Photochemistry of A1E, a Retinoid with a Conjugated Pyridinium Moiety: Competition between Pericyclic Photooxygenation and Pericyclization

Article abstract of DOI:10.1021/ja039048d

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible right (γ _irr = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum

Full text of DOI:10.1021/ja039048d

Published on Web 03/20/2004
Photochemistry of A1E, a Retinoid with a Conjugated
Pyridinium Moiety: Competition between Pericyclic
Photooxygenation and Pericyclization
Steffen Jockusch, Rex X. Ren,† Young Pyo Jang, Yasuhiro Itagaki,
Heidi R. Vollmer-Snarr,‡ Janet R. Sparrow,§ Koji Nakanishi,* and Nicholas J. Turro*
Contribution from the Department of Chemistry, Columbia UniVersity, 3000 Broadway,
New York, New York 10027
Received October 14, 2003; E-mail: kn5@columbia.edu; njt3@columbia.edu
Abstract: The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence.
Irradiation of A1E with visible light (λirr ) 425 nm) in methanol solutions resulted in pericyclization to form
pyridinium terpenoids. Although the quantum yield for this cyclization is low (∼10-4), nevertheless the
photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and
diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated
carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium
solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching
studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed
that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen
from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic
peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and
peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration
of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a
given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen
produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E,
then cyclization dominates.
Introduction
developed world. In a previous study we showed that A2E
undergoes photooxidation during irradiation with blue light (430
The chromophore A2E, 1 (Chart 1), a pyridinium bisretin-
oid,1,2 accumulates with age in retinal pigment epithelial cells
of the eye3-5 and in patients with a juvenile form of macular
degradation (Stargardt’s disease).6 Many studies suggest that
A2E contributes to age-related macular degeneration,7-9 which
is the leading cause of blindness among the elderly in the
nm), producing epoxide rings initially at positions 7, 8 and 7′,
8′ (2) under involvement of singlet oxygen.10 It was proposed
that, upon irradiation, A2E acts as a sensitizer for generation
of singlet oxygen from triplet oxygen, and that subsequently
the singlet oxygen reacts with A2E to produce epoxides. The
extent of epoxidation (number of epoxide rings per A2E
molecule) is dependent on the intensity and duration of
irradiation with blue light.8 MS analysis showed that epoxidation
proceeds to give the nonaoxirane 3.10 In a study in which A2E
epoxides were loaded into RPE cells, we found that the A2E
epoxides induce cell damage directly, independent of damage
by singlet oxygen.11
† Current address: Department of Chemistry, Wesleyan University,
Middletown, CT 06459.
‡ Current address: Department of Chemistry & Biochemistry, Brigham
Young University, Provo, UT 84602.
§ Department of Ophthalmology, Columbia University, New York, NY
10028.
(1) Sakai, N.; Decatur, J.; Nakanishi, K.; Eldred, G. E. J. Am. Chem. Soc.
1996, 118, 1559-1560.
(2) Ren, R. X.-F.; Sakai, N.; Nakanishi, K. J. Am. Chem. Soc. 1997, 119, 3619-
To elucidate the mechanism of photooxygenation of the
bisretinoid A2E, the nonphysiological monoretinoid A1E (4)
was synthesized. A1E possesses a structure identical to A2E
except that it has a single side arm, whereas A2E has two (Chart
1). The photophysics and photochemistry of A1E were studied
using a multitechnique approach, including steady-state and
time-resolved optical absorption and luminescence spectroscopy
3620.
(3) Eldred, G. E.; Lasky, M. R. Nature 1993, 361, 724-726.
(4) Parish, C. A.; Hashimoto, M.; Nakanishi, K.; Dillon, J.; Sparrow, J. Proc.
Natl. Acad. Sci. U.S.A. 1998, 95, 14609-14613.
(5) (a) Haralampus-Grynaviski, N. M.; Lamb, L. E.; Clancy, C. M. R.; Skumatz,
C.; Burke, J. M.; Sarna, T.; Simon, J. D. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 3179-3184. (b) Lamb, L. E.; Simon, J. D. Photochem.
Photobiol. 2004, 79, 127-136.
(6) Mata, N. L.; Weng, J.; Travis, G. H. Proc. Natl. Acad. Sci. U.S.A. 2000,
97, 7154-7159.
(7) Shaban, H.; Richter, C. Biol. Chem. 2002, 383, 537-545.
(8) Sparrow, J. R.; Zhou, J.; Ben-Shabat, S.; Vollmer, H.; Itagaki, Y.; Nakanishi,
K. InVest. Ophthalmol. Visual Sci. 2002, 43, 1222-1227.
(9) Roberts, J. E.; Kukielczak, B. M.; Hu, D.-N.; Miller, D. S.; Bilski, P.; Sik,
R. H.; Motten, A. G.; Chignell, C. F. Photochem. Photobiol. 2002, 75,
184-190.
(10) Ben-Shabat, S.; Itagaki, Y.; Jockusch, S.; Sparrow, J.; Turro, N. J.;
Nakanishi, K. Angew. Chem., Int. Ed. 2002, 41, 814-817.
(11) Sparrow, J.; Vollmer-Snarr, H. R.; Zhou, J.; Jang, Y. P.; Jockusch, S.;
Itagaki, Y.; Nakanishi, K. J. Biol. Chem. 2003, 278, 18207-18213.
9
4646
J. AM. CHEM. SOC. 2004, 126, 4646-4652
10.1021/ja039048d CCC: $27.50 © 2004 American Chemical Society

Photochemistry of A1E
A R T I C L E S
Chart 1. Structures of A2E, A1E, and Their Photoproducts
Scheme 1. Synthesis of A1E
and product studies after irradiation at various conditions using
MS and NMR techniques.
(Φf23 °C ) 0.002 ( 0.001). In agreement with the low fluores-
cence quantum yield, a short fluorescence lifetime was observed
(τf23 °C < 0.15 ns) which is shorter than the time resolution of
our instrumental setup. In a frozen ethanol matrix at 77 K the
fluorescence quantum yield and lifetime increase strongly (Φf77 K
> 0.6; τf77 K ) 1.7 ( 0.1 ns). The low fluorescence quantum
yield and lifetime at room temperature are consistent with
efficient trans-cis isomerization of the double bonds from the
singlet excited state of A1E, which was also observed for other
retinoids.14-16 In a matrix at 77 K the trans-cis isomerization
is suppressed, and therefore, a strong fluorescence was observed
in ethanol glass.
Results and Discussion
A1E was synthesized as shown in Scheme 1. Site-specific
deprotonation12,13 of 2,4-dimethylpyridine (7) with NaNH2 in
liquid ammonia, followed by addition of retinoidyl aldehyde 8,
led to alcohol 9, which was subjected to acid-catalyzed
elimination to give all-trans pyridyl retinoid 10. Alkylation of
10 with 2-iodoethanol in nitromethane under reflux afforded
the pyridinium substrate A1E (4).
A1E exhibits an intense optical absorption in methanol
solution centered at 417 nm (ꢀ ) 28950 M-1 cm-1). At room
temperature A1E shows only a weak fluorescence (λmax ) 560
nm; ethanol solution) with a low fluorescence quantum yield
To investigate the photochemistry, methanol solutions of A1E
were irradiated at 425 nm and the reaction was monitored via
(14) Pawlak, A.; Wrona, M.; Rozanowska, M.; Zareba, M.; Lamb, L. E.; Roberts,
J. E.; Simon, J. D.; Sarna, T. Photochem. Photobiol. 2003, 77, 253-258.
(15) Hochstrasser, R. M.; Narva, D. L.; Nelson, A. C. Chem. Phys. Lett. 1976,
43, 15-19.
(16) Doukas, A. G.; Junnarkar, M. R.; Chandra, D.; Alfano, R. R.; Callender,
R. H. Chem. Phys. Lett. 1983, 100, 420-424.
(12) Kaiser, E. M.; Bartling, G. J.; Thomas, W. R.; Nichols, S. B.; Nash, D. R.
J. Org. Chem. 1973, 38, 71.
(13) Levine, R.; Dimmig, D. A.; Kadunce, W. M. J. Org. Chem. 1974, 39, 3834.
9
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A R T I C L E S
Jockusch et al.
Figure 2. Optical absorption spectra recorded after different times of dark
reaction of A1E in the presence of the endoperoxide of 1,4-diphenylnaph-
thalene (17 mM) in solutions of methanol (a) and deuterated methanol (b)
at 25 °C.
type of photoproduct. Figure 1c shows the absorbance after
different irradiation times. With continuing irradiation, A1E
bleaches and a new hypsochromically shifted absorption appears
(λmax ) 363 nm) (6). In addition to the major peak at 363 nm,
a weak peak at 294 nm appeared. Because this absorption (λmax
) 294 nm) was also observed during photolysis in a methanol
solution (Figure 1a), we assign the peak at 294 nm to the
cycloreaction product 5. Irradiation of A1E in CHCl3 at 425
nm yielded a similar absorption spectrum with maxima at 294
nm (major peak) and 363 nm (minor peak) (Figure 1b), but the
peak intensities are reversed. In addition, the rate of disappear-
ance of the absorption of A1E is 13-fold slower in CHCl3
compared to the rate in CDCl3. This strong deuterium effect is
a signature of the involvement of singlet oxygen, whose lifetime
possesses an extraordinary dependence on the occurrence of CH
versus CD bonds in the solvent.19-21 Due to the longer lifetime
of singlet oxygen in CDCl3 (7 ms)22 compared to CHCl3 (0.23
ms),22 reactions involving singlet oxygen are expected to proceed
faster in deuterated solvents. A detailed structural analysis of
the reaction product 6 associated with the absorption at 363
nm will be given below. In photolysis experiments of A1E in
deoxygenated CDCl3 solutions (deoxygenation by five freeze-
pump-thaw cycles) the cyclization product 5 was observed
almost exclusively, showing that molecular oxygen is necessary
to generate 6.
Figure 1. Optical absorption spectra recorded after irradiation of A1E in
air-saturated solutions of methanol (6.7 × 10-5 M) (a), chloroform (3.9 ×
10-5 M) (b), and deuterated chloroform (3.4 × 10-5 M) (c) at λex ) 425
nm.
UV-vis absorption spectroscopy. Figure 1a shows the absor-
bance after different irradiation times. With continuing irradia-
tion, the band at 417 nm decreases and two new hypsochro-
mically shifted maxima appear (λmax at 288 nm and λmax at 356
nm). In addition, two isosbestic points at 308 and 260 nm were
observed. To investigate the structure of the photoproduct,
preparative irradiations were performed employing a 1000 W
Xe lamp in conjunction with an aqueous filter solution (1 M
NaNO2; λirr > 400 nm).17 NMR analysis showed that irradiation
in methanol yields a single diastereomeric bicyclic pyridinium
drimadiene (racemic) (5) with an almost quantitative yield. The
structure of 5 was confirmed on the basis of NOE studies (for
details see the Supporting Information).
As a direct test of the involvement of singlet oxygen in the
formation of 6, singlet oxygen was generated from the decom-
position of the aromatic endoperoxide 11.23 The latter was
selected because of its convenient half-life (approximately 5 h
at 25 °C), and the fact that it decomposes nearly exclusively
into 1,4-dimethylnaphthalene (12) and singlet oxygen (eq 1).24
The quantum yield for this photoreaction in methanol solution
was estimated (Φ ∼ 10-4) on the basis of the disappearance of
4 by employing Aberchrome 540 as actionmeter.18 No change
in quantum yield of the photoproduct was observed, whether
the methanol solution was deoxygenated by argon bubbling or
saturated with oxygen. Furthermore, irradiation of A1E in
solvents such as acetonitrile and water gave the same photo-
product.
Solutions of A1E and endoperoxide in CH3OH and CD3OD
were stored in the dark for 12 h at room temperature. Figure 2
In contrast, irradiation of A1E in air-saturated deuterated
chloroform (CDCl3) at 425 nm produced a completely different
(19) Rodgers, M. A. J. J. Am. Chem. Soc. 1983, 105, 6201-6205.
(20) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1983, 105, 3423-3430.
(21) Hurst, J. R.; Schuster, G. B. J. Am. Chem. Soc. 1983, 105, 5756-5760.
(22) Schmidt, R.; Afshari, E. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 788-
794.
(23) Wasserman, H. H.; Larsen, D. L. J. Chem. Soc., Chem. Commun. 1972, 5,
253-254.
(24) Adam, W.; Prein, M. Acc. Chem. Res. 1996, 29, 275-283.
(17) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Mercel Dekker: New York, 1993.
(18) Wintgens, V.; Johnston, L. J.; Scaiano, J. C. J. Am. Chem. Soc. 1988, 110,
511.
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Photochemistry of A1E
A R T I C L E S
Figure 3. 1H NMR spectra of A1E (top) and 6 (bottom) (400 MHz, CD3-
OD). The upfield-shifted protons of the 7 and 8 positions correlated with
115.6 and 82.6 ppm of the 13C NMR resonance in the HSQC study,
respectively.
Figure 4. Right: phosphorescence spectrum of singlet oxygen generated
by sensitization with A1E (λex ) 425 nm) in CCl4 at 25 °C. Left: excitation
spectrum for the singlet oxygen phosphorescence (λem ) 1270 nm), which
resembles the A1E absorption spectrum.
shows optical absorption spectra after different reaction times.
In CD3OD (Figure 2b) the reaction proceeds approximately
8-fold faster than in CH3OH (Figure 2a), as is expected due to
the longer lifetime of singlet oxygen in CD3OD (270 µs)25
compared to CH3OH (9.5 µs).22 In both solvents, CH3OH and
CD3OD, the same reaction product (6) was formed as shown
by mass spectrometry.
To investigate the structure of the product 6, the reaction of
A1E with singlet oxygen (generated from endoperoxide) (eq
2) was performed on a preparative scale (A1E, 46 mg;
Figure 5. Transient optical absorption spectra of A1E recorded 0.6, 1.3,
2.2, and 4.3 µs following laser excitation (400 nm, ca. 7 mJ/pulse) of argon-
saturated benzene solutions at 23 °C.
endoperoxide, 200 mg; CH3OH, 2 mL; 25 °C). The high
endoperoxide concentration compared to that of the experiment
described in Figure 2a was chosen to increase the conversion
of A1E in CH3OH solutions. After 20 h, approximately 80%
of A1E was converted into products (determined by UV-vis
spectroscopy). The oxygenation product 6 was isolated by
preparative HPLC. FAB-MS studies showed that two oxygen
atoms were incorporated into A1E by oxidation (see the
spectrum (Figure 4, left) is very similar to the optical absorption
spectrum of A1E. Therefore, we conclude that A1E is the active
species whose absorption results in an excited state that
generates singlet oxygen. Given the very short fluorescence
lifetime (S1 state) of A1E (see above), we conclude that the
triplet state (T1) is probably the excited state of A1E that
produces singlet oxygen.
1
Supporting Information). NMR studies of 6 including H and
13C NMR, magnitude COSY, and HSQC revealed the structure
of 6 as a cyclic 5,8-peroxide (Figure 3).
Laser flash photolysis experiments were performed to deter-
mine if triplet states of A1E are involved in the singlet oxygen
sensitization. Irradiation of argon-saturated benzene solutions
of A1E with laser pulses (400 nm) afforded readily detectable
transient absorption spectra with a broad absorption from 480
to 620 nm (Figure 5), which decayed by pseudo-first-order
kinetics (τ ) 1 µs). This transient was assigned to the triplet
state of A1E (eq 3), because the transient was efficiently
quenched by the triplet quencher canthaxanithin, a carotin (ET
) 88-105 kJ/mol),27 with a diffusion-controlled rate constant
(kq ≈ 1 × 1010 M-1 s-1). In addition, the transient was also
The large deuterium isotope effect in the photooxidation and
thermal oxidation utilizing endoperoxide demands the involve-
ment of singlet oxygen in both instances. Thus, we conclude
that irradiation of A1E in the presence of oxygen must lead to
the production of singlet oxygen through the quenching of an
excited state of A1E. To demonstrate that singlet oxygen can
be generated by electronically excited states (S1 or T1) of A1E,
the phosphorescence of singlet oxygen centered at 1270 nm was
used as a spectroscopic probe.26 Air-saturated solutions of A1E
in CCl4 were irradiated at 425 nm, and the photoluminescence
spectra were recorded from 1100 to 1400 nm. The resulting
spectrum shown in Figure 4 (right) is consistent with that
reported for singlet oxygen phosphorescence.26 To determine
the species responsible for producing the phosphorescence,
luminescence excitation spectra were recorded. The excitation
quenched by molecular oxygen (kO ≈ 1 × 1010 M-1 s-1
)
2
probably with simultaneous formation of singlet oxygen (eq 4).
Subsequently, the generated singlet oxygen could react with A1E
(eq 5).
The rate constant for quenching of singlet oxygen by A1E
(eq 5) was determined by luminescence quenching experiments.
(25) Toumaire, C.; Croux, S.; -T., M. M.; Beck, I.; Hocquaux, M.; Braun, A.
M.; Oliveros, E. J. Photochem. Photobiol., B 1993, 205-215.
(26) Khan, A. U.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 6047-
6049.
(27) Rodgers, M. A. J.; Bates, A. L. Photochem. Photobiol. 1980, 31, 533-
537.
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A R T I C L E S
Jockusch et al.
A1E
9
hν8 1(A1E)* f 3(A1E)*
(3)
(4)
(5)
kO2
3(A1E)* + 3O2 8 A1E + 1O2
kA1E
A1E + 1O2
8 6
CCl4 solutions of the endoperoxide of 1,4-dimethylnaphthalene
show a strong chemiluminescence of singlet oxygen centered
at 1270 nm (Figure 6, left). In the presence of A1E the
luminescence intensity was gradually reduced with increasing
A1E concentration, suggesting an efficient quenching (Figure
6, left). The rate constant of the singlet oxygen quenching (eq
5) was determined by a Stern-Volmer treatment of the
luminescence intensities.28 The lifetime of singlet oxygen in
CCl4 under our experimental conditions (high concentrations
of the endoperoxide) is expected to be significantly shorter than
in pure CCl4 (59 ms; sensitization with 5,10,15,20-tetraphe-
nylporphine).22 Therefore, the singlet oxygen lifetime (τ ) 3.4
ms) was determined by luminescence quenching with 1-methyl-
1-cyclohexane using its known quenching rate constant (3.6 ×
105 M-1 s-1).29 This lifetime led to an estimation of the rate
Figure 6. Left: chemiluminescence spectra of singlet oxygen generated
from the endoperoxide of 1,4-diphenylnaphthalene (2.0 mM) in the presence
of A1E at different concentrations (0-55 µM) in CCl4 at 25 °C. Right:
the corresponding Stern-Volmer plot to determine the quenching rate
constant, where I0 is the intensity of the chemiluminescence at 1270 nm in
the absence of A1E and IA1E is the intensity in the presence of A1E.
constant for the quenching of singlet oxygen by A1E of kA1E
≈
4 × 107 M-1 s-1
.
Discussion
Scheme 2. Photocyclization of A1E
The photochemistry of A1E, a retinoid, was investigated in
different solvents. Two major reaction products were ob-
served: the cyclization product pyridinium terpenoid 5 and the
cyclic 5,8-peroxide 6. Deuterium solvent effects (Figure 1b,c),
experiments utilizing an endoperoxide (Figure 2), phosphores-
cence (Figure 4), and chemiluminescence quenching experi-
ments (Figure 6) show that singlet oxygen reacts with A1E to
form the peroxide 6. The formation of similar cyclic peroxides
was also observed for other retinoids.30 The singlet oxygen was
generated by quenching photoexcited states of A1E, which was
demonstrated by the observation of the singlet oxygen phos-
phorescence after photoexcitation of A1E (Figure 4). Theoreti-
cally, singlet oxygen can be generated from both electronic
excited states, S1 and T1. Laser flash photolysis experiments
showed the formation of triplet excited states of A1E (Figure
5), which were quenched by molecular oxygen with a diffusion-
controlled rate constant, and probably resulted in the formation
of singlet oxygen. Singlet excited states are also known to be
quenched by oxygen under formation of singlet oxygen.31
Nevertheless, because of the short singlet lifetime of A1E (τf23 °C
< 0.15 ns), oxygen quenching of the S1 state is the unlikely
source of singlet oxygen. Therefore, we propose that the singlet
oxygen is generated from quenching of triplet excited states of
A1E (which are generated after photoexcitation and intersystem
crossing), by molecular oxygen. Afterward, the singlet oxygen
reacts with A1E to form the cyclic 5,8-peroxide 6, or the singlet
oxygen is deactivated (e.g., by solvent molecules) to ground-
state triplet oxygen. As expected, the highest yields of 6 were
observed in solvents with long singlet oxygen lifetime, such as
CCl4 and CDCl3. In addition, an increase in concentration of
A1E also increases the yield of 6 (see the Supporting Informa-
tion).
If the singlet oxygen is quenched by the solvent faster than
it can react with A1E, pericyclization (Scheme 2) dominates.
Because no change in the quantum yield of the formation of
the photoproduct 5 was observed in the presence or absence of
oxygen, we propose that the pericyclization occurs from the
singlet excited state. The singlet excited state is too short lived
(τf23 °C < 0.15 ns) to be quenched by oxygen, whereas the triplet
23 °C
state lives long enough for efficient oxygen quenching (τT
) 1 µs; laser flash photolysis). Mechanistically, the pericy-
clization should most likely involve the initial photoisomeriza-
tion of the C7-C8 double bond, leading to intermediate 13,
followed by ring closure (to produce 5) in a separate photo-
chemical step (Scheme 2). Similar cyclization reactions were
reported for vitamin A derivatives.32 It is known that photo-
chemical cyclizations in the vitamin A series are limited to the
6π-electron reaction of the C5-C6, C7-C8, and C9-C10
double bonds regardless of the length of the polyene chains.32
The quantum yield for the cyclization reaction is very low (Φ
≈ 10-4), probably because of efficient photoisomerization of
(28) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/
Plenum Publishers: New York, 1999.
(29) Monroe, B. M. J. Am. Chem. Soc. 1981, 103, 7253.
(30) Montenegro, M. A.; Nazareno, M. A.; Durantini, E. N.; Borsarelli, C. D.
Photochem. Photobiol. 2002, 75, 353-361.
(31) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991.
(32) Liu, R. S. H.; Asato, A. E. Tetrahedron 1969, 40, 1931-1969.
9
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Photochemistry of A1E
A R T I C L E S
(7) with NaNH2 in liquid ammonia followed by addition of retinoidyl
aldehyde 8 led to alcohol 9, which was subjected to acid-catalyzed
elimination to give all-trans pyridyl retinoid 10. Alkylation with
2-iodoethanol, or methyl iodide in nitromethane under reflux, or
treatment with methyl triflate in diethyl ether at room temperature,
afforded pyridinium substrate A1E (4).
The oxidation product of A1E (6) was isolated and purified by HPLC
(HP 1100 series with DAD detector) equipped with a reversed-phased
preparative column (Vydac, C18, 10 µm, 22 × 250 mm) and using a
gradient solvent system mode (water/methanol with 0.1% TFA, 25:75
to 5:95).
1H NMR spectra were recorded using a Varian VXR 400 spectrom-
eter at 400 MHz. 13C NMR spectra were recorded using a Varian
Gemini 300 MHz spectrometer at 75 MHz in CDCl3. NOE difference
spectra were performed on a 500 MHz Bruker NMR spectrometer.
Chemical shifts are given in δ (ppm) using the solvent as the internal
reference, and the coupling constants (J) are in hertz (Hz). Low-
resolution and high-resolution FAB mass spectra were measured on a
JEOL JMS-HX110/110A tandem mass spectrometer (Tokyo, Japan)
using an N-nitrobenzyl alcohol (NBA) matrix and Xe gas (6 kV). ESI
mass spectra were obtained on a Q-TOF (Micromass, Manchester,
U.K.). UV-vis spectra were recorded on an Agilent 8453 or 8452A
spectrometer. Luminescence spectra were recorded on a Fluorolog-2
spectrometer (SPEX) in conjunction with an EO-817L Ge-diode detector
(North Coast Scientific Corp.) The laser flash photolysis experiments
were performed by Elizabeth Gaillard at Northern Illinois University,
DeKalb. The setup employed the pulses from a Continuum Powerlite
9010 injection-seeded Nd:YAG-pumped Sunlite optical paramagnetic
oscillator (400 nm; ∼7 mJ/pulse) and a computer-controlled system
described elsewhere.35
the 7-cis isomer 13 back to the all-trans isomer of A1E, which
competes with the cyclization reaction. For all the photoreactions
carried out on A1E, the UV-vis spectra corresponding to the
all-trans pyridinium retinoid starting material (Figure 1, first
spectrum in a series) do not pass the isosbestic points. This
should indicate a very rapid photoisomerization of the C7-C8
double bond with much higher quantum efficiencies to the
photostationary state, which is a mixture of the all-trans isomer
of A1E and the cis isomer 13 (Figure 1, second spectrum in a
series). The isosbestic points observed in the experiments (Figure
1a) indicate that the 6π-electropericyclic reaction is indeed a
unimolecular process33 with respect to the initial C7-C8 double
bond isomerization. The diastereospecificity originating from
the conrotatory mode can be accounted for by the Woodward-
Hoffmann rule34 regarding pericyclic reactions of the conjugated
triene moiety under photochemical conditions.
Summary and Conclusions
The photochemistry of the retinoid analogue A1E was
investigated in different solvents. Irradiation of A1E with visible
light (λirr ) 425 nm) in methanol solutions resulted in pericy-
clization to form pyridinium terpenoids 5. Although the quantum
yield for this cyclization is low (∼10-4), the photochemical
transformation occurs with quantitative chemical yield and with
remarkable chemioselectivity and diastereoselectivity. Con-
versely, irradiation of A1E under the same irradiation conditions
in air-saturated carbon tetrachloride or deuterated chloroform
produced a cyclic 5,8-peroxide (6) as the major product.
Deuterium solvent effects, experiments utilizing endoperoxide,
phosphorescence, and chemiluminescence quenching studies
strongly support the involvement of singlet oxygen in the
endoperoxide formation. We propose that, upon irradiation, in
the presence of oxygen, A1E acts as a sensitizer for generation
of singlet oxygen from triplet oxygen present in the solution;
the singlet oxygen produced reacts with A1E to produce the
cyclic peroxide. Thus, the photochemistry of A1E is character-
ized by two competing reactions, cyclization and peroxide
formation. Which reaction dominates depends on the concentra-
tions of oxygen and A1E and the lifetime of singlet oxygen in
the solvent employed. If the concentrations of oxygen and A1E
are high enough and the lifetime of singlet oxygen in the solvent
is long enough, then the peroxide is the major reaction product.
Even if the oxygen concentration is high enough to quench the
excited states of A1E to produce singlet oxygen, if the singlet
oxygen produced is quenched by the protonated solvent
molecules faster than singlet oxygen reacts with A1E, then
cyclization dominates.
The quantum yields were obtained by continuous irradiation of a 3
mL solution in a 1 × 1 cm quartz cell at 417 nm (15 nm bandwidth,
30 mW/cm2) employing a Xe lamp (LX300 UV) in conjunction with
a monochromator (Kratos, Schoeffel Instruments). The changes in
optical absorption were measured on an Agilent 8452A spectropho-
tometer. The absorbed dose was determined by actinometry using
Aberchrome 540 (Aberchromics Ltd., U.K.).18 The preparative irradia-
tions were performed with visible light using a Xe lamp (XBO 1000,
Hanovia) and a filter 1 M NaNO2(aq) solution to block the UV light,
giving cutoff at about 400 nm.17
1
NMR of 10. H NMR (CDCl3, ppm) δ 8.33 (d, 1H, J ) 5.1 Hz),
7.28 (dd, 1H, J ) 15.5, 11.4 Hz), 7.06 (s, 1H), 7.02 (d, 1H, J ) 5.1
Hz), 6.36 (d, 1H, J ) 15.4 Hz), 6.22 (d, 1H, J ) 16 Hz), 6.15 (d, 1H,
J ) 11.4 Hz), 6.10 (d, 1H, J ) 16 Hz), 2.48 (s, 3H), 1.95 (m, 5H),
1.61 (m, 5H), 1.41 (m, 2H), 0.97 (s, 6H); 13C NMR (CDCl3, ppm) δ
158.5, 149.2, 145.2, 139.1, 137.6, 137.1, 129.8, 129.3, 129.1, 128.9,
128.5, 119.9, 117.6, 39.5, 34.2, 33.0, 29.0, 24.3, 21.7, 19.1, 12.8.
1
NMR of A1E (4). H NMR (CDCl3, ppm) δ 9.0 (d, 1H, J ) 6.7
Hz), 7.72 (m, 2H), 7.58 (s, 1H), 6.52 (m, 2H), 6.3 (m, 2H), 4.8 (t, 2H,
J ) 4.6 Hz), 4.2 (t, 2H, J ) 4.6 Hz), 2.9 (s, 3H), 2.2 (s, 3H), 2.1 (m,
2H), 1.76 (s, 3H), 1.62 (m, 2H), 1.52 (m, 2H), 1.1 (s, 6H); 13C NMR
(CDCl3, ppm) δ 153.7, 153.3, 146.3, 145.3, 138.3, 137.5, 136.5, 132.3,
131.7, 128.7, 125.1, 124.8, 121.1, 59.8, 58.7, 39.6, 34.2, 33.2, 28.9,
21.7, 21.5, 19.0, 13.7.
Previously, we found that the bisretinoid A2E (1) forms
epoxides (2, 3) upon reaction with singlet oxygen.10 In this study
we showed that the monoretinoid A1E forms a cyclic 5,8-
peroxide (6) in the reaction with singlet oxygen. The cause of
the entirely different reaction products of A2E and A1E with
singlet oxygen is currently under investigation and the subject
of an upcoming paper.
1
NMR of 5. H NMR (CDCl3, ppm) δ 9.15 (d, 1H, J ) 6.7 Hz),
7.75 (d, 1H, J ) 6.7 Hz), 7.65 (s, 1H), 7.05 (dd, 1H, J ) 15.5, 11 Hz),
6.6 (d, 1H, J ) 15.5 Hz), 5.9 (d, 1H, 5.7 Hz), 5.84 (m, 1H), 4.9 (t, 2H,
J ) 4.6 Hz), 4.2 (m, 2H), 3.0 (m, 4H), 1.72 (s, 3H), 1.7-1.2 (m, 6H),
1.17 (s, 3H), 1.15 (s, 3H), 1.1 (s, 3H); 13C NMR (CDCl3, ppm) δ 154.5,
152.9, 151.8, 146.5, 146.3, 132.3, 128.4, 125.8, 121.2, 117.8, 59.7,
59.1, 58.3, 41.0, 39.9, 38.9, 35.4, 31.9, 31.8, 21.9, 21.5, 18.4, 17.8.
Experimental Section
Synthesis of A1E. The requisite precursor was synthesized as shown
in Scheme 1. Site-specific deprotonation12,13 of 2,4-dimethylpyridine
Acknowledgment. We thank Elizabeth R. Gaillard (Depart-
ment of Chemistry and Biochemistry, Northern Illinois Uni-
versity, DeKalb, IL) for performing the laser flash photolysis
experiments. The studies were supported by NIH Grants
(33) Trehan, A.; Liu, R. S. H. Tetrahedron Lett. 1988, 29, 419-422.
(34) Woodward, R. B.; Hoffman, R. The ConserVation of Orbital Symmetry;
Academic Press: New York, 1970.
(35) Harper, W. S.; Gaillard, E. R. Photochem. Photobiol. 2001, 73, 71-76.
9
J. AM. CHEM. SOC. VOL. 126, NO. 14, 2004 4651

A R T I C L E S
Jockusch et al.
GM34509 (to K.N.), EY12951 (to J.R.S.), and IF32EY06750-
01A1 (to R.X.R.) and by the Macula Vision Research Founda-
tion (J.R.S.). H.R.V.-S. was supported by a National Eye
Institute Vision Science training grant to Columbia University.
N.J.T. and S.J. thank the NSF for their generous support by
Grant CHE-01-10655.
Supporting Information Available: Synthetic protocols, mass
spectrometry of 6, and A1E concentration dependence of
photooxygenation (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA039048D
9
4652 J. AM. CHEM. SOC. VOL. 126, NO. 14, 2004