Photoinduced selective transformation of n-acyl-α- dehydro(9-phenanthryl)alaninamides into their cyclization intermediates in a polar protic solvent

Article abstract of DOI:10.3987/COM-12-12449

Irradiation of the title compounds [(Z)-1] in methanol afforded trans- 3,8a-dihydro-2-dibenzo[f,h]quinolinone derivatives (trans-2) in preference to cis-2, all of which serve as precursors of the corresponding 3,4- dihydrodibenzoquinolinones. An analysis of the substituent and solvent effects on the cyclization occuring from the excited-state (E)-1 showed that the reactivity of this (E)-isomer is greatly influenced by the steric bulk of the alkyl group attached to the N′-amide nitrogen of the starting 1. In addition, this analysis confirmed that the stabilization of the excited-state (E)-1 (having a zwitterionic structure), by hydrogen-bonding and charge-transfer interactions with methanol, is an essential prerequisite for the occurrence of the photocyclization reaction selectively forming trans-2 and cis-2.

Full text of DOI:10.3987/COM-12-12449

HETEROCYCLES, Vol. 85, No. 5, 2012
1053
HETEROCYCLES, Vol. 85, No. 5, 2012, pp. 1053 - 1060. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 14th February, 2012, Accepted, 19th March, 2012, Published online, 27th March, 2012
DOI: 10.3987/COM-12-12449
PHOTOINDUCED SELECTIVE TRANSFORMATION OF N-ACYL-!-
DEHYDRO(9-PHENANTHRYL)ALANINAMIDES
INTO
THEIR
CYCLIZATION INTERMEDIATES IN A POLAR PROTIC SOLVENT
Yuki Kawanishi, Hiroko Yoshiie, Tetsutaro Igarashi, and Tadamitsu
Sakurai*
Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa
University, Kanagawa-ku, Yokohama 221-8686, Japan
Abstract – Irradiation of the title compounds [(Z)-1] in methanol afforded trans-
3,8a-dihydro-2-dibenzo[f,h]quinolinone derivatives (trans-2) in preference to
cis-2, all of which serve as precursors of the corresponding 3,4-
dihydrodibenzoquinolinones. An analysis of the substituent and solvent effects
on the cyclization occuring from the excited-state (E)-1 showed that the
reactivity of this (E)-isomer is greatly influenced by the steric bulk of the alkyl
group attached to the N´-amide nitrogen of the starting 1. In addition, this
analysis confirmed that the stabilization of the excited-state (E)-1 (having a
zwitterionic structure), by hydrogen-bonding and charge-transfer interactions
with methanol, is an essential prerequisite for the occurrence of the
photocyclization reaction selectively forming trans-2 and cis-2.
Recently, photoinduced electron transfer (PET) has attracted much attention as a means for activating
organic molecules, owing to the potential for PET-initiated reactions of these molecules, to pave the way
for the construction of various heteroatom-containing ring systems exhibiting potent pharmacological
activities.1
In the course of a systematic study on the PET reactions of N-acyl-!-
dehydroarylalaninamides and N-acyl-!-dehydroarylalanine alkyl esters, we found that these derivatives
readily undergo one-electron reduction in the presence of a tertiary aliphatic amine to enable the highly
selective construction of 3,4-dihydroquinolinone and 4,5-dihydrooxazole ring systems, respectively,
through the corresponding reactive radical ion pair intermediates.2,3 However, all attempts to isolate
and characterize the cyclization intermediate, the presumed precursor of the substituted 3,4-
dihydroquinolinones, were unsuccessful.2 The elucidation of the structure and reactivity of this
hypothetical intermediate would help to unravel the mechanism for the novel PET-initiated cyclization
reaction, leading to these six-membered heterocyclic compounds. In other words, if the cyclized
intermediate could be isolated, its stability and stereochemistry could be determined.

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HETEROCYCLES, Vol. 85, No. 5, 2012
In previous studies, we reported that the selectivity for the formation of the 3,4-
dihydrobenzoquinolinone derivatives is decreased with an increase in the steric bulk of the alkyl group
attached to the N´-amide nitrogen of the starting !-dehydronaphthylalaninamides, the reactivities of
which are much greater in methanol than in acetonitrile.2a In addition, the 1H NMR spectral analysis of
the photocyclization processes for these naphthylalaninamides confirmed that the naphthalene ring is not
able to stabilize a cyclization intermediate enough to permit its isolation. On the basis of the
reasonable assumption that the !-electrons at the 9- and 10-positions on the phenanthrene ring are less
delocalized than those on the naphthalene ring, we introduced a 9-phenanthryl group into the aryl moiety
of the starting !-dehydroarylalaninamides for the purpose of enhancing the stability of the cyclization
intermediate. In this communication, we report the effects of the substituent and solvent on the
photoreactivity of (Z)-N-acyl-!-dehydro(9-phenanthryl)alaninamides [(Z)-1a–d, Chart 1] as well as on
the composition and stability of the cyclization intermediates formed as the precursors of the
corresponding 3,4-dihydroquinolinone derivatives.
H
N
The starting !-dehydrophenanthrylalaninamides of the (Z)-
O
O
R1
configuration were prepared in 50–57% overall yields by a
Knoevenagel-type condensation between 9-phenanthrene-
carbaldehyde and N-benzoylglycine [(Z)-1a–c] or N-
acetylglycine [(Z)-1d] in acetic anhydride containing sodium
acetate, followed by ring-opening reactions of the resulting 2-
substituted (Z)-4-(9-phenanthrylmethylene)-5(4H)-oxazolone,
mediated by methylamine [(Z)-1a,d], benzylamine [(Z)-1b], or
ammonia [(Z)-1c] in THF.2,4 To examine the distribution and
composition of the photoproducts generated under the PET
conditions, a nitrogen-saturated methanol solution (500 mL)
N
H
R2
(Z)-1a (R1 = Me, R2 = Ph
(Z)-1b (R1 = CH2Ph, R2 = Ph
(Z)-1c (R1 = H, R2 = Ph
)
)
)
(Z)-1d (R1 = R2 = Me
)
Chart 1
containing (Z)-1a (4.0 " 10–3 mol dm–3) and triethylamine (TEA, 0.10 mol dm–3) was irradiated at
wavelengths longer than 280 nm for 20 min at room temperature (conversion: 100%, light source: 400
W high-pressure Hg lamp). The resulting reaction mixture was immediately subjected to preparative
thin layer chromatography using silica gel (eluent: EtOAc-hexane or EtOAc-CHCl3). Standard workup
led to the isolation of trans-2a [an equimolar mixture of (3S,8aS)- and (3R,8aR)-3-benzoylamino-3,8a-
dihydro-1-methyl-2(1H)-dibenzo[f,h]quinolinones, 60% yield] and cis-2a [an equimolar mixture of
(3S,8aR)- and (3R,8aS)-3-benzoylamino-3,8a-dihydro-1-methyl-2(1H)-dibenzo[f,h]quinolinones, 18%
yield], along with additional minor products.5 From the mixture of minor products, we isolated 3-
benzoylamino-3,4-dihydro-1-methyl-2(1H)-dibenzo[f,h]quinolinone (3a, <1%) and its structural isomers
by means of reverse-phase preparative HPLC (mobile phase: MeCN-H2O, 3:2 v/v).6 A summary of
these results is shown in Scheme 1. The structures of the above products were confirmed by mass

HETEROCYCLES, Vol. 85, No. 5, 2012
1055
spectral analysis as well as 1H difference NOE (0% for H3-H8a of trans-2a and 5% for H3-H8a of cis-2a
1
1
in CDCl3) and H-1H COSY NMR spectral analyses. In addition to the H NMR and mass spectral
analyses of the structural isomers of dihydrodibenzoquinolinone derivative 3a, HPLC analysis of these
isomers suggested the presence of at least two isomers possessing the same molecular weight as 3a
([M]+ 380.43, MALDI TOF-MS). As shown in Scheme 1, the same product distribution as that of (Z)-
1a was observed for (Z)-1b–d. On the other hand, the (E)-isomer of 1a was isolated from the reaction
mixture obtained by 30-min irradiation of a nitrogen-saturated methanol solution containing (Z)-1a (4.0
" 10–3 mol dm–3) and benzophenone (2.0 " 10–2 mol dm–3) at wavelengths longer than 340 nm (400 W
high-pressure Hg lamp) by a similar workup.5,7
NHCOR2
O
NHCOR2
O
NHCOR2
O
H
H
H
H
H
H
H
h!
(Z)-1a–d
N
N
N
R1
R1
H
R1
H
MeOH
trans-2a–d
cis-2a–d
3a–d
a: R1 = Me, R2 = Ph, b: R1 = CH2Ph, R2 = Ph, c: R1 = H, R2 = Ph, d: R1 = R2 = Me
Scheme 1
As described above, we isolated the predicted cyclization intermediates, trans-2a and cis-2a, as the
major photoproducts, constituting 92% of the product mixture (1H NMR spectral analysis). These
intermediates were stable for a long period of time in the absence of TEA, but slowly converted into 3a
and its isomers in the presence of this amine, thus providing strong evidence that 2a is the precursor of
3a. In addition, this finding led us to predict that the irradiation of a methanol solution of (Z)-1a in the
absence of TEA may yield trans-2a and cis-2a selectively, without forming any other products. Since
these photoproducts underwent negligible decomposition under the irradiation conditions tested, the
progress of the observed photocyclization was monitored by 1H NMR spectrometry. The results shown
in Table 1 confirm that 2a is selectively and efficiently formed, being consistent with our prediction.8
Taking into account that the excited-state (E)-1 is highly reactive and serves as the precursor of 2, we
assumed that the (Z)-isomer adopts a hydrogen bond- and charge-transfer-assisted zwitterion structure
(shown in Figure 1), which is highly susceptible to isomerization to the (E)-isomer. As suggested in
the introductory section, the less delocalized !-electrons at the 9- and 10-positions on the phenanthrene
ring (as compared to !-electrons on the naphthalene ring) are more likely to migrate to the N´-amide
carbonyl oxygen through conjugation in the excited state.
On the basis of the above consideration, we propose a mechanism that explains the observed product

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HETEROCYCLES, Vol. 85, No. 5, 2012
Table 1. Effects of the substituent and solvent on the conversion of 1 and the composition of
each compound obtained by the irradiation of (Z)-1 (1.0
x
10–3 mol dm–3) at room temperature
Composition (%)
(Z)-1
Solvent
MeOH
Irradiation
time (min)
Conversion
(%)
trans-2
0
cis-2
0
3
(Z)-1
100
7
(E)-1
0
1a
0
5
0
87
100
44
98
99
0
0
6
64
23
26
13
28
32
0
0
0
0
4
0
0
10
10
10
10
10
0
0
74
1b
1c
1d
1a
MeOH
MeOH
MeOH
MeCN
23
1
33
1
31
66
0
1
67
79
21
0
Me
O
H
*
H
N
H
N
O
O
R1
R1
O
O
N
H
R2
N
H
R2
H
O
Me
R1 = Me, CH2Ph, H R2 = Ph, Me
Figure 1. Schematic illustration of hydrogen-bonding and charge-transfer
interactions between (Z)-1 and methanol in the excited state
distribution (Scheme 2). The nucleophilic attack of the R1NH nitrogen at the carbocation at the 10
position on the ring, which should occur in competition with the deactivation of the excited-state (E)-1
to the ground-state 1, may afford a zwitterionic cyclization intermediate, and subsequent proton shift and
keto-enol tautomerization readily converts this intermediate into 2. The fact that the cyclization of (E)-
1a in methanol does not occur without irradiation suggests the negligible contribution of a zwitterion
structure in the ground-state (E)-isomer. Since the conjugated carbonyl oxygen in the excited-state
(E)-1 is also potentially capable of participation in the cyclization step, we considered the possibility that
the isolated cyclization intermediate does not possess a lactam ring but an iminolactone ring, as depicted
in Scheme 2. The exocyclic carbon-nitrogen double bond in this intermediate is known to be
susceptible to acid-catalyzed hydrolysis that transforms this double bond into the carbon-oxygen double
bond.9 Thus, the observation that 2a is very stable for a long period of time in 75 vol% DMSO-H2O

HETEROCYCLES, Vol. 85, No. 5, 2012
1057
ꢀ
O
O
R1NH
O
R2
R2
O
NH
NH
O
O
R2
N
H
NHR1
NHR1
hv
hv
(E)-1
(Z)-1
R1 = Me, CH2Ph, H
R2 = Ph, Me
O
O
H
R2
R2
NH
NH
O
O
1) H+ shift
cyclization
NH–R1
N
R1
2) tautomerization
H
H
2
O
H
O
R2
R2
NH
NH
NHR1
N
cyclization
tautomerization
R1
O
H
O
H
Scheme 2
containing 0.5 mol dm–3 HCl at room temperature (1H NMR spectral analysis) rules out the possibility
that the conjugated carbonyl oxygen is involved in the cyclization step described above. It is likely that
the intramolecular hydrogen bond formed between the R2C(=O)NH hydrogen and the N´-amide
carbonyl oxygen enables the exclusive participation of the R1NH nitrogen in this step. Another feature
of the process highlighted by the results given in Table 1 is the preferential formation of trans-2 with
respect to cis-2. To elucidate the reason for this bias, the energy-minimized conformations of trans-2a
and cis-2a were determined by MM2 and PM5 calculations.10 A comparison of the heats of formation
for these two conformations (estimated by the latter calculation), as depicted in Figure 2, established that
the former isomer is thermodynamically more stable than the latter. Therefore, we conclude that the
photochemical conversion of the excited-state (E)-1 into 2 is a thermodynamically controlled process.
Next, we focused our attention on the effects of the substituent and solvent on the photoreactivity of 1 as
well as on the distribution and composition of the photoproducts, which are summarized in Table 1.

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HETEROCYCLES, Vol. 85, No. 5, 2012
trans-2a (!Hf = 19.84 kJ mol–1)
cis-2a (!Hf = 28.70 kJ mol–1)
Figure 2. Energy-minimized conformations of trans-2a and cis-2a
and their heats of formation (!Hf)
While there was a tendency for the photoreactivity to decrease with an increase in the steric bulk of the
substituent R1: 1a (R1 = Me) ! 1c (R1 = H) >> 1b (R1 = CH2Ph), the thermodynamically more stable
trans-isomers were preferentially formed in all derivatives. This increase in the steric bulk may
enhance the energy barrier to the photocyclization of the excited-state (E)-1 but may exert only a minor
effect on the nucleophilicity of the R1NH nitrogen, resulting in an acceleration of the deactivation of this
(E)-isomer. Interestingly, irradiation of (Z)-1c under the same conditions as those for (Z)-1a afforded a
minor amount of the dihydrodibenzoquinolinone derivative 3c in addition to the expected cyclization
intermediates trans-2c and cis-2c. A control experiment showed that these intermediates were less
stable and slowly converted into 3c even in the absence of TEA at room temperature, suggesting that a
decrease in the steric bulk of the substituent R1 lowers the energy barrier for hydrogen migration from
the 8a-position to the 4-position on the dihydrodibenzoquinolinone ring. In contrast, the replacement
of the benzoyl group in 1a by the less bulky and less electron-withdrawing acetyl group 1d exerted a
negligible effect on the photoreactivity of 1, though it decreased the selectivity of trans-2 formation to
some extent (74%–67%). Surprisingly, when a nitrogen-saturated acetonitrile solution of (Z)-1a was
irradiated at wavelengths longer than 280 nm in the absence of TEA, it underwent only
photoisomerization to the (E)-isomer without giving any products (Table 1). Since the polar aprotic
solvent, acetonitrile, almost has the same polarity as that of methanol, this finding provided a strong
piece of evidence, supporting the conclusion that the stabilization of a zwitterion structure [for the
excited-state (E)-1] by hydrogen-bonding and charge-transfer interactions with methanol is a vital
prerequisite for the occurrence of the observed photocyclization reaction.

HETEROCYCLES, Vol. 85, No. 5, 2012
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ACKNOWLEDGMENTS
This research was partly supported by “Scientific Frontier Research Project” from the Ministry of
Education, Culture, Sports, Science and Technology in Japan.
REFERENCES AND NOTES
1. a) ‘Synthetic Organic Photochemistry,’ ed. by W. M. Horspool, Plenum, New York, 1984; b)
‘Synthetic Organic Photochemistry,’ ed. by A. G. Griesbeck and J. Mattay, Marcel Dekker, New
York, 2005; c) A. G. Griesbeck, N. Hoffmann, and K. Warzecha, Acc. Chem. Res., 2007, 40, 128.
2. a) K. Maekawa, T. Igarashi, K. Kubo, and T. Sakurai, Tetrahedron, 2001, 57, 5515; b) T.
Motohashi, K. Maekawa, K. Kubo, T. Igarashi, and T. Sakurai, Heterocycles, 2002, 57, 269; c) K.
Maekawa, H. Kajiwara, Y. Iseya, T. Igarashi, and T. Sakurai, Heterocycles, 2003, 60, 637; d) K.
Maekawa, A. Shinozuka, M. Naito, T. Igarashi, and T. Sakurai, Tetrahedron, 2004, 60, 10293; e) K.
Maekawa, K. Fujita, K. Iizuka, T. Igarashi, and T. Sakurai, Heterocycles, 2005, 65, 117.
3. a) K. Maekawa, T. Sasaki, K. Kubo, T. Igarashi, and T. Sakurai, Tetrahedron Lett., 2004, 45, 3663;
b) K. Maekawa, N. Hishikawa, K. Kubo, T. Igarashi, and T. Sakurai, Tetrahedron, 2007, 63, 11267;
c) Y. Sato, A. Yoshida, T. Igarashi, and T. Sakurai, Heterocycles, 2010, 81, 997.
4. a) Y. S. Rao and R. Filler, Synthesis, 1975, 749; b) K. Noda, Y. Shimohigashi, and N. Izumiya, ‘The
Peptides–Analysis, Synthesis, Biology,’ ed. by E. Gross and J. Meienhofer, Academic Press, New
York, 1983, pp. 285–339.
1
Data for (Z)-1a: mp 236.5–237.0 °C; IR (KBr) "/cm–1 = 3300, 3218, 1633; H NMR (600 MHz,
DMSO-d6) # = 2.76 (3H, d, J = 4.1 Hz), 7.39 (2H, dd, J = 7.6, 7.6 Hz), 7.48 (1H, dd, J = 7.6, 7.6
Hz), 7.58 (1H, dd, J = 7.6, 8.2 Hz), 7.66 (1H, dd, J = 7.6, 8.2 Hz), 7.68 (1H, dd, J = 7.6, 8.2 Hz),
7.72 (1H, dd, J = 7.6, 8.2 Hz), 7.75 (1H, s), 7.75 (1H, d, J = 8.2 Hz), 7.81 (2H, d, J = 7.6 Hz), 7.92
(1H, s), 8.09 (1H, d, J = 8.2 Hz), 8.23 (1H, q, J = 4.1 Hz), 8.80 (1H, d, J = 8.2 Hz), 8.86 (1H, d, J =
13
8.2 Hz), 9.78 (1H, s); C NMR (150 MHz, DMSO-d6) # = 26.4, 122.8, 123.3, 125.1, 126.2, 126.9
(2C), 127.0, 127.1, 127.3, 127.8 (2C), 128.1 (2C), 128.5, 129.7, 129.8, 130.0, 130.2, 130.8, 131.5,
132.9, 133.9, 165.1, 166.3. Anal. Calcd for C25H20N2O2: C, 78.93; H, 5.30; N, 7.36. Found: C, 78.97;
H, 5.22; N, 7.27.
5. Data for trans-2a: mp 204.0–205.0 °C; IR (KBr) "/cm–1 = 3253, 1657; 1H NMR (600 MHz, DMSO-
d6): # = 3.18 (3H, s), 5.20 (1H, ddd, J = 2.5, 5.0, 8.0 Hz), 5.40 (1H, dd, J = 2.0, 5.0 Hz), 5.87 (1H,
dd, J = 2.0, 2.5 Hz), 7.30 (1H, d, J = 7.5 Hz), 7.39–7.52 (7H, m), 7.57 (1H, dd, J = 7.0, 7.5 Hz), 7.88
(1H, d, J = 7.5 Hz), 7.94 (2H, d, J = 7.0 Hz), 7.99 (1H, d, J = 7.5 Hz), 9.05 (1H, d, J = 8.0 Hz); 13C
NMR (150 MHz, DMSO-d6) # = 35.4, 47.2, 62.5, 120.9, 122.8, 123.7, 124.4, 125.3, 127.4 (2C),
127.7,128.0, 128.3 (2C), 128.6, 129.1, 131.4, 132.5, 133.1, 133.4, 133.9, 134.0, 136.1, 166.0, 166.9.
HR EI-MS m/z calcd for C25H20N2O2: 403.1422 [M + Na]+. Found: 403.1421.

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HETEROCYCLES, Vol. 85, No. 5, 2012
Data for cis-2a: mp 120.0–121.0 °C; IR (KBr) "/cm–1 = 3276, 1669; 1H NMR (600 MHz, DMSO-d6)
# = 3.20 (3H, s), 5.24 (1H, ddd, J = 4.0, 4.0, 8.0 Hz), 5.39 (1H, dd, J = 2.0, 4.0 Hz), 5.94 (1H, dd, J =
2.0, 4.0 Hz), 7.34 (1H, dd, J = 5.0, 5.0 Hz), 7.39–7.43 (5H, m), 7.45–7.51 (2H, m), 7.59 (1H, d, J =
7.5 Hz), 7.80 (2H, d, J = 7.5 Hz), 7.85–7.87 (1H, m), 7.99 (1H, d, J = 7.5 Hz), 9.07 (1H, d, J = 8.0
Hz); 13C NMR (150 MHz, DMSO-d6) # = 37.3, 48.4, 62.4, 119.6, 124.0, 124.6, 125.4, 125.5, 127.9,
128.0 (2C), 128.3, 128.7 (2C), 129.0, 129.8, 131.8, 133.1, 133.4, 134.5, 134.6, 134.7, 136.8, 166.6,
167.2. HR EI-MS m/z calcd for C25H20N2O2: 403.1422 [M + Na]+. Found: 403.1365.
Data for (E)-1a: mp 180.0–181.0 °C; IR (KBr) "/cm–1 = 3421, 3253, 1647; 1H NMR (600 MHz,
DMSO-d6) # = 2.45 (3H, d, J = 4.5 Hz), 7.34 (1H, s), 7.56 (2H, dd, J = 7.0, 8.0 Hz), 7.60–7.76 (6H,
m), 7.88 (1H, d, J = 8.0 Hz), 7.93 (1H, q, J = 5.0 Hz), 8.01 (2H, d, J = 7.0 Hz), 8.15 (1H, d, J = 8.0
Hz), 8.81 (1H, d, J = 8.0 Hz), 8.89 (1H, d, J = 8.0 Hz), 10.33 (1H, s); 13C NMR (150 MHz, DMSO-
d6) # = 26.4, 116.6, 123.3, 123.8, 125.9, 126.5, 127.36, 127.39, 127.4, 127.6, 128.4 (2C), 128.9 (2C),
129.0, 129.9, 130.3, 131.2, 131.3, 131.7, 132.4, 134.4, 135.8, 165.5, 165.6. HR EI-MS m/z calcd for
C25H20N2O2: 403.1422 [M + Na]+. Found: 403.1389.
6. Data for 3a: 1H NMR (600 MHz, DMSO-d6) # = 3.36 (1H, dd, J = 14.5, 16.0 Hz), 3.44 (3H, s), 3.69
(1H, dd, J = 6.0, 16.0 Hz), 4.95 (1H, ddd, J = 6.0, 8.0, 14.5 Hz), 7.54 (2H, dd, J = 7.0, 7.5 Hz), 7.60
(1H, dd, J = 7.5, 7.5 Hz), 7.67–7.76 (4H, m), 8.00 (2H, d, J = 7.0 Hz), 8.06 (1H, d, J = 7.5 Hz), 8.15
(1H, d, J = 7.5Hz), 8.87 (2H, d, J = 8.0 Hz), 8.91 (1H, d, J = 8.0 Hz); 13C NMR (150 MHz, DMSO-
d6) # = 37.9, 49.1, 67.0, 121.8, 123.2, 123.7, 123.8, 124.2, 124.9, 126.1, 126.6, 126.7, 127.4 (2C),
127.6, 127.9, 128.4 (2C), 129.3, 130.5, 131.5, 134.0, 135.3, 166.2, 171.2. MALDI TOF-MS m/z
calcd for C25H20N2O2: 380.47 [M]+. Found: 380.31.
7. H. Hoshina, H. Tsuru, K. Kubo, T. Igarashi, and T. Sakurai, Heterocycles, 2000, 53, 2261.
8. To monitor the progress of the reaction for (Z)-1a, nitrogen-saturated methanol solutions of (Z)-1a
(1.0 " 10–3 mol dm–3, 10 mL " 3) were irradiated in parallel using a merry-go-round irradiation
apparatus (light source: 400 W high-pressure Hg lamp, filter: Pyrex glass) for a given period of time
at room temperature. After irradiation, the solutions were concentrated in vacuo to dryness and the
1
resulting residues were subjected to H NMR spectral analysis. The composition of the
photoproducts was estimated on the basis of the area ratio of the signals characteristic of each
product.
9. B. A. Cunningham and G. L. Schmir, J. Am. Chem. Soc., 1966, 88, 551.
10. MM2 and PM5 calculations were performed by using CAChe 5.0 for Windows available from
Fujitsu Ltd, 2002.