2 H).
Anal. calc. for C16H16N2O2: C 71.62, H 6.01, N 10.44; found: C 71.61, H 6.01, N 10.35. X-Ray crystallography: see
Sect. 5 and Fig. 1.
Data of 3 (anti-isomer)3). Isolated yield: 42%. Colorless solid. M.p. 110.0 ± 111.08. [a]2D5 ˆ ‡197.4 and
À 199.6 (c ˆ 0.1, MeCN) for (‡)- and (À)-3, resp. CD: Two perfectly symmetrical curves for the antipodes were
observed. 1H-NMR (CDCl3): 7.42 (dd, J ˆ 8.5, 2.5, 1 H); 6.99 (dd, J ˆ 8.5, 2.5, 1 H); 6.84 (d, J ˆ 2.5, 1 H); 6.69
(d, J ˆ 8.5, 1 H); 6.58 (d, J ˆ 2.5, 1 H); 6.54 (d, J ˆ 8.5, 1 H); 5.55 ± 5.44 (m, 2 H); 4.16 ± 4.08 (m, 3 H); 3.88 ±
3.84 (m, 1 H); 2.65 ± 2.51 (m, 2 H); 2.30 ± 2.21 (m, 2 H). Anal. calc. for C16H16N2O2: C 71.62, H 6.01, N 10.44;
found: C 71.92, H 6.20, N 10.34.
3.2. 10,13-Dioxa-8-azatetracyclo[12.2.2.26,9.02,5]eicosa-6,8,14,16,17,19-hexaene (6). Prepared according to
GP 2. Isolated yield: 78%. Colorless solid. M.p. 126.4 ± 127.28. 1H-NMR (CDCl3): 7.26 (dd, J ˆ 7.9, 2.1, 1 H);
6.88 (d, J ˆ 2.1, 1 H); 6.72 (dd, J ˆ 8.5, 2.4, 1 H); 6.63 (dd, J ˆ 8.5, 2.4, 1 H); 6.43 (d, J ˆ 7.9, 1 H); 6.41 (dd, J ˆ 8.5,
2.4, 1 H); 6.28 (dd, J ˆ 8.5, 2.4, 1 H); 5.49 (dd, J ˆ 14.7, 5.8, 1 H); 4.63 (dd, J ˆ 14.7, 5.8, 1 H); 4.40 (dd, J ˆ 14.7,
5.8, 1 H); 4.17 ± 4.11 (m, 2 H); 3.99 ± 3.97 (m, 1 H); 2.62 ± 2.49 (m, 2 H); 2.36 ± 2.29 (m, 2 H). HR-EI-MS:
‡
‡
267.1259 (M , C17H17NO2 ; calc. 267.1259). X-Ray crystallography: see Sect. 5 and Fig. 1.
3.3. 8,15-Dimethyl-10,13-dioxatetracyclo[12.2.2.26,9.02,5]eicosa-6,8,14,16,17,19-hexaene (isomers 9 ± 11). Pre-
pared according to GP 2.
Data of cis,endo,syn-Isomer 9. Isolated yield: 27%. Colorless solid. M.p. 176.0 ± 176.98. 1H-NMR (CDCl3):
6.64 (s, 2 H); 6.43 (d, J ˆ 8.2, 2 H); 6.31 (d, J ˆ 8.2, 2 H); 4.34 (s, 4 H); 4.10 ± 4.08 (m, 2 H); 2.49 (m, 4 H); 2.18
(s, 6 H). Anal. calc. for C20H22O2: C 81.60, H 7.53; found: C 81.48, H 7.60. X-Ray crystallography: see Sect. 5 and
Fig. 1.
Data of cis,exo,syn-Isomer 10. Isolated yield: 3%. Colorless solid. 1H-NMR (CDCl3): 6.72 (d, J ˆ 8.2, 2 H);
6.54 (d, J ˆ 8.2, 2 H); 6.28 (s, 2 H); 4.34 (s, 4 H); 4.06 ± 4.03 (m, 2 H); 2.49 (m, 4 H); 2.10 (s, 6 H). HR-EI-MS:
‡
‡
294.1630 (M , C20H22O2 ; calc. 294.1620).
Data of cis,anti-Isomer 11. Isolated yield: 29%. Colorless solid. M.p. 101.3 ± 102.18. 1H-NMR (CDCl3): 6.72
(d, J ˆ 2.1, 1 H); 6.51 (d, J ˆ 8.3, 1 H); 6.46 (dd, J ˆ 8.3, 2.1, 1 H); 6.40 (d, J ˆ 8.3, 1 H); 6.33 (d, J ˆ 2.1, 1 H);
6.08 (dd, J ˆ 8.3, 2.1, 1 H); 4.43 ± 4.41 (m, 2 H); 4.36 ± 4.28 (m, 2 H); 4.11 ± 4.08 (m, 1 H); 4.06 ± 3.95 (m, 1 H);
2.56 ± 2.37 (m, 4 H); 2.16 (s, 3 H); 2.06 (s, 3 H). Anal. calc. for C20H22O2: C 81.60, H 7.53; found: C 81.72, H 7.70.
3
)
For the X-ray crystal structure of a homologue of 3, see [6].

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4. Photoreaction in the Presence of a Triplet Quencher. Into a 50-ml Pyrex test tube were placed compound 1
(5 mg, 20 mmol) and a defined amount of penta-1,3-diene (cis/trans mixture) dissolved in O2-free MeCN (10 ml)
(Table 1). The test tube was set at a carousel having a water-cooling lamp socket, and irradiated for 5 min from a
distance of 3 cm by a 400-W high-pressure Hg lamp. The mixture was then kept in the dark and analyzed by
HPLC. The amounts of remaining starting material 1 and products were obtained from calibration curves.
Related experiments were run similarly.
5. X-Ray Crystal-Structure Analyses. 5.1. Crystal Growth. A soln. of 2 (5.0 mg) was prepared in acetone
(0.1 ml) at 508, and then allowed to stand at r.t. for 1 d. This resulted in transparent, prismatic crystals. For
compound 6 (10.0 mg), transparent, prismatic crystals were obtained from a MeOH soln. (0.5 ml) by slow
evaporation of the solvent under N2 atmosphere. In the case of 10 (10.0 mg), mica-like, transparent crystals were
obtained from a binary soln. in MeOH (0.3 ml) and CH2Cl2 (0.1 ml) at r.t. by slow evaporation under N2
atmosphere.
5.2. Data Collection and Refinement. The crystallographic data were recorded on a Rigaku AFC7S
instrument. The structures were solved by direct methods (Table 3), and expanded using Fourier techniques
(DIRDIF-94 program system). Crystallographic data (excluding structure factors) for compounds 2, 6, and 9
(see Fig. 1) have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary
publication numbers CCDC-258070, -258071, and -258135, resp. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK(fax: ‡ 44-(0)1223-336033; e-mail:
data_request@ccdc.cam.ac.uk; internet: http://www.ccdc.cam.ac.uk/data_request/cif).
Table 3. Crystal Data of Compounds 2, 6, and 9
2
6
9
Formula
Mr
C16H16N2O2
268.31
C17H17NO2
267.33
C20H22O2
294.39
T (8C)
20.0
20.0
20.0
F(000)
568
568
632
Crystal size (mm)
0.20 Â 0.20 Â 0.30
0.20 Â 0.20 Â 0.30
0.20 Â 0.20 Â 0.30
m (mmÀ1
)
0.093
0.086
0.078
Crystal system
Lattice parameters:
a [Š]
b [Š]
c [Š]
b (8)
V (Š3)
Space group
Z
Dcalc. [g/cm3]
2qmax (8)
monoclinic
orthorhombic
monoclinic
7.739(2)
15.377(3)
11.167(3)
105.50(2)
1280.6(5)
P21/c
15.236(5)
7.318(5)
12.137(7)
±
1353(2)
Pca21
4
8.215(2)
11.768(1)
16.464(2)
98.78(1)
1572.8(4)
P21/a
4
4
1.392
55.0
1.312
55.0
1.243
55.0
Refl. collected
Independent refl.
No. of variables
R1 [I > 2s(I)]
wR2 (all data)
Goodness of fit on F2
3279
2945
182
0.071
0.173
1.15
1823
1632
183
0.048
0.129
1.09
4065
3624
200
0.070
0.184
1.17
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Received January 11, 2005