Reversal of regioselectivity (straight vs. cross ring closure) in the intramolecular [2+2] photocycloaddition of phenanthrene derivatives

Article abstract of DOI:10.1039/c000179a

Intramolecular [2+2] photocycloaddition of aromatic chain imides possessing bis-phenanthrene moieties afforded straight and cross ring closure products. The ratio of cycloadducts was dependent on reaction time and temperature, which resulted in a reversal of regioselectivity. The reaction was proved to involve a retro cycloaddition. The cross ring closure products were formed predominantly in the early stage of the reaction at lower reaction temperature. In contrast, the straight ring closure products were predominant at higher reaction temperature with prolonged irradiation.

Full text of DOI:10.1039/c000179a

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Reversal of regioselectivity (straight vs. cross ring closure) in the
intramolecular [2+2] photocycloaddition of phenanthrene derivatives†
Shigeo Kohmoto,*a Shugo Hisamatsu,a Hakuei Mitsuhashi,a Masahiro Takahashi,a Hyuma Masu,b
Isao Azumaya,b Kentaro Yamaguchib and Keiki Kishikawaa
Received 12th January 2010, Accepted 2nd March 2010
First published as an Advance Article on the web 12th March 2010
DOI: 10.1039/c000179a
Intramolecular [2+2] photocycloaddition of aromatic chain imides possessing bis-phenanthrene
moieties afforded straight and cross ring closure products. The ratio of cycloadducts was dependent on
reaction time and temperature, which resulted in a reversal of regioselectivity. The reaction was proved
to involve a retro cycloaddition. The cross ring closure products were formed predominantly in the
early stage of the reaction at lower reaction temperature. In contrast, the straight ring closure products
were predominant at higher reaction temperature with prolonged irradiation.
Introduction
Regio- and stereo-controlled organic photochemical reactions
have been studied intensively not only from mechanistic view-
points but also from a synthetic point of view.1 The factors
generally utilized to affect selectivities are reaction temperature,2
solvent polarity,3 irradiation wavelength,4 excited spin states,5
and reaction phases.6 In some cases, high selectivities have been
attained including the reversal of the selectivities by varying the
above-mentioned conditions. However, the successful examples
are limited to diastereoselective [2+2] photocycloadditions. More-
over, the majority of the examples of the reversal of selectivities
were reported in the stereoselective Paterno`-Bu¨chi reaction.5,7 In
this paper, we present the rare example of the regio-controlled
intramolecular [2+2] photocycloaddition reaction with the rever-
sal of regioselectivity depending on the reaction temperature and
irradiation time. In addition, the reaction involves a rare example
of cross ring closure in intramolecular [2+2] photocycloaddition
of aromatic compounds.
Scheme 1 Photocycloaddition of a phenanthrene–phenanthrene system.
Results and discussion
Aromatic chain imides 1 possessing two phenanthrene moieties
linked with iminodicarbonyl were subjected for photochemical
study. Irradiation of 1 with a high pressure Hg lamp under
argon atmosphere through Pyrex filter (>290 nm) resulted in the
formation of two cycloadducts 2 and 3 in an almost quantitative
yield (Scheme 1). The product ratio varied depending on the
reaction temperature and irradiation time. The structure of 2a and
3a were unequivocally determined by single crystal X-ray analysis
of them. Their X-ray structures are shown in Fig. 1 together
with that of 1a which adopts a cross-oriented conformation.
The cycloadducts are regioisomers of the intramolecular [2+2]
photocycloaddition. The compound 2a is the straight ring closure
[2+2] cycloadduct and 3a is the product of cross ring closure.
In general photocycloaddition of a compound possessing
two double bonds linked with a propylene chain affords a
cyclopentane ring fused cyclobutane derivative via the straight
ring closure known as the “rule of five” ring closure.8 The regio
chemistry of the cycloadducts was interpreted in terms of the
stability of diradical intermediates.9 In some cases, the cross
ring closure10 were reported. It gives a bicyclo[3.1.1]heptane ring
system. Up to now, this cross ring closure is limited to aliphatic
[2+2] photocycloaddition. In the photocycloaddition of aliphatic
imides, an exclusive formation of cross adducts was reported in
the reaction of dimethylacrylimides.11 Aromatic chromophores,
such as naphthalene,12 phenanthrene,13 and coumarin14 have
aDepartment of Applied Chemistry and Biotechnology, Graduate School of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,
Japan. E-mail: kohmoto@faculty.chiba-u.jp; Fax: +81 43 290 3422; Tel: +81
43 290 3420
bFaculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri
University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
† Electronic supplementary information (ESI) available: Spectroscopic
data (1b, 2b, 3b, 4 and 5) and copies of 1H and 13C NMR spectra.
Crystallographic data of 1a, 2a and 3a in CIF format. CCDC reference
numbers 759325 (for 1a), 606845 (for 2a) and 606846 for (3a). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c000179a
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[2+2] photocycloaddition. No other product was found. However,
prolonged irradiation in acetone-d6 or in acetonitrile-d3 caused
slight decomposition. A better yield of the cross adduct 3 was
obtained at lower reaction temperature. The isomer ratios of
2a : 3a = 23 : 77 and 2b : 3a = 32 : 68 were observed for 1a and
1b, respectively, after irradiation for 3 min in acetone-d6 at -78 ◦C
(entries 6 and 12). When the irradiation was carried out in a melted
state at 230~260 ◦C, the cycloadduct 2a was the sole product (entry
8). No reaction occurred in solid state at ambient temperature.
Because of the cross-oriented conformation of 1a observed in
its X-ray structure (Fig. 1a), it is difficult to cyclise in the 1,5-
cycloaddition manner. Even though the distances between the two
carbon atoms (d1 and d2) to be reacted in the 1,6-cycloaddition are
˚
both 3.3 A which is within the distance allowed for cycloaddition
in Schmidt rule,15 the angles between the double bond (q1 and q2)
are rather narrow (58◦).
The reaction profile of the photoreaction of 1a at 50 ◦C in
1
benzene-d6 was monitored by H NMR spectroscopy (Fig. 2a).
It showed that both products were generated immediately. The
amount of 3a decreased and that of 2a increased gradually with
increasing irradiation time. The results clearly show that the
reaction involves retro [2+2] photocycloaddition of 3a to give
2a. Prolonged irradiation resulted in the photo stationary state
with the ratio of 2a : 3a = 80 : 20. The cycloadducts 2a and 3a
are thermally stable. Heating of them for a day at 50 ◦C in
the dark caused no interconversion. In order to confirm this,
the photoreaction of 2a and 3a were carried out independently
under the identical photoreaction conditions. Irradiation of 2a
resulted in the immediate formation of 3a. Formation of 1a was
not detected. Then, the amount of 3a decreased and that of 2a
increased gradually with increasing irradiation time. Finally it
reached to the photo stationary state with the identical isomer ratio
as started from 1a (Fig. 2b). Similarly, irradiation of 3a afforded
the same isomer ratio at the photo stationary state (Fig. 2c). The
results indicate that the reaction involves the retro cycloadditions
of both 2a and 3a. Equilibrium exists under irradiation. The
reaction profiles of 1a and 2a are unusual as those involving an
equilibration. Normally, both products should be generated with a
constant ratio all through the reaction period in an equilibration.
The amount of 3a should not jump in the early stage of the reaction
and after that it should not decrease if the reaction involves
equilibrium. Nevertheless the photo stationary state exists in the
reaction. The same product ratio was obtained starting from any of
1a, 2a and 3a. The explanation for this observation was found from
the inspection of their UV spectra. Fig. 3 shows the uv spectra of
1a, 2a and 3a. The absorption of 2a and 3a over 300 nm, the region
covers the line spectrum (313 nm) emitted from a high pressure
Hg lamp, is weaker than that of 1a. Therefore, most of the light
will be absorbed by 1a. Because of this, the photocycloaddition
of 1a is much more efficient than the retro cycloadditions of
2a and 3a in the early stage of the reaction, which results in
the immediate formation of the favourable product 3a. While
the reaction proceeds the amount of 3a gradually decreases and
finally it reaches to the photo stationary state after the irradiation
of a reasonably long period of time. In the case of the retro
cycloaddition of 2a, the resulting 1a is immediately converted to
3a which will be accumulated in the beginning of the reaction and
after that the reaction proceeds in a similar way as observed in the
reaction profile of 1a.
Fig. 1 Single crystal X-ray structure of 1a (a), 2a (b) and 3a (c). Ellipsoids
are presented in 50% probability level while isotropic H-atom thermal
parameters are represented by spheres of arbitrary size.
been known to give [2+2] photocycloadducts in both inter- and
intramolecular ways. To the best of our knowledge, this is the first
example of the cross ring closure between aromatic chromophores
in intramolecular [2+2] photocycloaddition.
The product ratio was reaction temperature and time dependent
(Table 1). Irradiation was carried out in deuterated solvents with
bubbling of argon prior to irradiation. The product ratio was
determined based on the integration of both products in the
1
reaction mixture by H NMR spectroscopy. Both products were
generated immediately in the early stage of the reaction with the
cross cycloadduct as the major product in the case of 1a. Even
irradiation for 30 s caused completion or almost completion of
the reaction (entries 1, 3, and 7). Prolonged irradiation resulted
in predominant formation of 2. The isomer ratios of 2a : 3a =
80 : 20 and 2b : 3b = 97 : 3 were observed after irradiation for
10 h and 7 h respectively, for 1a and 1b in benzene-d6 at 50 ◦C
(entries 2 and 10). The results indicate the existence of the retro
Table 1 Temperature dependent product ratios in the photocycloaddition
of 1a
Entry
compd.
Temp./
◦Cb
Time/ Ratio
Conversion
Solvent
minc
(2a : 3a) (%)
1
2
3
4
5
6
7
8
1a C6D6
50
50
50
50
0
-78
50
0.5
600
0.5
3
3
3
0.5
5
0.5
420
1
37 : 63
80 : 20
44 : 56
48 : 52
40 : 60
23 : 77
42 : 58
100 : 0
71 : 29
97 : 3
100
100
85
100
100
85
90
46
70
100
53
1a C6D6
1a (CD3)2CO
1a (CD3)2CO
1a (CD3)2CO
1a (CD3)2CO
1a CD3CN
1a melted state 230~260
1b C6D6
1b C6D6
9
50
50
-78
-78
10
11
12
1b (CD3)2CO
1b (CD3)2CO
30 : 70
32 : 68
3
100
a Irradiation was carried out with a high pressure Hg lamp under argon
atmosphere through Pyrex filter (>290 nm). The regioisomer ratio was
determined by 1H NMR spectroscopy. The concentration of 1a: ca. 10-2M.
b Reaction temperature. c Irradiation time
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carried out with high pressure Hg lamp through combination of
glass filters (340 nm < l < 390 nm) to choose the line spectra
at 365/366 nm for excitation. Both cycloadducts 2a and 3a were
inert within experimental errors at the early stage of the reaction
(irradiation for 3 min) while 95% of 1a was reacted in the same
irradiation period. However, prolonged irradiation (3 h) of 2a and
3a caused reaction with conversion of 19 and 38%, respectively.
Because of the existence of slight absorption of 2a and 3a at this
wavelength, they reacted slowly.
The conformation of 1 is responsible for the regiochemistry of
the cycloadducts. If the orientation of two phenanthrene moieties
is parallel (straight orientation), then the straight ring closure
occurs to give 2. On the contrary, the cross orientation of them
affords the cross ring closure adduct 3 as shown in Scheme 1. It is
reasonable to consider that the reactivity of these conformations
should be responsible for the regioselectivity. The cross oriented
1 should be more stable than the straight oriented 1. In order
to survey the relative stability of the starting conformations, the
DFT (B3LYP/6-31G(d)) calculation was carried out using the
Gaussian 03 program package.16 It is calculated that the cross-
oriented conformations of 1a and 1b are more stable than those
of the corresponding straight-oriented conformations in 2.3 and
2.4 kcal mol-1, respectively. To investigate the conformational
change between them, a variable temperature 1H NMR study
of 1a in CDCl3 was carried out. Lowering the temperature
down to 218 K no spectral change of the aromatic protons was
observed except slight broadening of them. The results indicate
that conformational change of the phenanthryl moieties was not
restricted at this temperature.
The mechanism of the photodimerization of phenanthrene
derivatives was well investigated by Lewis et al.17 Depending on
the electron-withdrawing substituents at the 9-position of phenan-
threne, they found three types of reactions. Methyl phenanthrene-
9-carboxylate undergoes self-quenching and dimerization via
normal excimer mechanism, N,N-dimethylphenanthrene fails
to undergo either self-quenching or photodimerization and
9-cyanophenanthrene fails to undergo self-quenching and under-
goes dimerization predominantly via excitation of a ground-state
p-complex. In our case, fluorescence self-quenching was observed
for 1a and 1b. They are completely non-fluorescent in a dilute
solution. This is due to the tethered structure of them. Self-
quenching easily occurs between the facing phenanthrene moieties.
Our results, the occurrence of self-quenching and dimerization,
indicated the possible involvement of normal excimer mechanism
for their dimerization.
Fig. 2 Reaction profiles of the photoreaction of 1a (a), 2a (b) and 3a (c)
in benzene-d6 at 50 ◦C monitored by 1H NMR spectroscopy. Green circles,
blue squares, and red circles indicate the progress of the reaction for 1a, 2a
and 3a, respectively.
In order to elucidate the excited states of the retro cycloaddition,
the photochemical reactions of 1a, 2a and 3a were carried
out with a triplet sensitizer. Irradiation was performed with
benzophenone or Michler’s ketone with high pressure Hg lamp
through combination of glass filters (340 nm < l < 390 nm) using
a merry-go-round apparatus. The results are shown in Table 2.
Without a sensitizer, no or almost no retro reaction occurred
for 2a and 3b, respectively, because the reaction was still in the
early stage. The intensity of light for irradiation was extremely
reduced compared to that utilized in the above-mentioned selective
irradiation experiments through filters. In contrast to the non-
sensitized reaction, the formation of 1a was observed in the
sensitized reaction of 2a and 3a. The results indicate that the retro
cycloaddition proceeds via triplet excited state. The appearance of
Fig. 3 Absorption spectra of 1a, 2a and 3a in acetonitrile.
In order to exclude the retro cycloaddition, selective excitation
of 1a was examined. Independent irradiation of 1a, 2a and 3a was
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Table 2 Product ratios in the photosensitized reactions at photo station-
Experimental
ary statea
1
General methods
Starting material
Sensitizerb
Product ratio 1a : 2a : 3a
All reactions were performed under nitrogen atmosphere. All
commercially available reagents were used without further pu-
rification. All solvents used were special grade. Column chro-
matography was performed with silica gel (0.063–0.2 mm).
Solvents were evaporated under reduced pressure. All yields
given refer to as isolated yields. Melting points were uncor-
rected. NMR spectra were recorded on 300 and 500 MHz
spectrometers. MS experiments were performed on high resolution
magnetic sector mass spectrometers. Crystallographic data were
collected on a CCD detector with graphite monocromated Mo-
1a
1a
1a
2a
2a
2a
3a
3a
3a
None
A
B
None
A
B
None
A
B
0 : 38 : 62
15 : 40 : 45
88 : 10 : 2
0 : 100 : 0
17 : 43 : 41
87 : 12 : 1
0 : 4 : 96
17 : 37 : 46
87 : 9 : 4
a Irradiation (340 nm < l < 390 nm) was carried out for 8 h in benzene-
d6 at ca. 20 ◦C. b With one equivalent amount of benzophenone (A) or
Michler’s ketone (B).
˚
Ka (l = 0.71073 A). The crystal structure was solved by direct
methods SHELXS-97 and refined by full-matrix least-squares
SHELXL-97.20 All non-hydrogen atoms were refined anisotrop-
ically. All hydrogen atoms were included as their calculated
positions.
1a in the reactions of 2a and 3a also indicates that the cycloaddition
of 1a involves singlet excited state, because 1a was completely
consumed in the reaction of 1a.
As the intermediates of photocycloaddition18,19 we postulate di-
radicals via the stepwise mechanism. A diradical intermediate was
proposed by us in the similar photocycloaddition of naphthalene
derivatives in which the diradical intermediate was trapped by
molecular oxygen to afford a novel 1,8-endoperoxide.19
It is noteworthy to mention that photocycloaddition of the imide
possessing phenanthrene and naphthalene moieties 4 afforded an
exclusive formation of the [2+2] cycloadduct 5 via the straight
ring closure (Scheme 2). The structure of 5 was determined
based on the cis coupling constant between protons Ha and
Hb (8.9 Hz). The conjugation by the extra phenylene ring in
1 could stabilise the diradical intermediate leading to the cross
cycloadduct 3. However, the stability of the diradical intermediate
responsible for the formation of the cross cycloadduct might not
be enough in the case of 4. This could be the reason why we
observed the cross [2+2] photocycloaddition in the phenanthrene–
phenanthrene and not in the phenanthrene–naphthalene
system.
2
Synthesis of N-(phenanthrene-9-carbonyl)-N-propyl-
phenanthrene-9-carboxamide (1a)
To a solution of N-propyl-9-phenathrylcarboxamide (302 mg,
1.15 mmol), triethyl amine (0.15 mL, 2.03 mmol), and DMAP
(14 mg, 0.11 mmol) in toluene (40 mL) was added a toluene
solution (10 mL) of phenantroyl chloride prepared from the
reaction of phenanthroic acid (305 mg, 1.37 mmol) and thionyl
chloride. After addition of toluene (15 mL), the resulting mixture
was refluxed for a day and was quenched with aqueous 1
N HCl solution. Ethyl acetate (100 mL) was added to this
solution and the resulting organic layer was separated and washed
with saturated aqueous solution of NaHCO3. After drying over
anhydrous MgSO4, solvent was evaporated and the residue was
chromatographed on silica gel eluted with hexane–ethyl acetate
(3/1) to afford 262 mg (49%) of 1a as a colourless crystal. Mp:
168–170 ◦C (ethyl acetate–hexane); UV-vis (CH3CN) 247 (74100),
312 (sh) nm (9190); IR (KBr) 3044 (w), 2956 (w), 1707 (m), 1657
(s), 1449 (m) 1337 (m) cm-1; (500 MHz, CDCl3) d 1.26 (q, J =
7.5 Hz, 3H), 2.14 (sext, J = 7.5, 2H), 4.32 (t, J = 7.5 Hz, 2H),
7.06 (td, J = 7.3, 1.5 Hz, 2H), 7.09 (td, J = 7.3, 1.5 Hz, 2H), 7.36
(td, J = 7.4, 1.3 Hz, 2H), 7.40 (td, J = 7.5, 1.6 Hz, 2H), 7.47 (s,
2H), 7.49 (dd, J = 7.6, 1.6 Hz, 2H), 7.77 (dd, J = 8.0, 1.5 Hz,
2H), 7.95 (dd, J = 8.0, 1.5 Hz, 2H), 8.03 (d, J = 8.0 Hz, 2H);
13C NMR (125 MHz) d 11.9, 22.3, 47.0, 121.83, 121.85, 125.4,
126.36, 126.44, 126.7, 127.0, 127.4, 127.7, 129.0, 129.2, 129.4,
130.3, 133.9, 173.6; HRMS (FAB), calcd for C33H25NO2 (M+)
467.1885, found 467.1849. Crystal data for 1a: C33H25NO2, M =
467.54, Orthorhombic, space group Pna21 (no. 33), a = 9.489(2),
Scheme 2 Photocycloaddition of a phenanthrene–naphthalene system.
3
˚
b = 27.726(5), c = 9.063(2), V = 2384.5(8) A , Z = 4, T = 200 K,
Dc = 1.302 Mg m-3, m = 0.080 mm-1, 11320 reflections measured,
4220 unique (Rint = 0.0336) which were used in all calculations.
The final R1 and wR2 were 0.0487 and 0.0860 (all data). CCDC
759325.†
Conclusion
We have demonstrated a novel example of intramolecular cross
[2+2] photocycloaddition of aromatic compounds and an un-
usual reversal of regioselectivity affected by reaction temperature
and irradiation time. The straight and cross cycloadducts were
obtained preferably at higher and lower reaction temperature,
respectively.
3
Photocycloaddition
3.1 Photocycloaddition of 1a. Acetone solution (25 mL) of
1a (85 mg, 0.18 mmol) was irradiated with a 450 W high-
pressure Hg lamp under argon atmosphere through Pyrex filter
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◦
for 2 h at -78 C. After evaporation of solvent, the residue was
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3311.
chromatographed on silica gel eluted with hexane–ethyl acetate
(5/1) to give 2a (20 mg, 24%) and 3a (56 mg, 66%) as colourless
crystals. Cycloadduct 2a: Mp: 204–206 ◦C (colorless crystals from
ethyl acetate–hexane); UV-vis (CH3CN) 274 nm (11900); IR (KBr)
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3072 (w), 2964 (w), 1771 (m), 1702 (s), 1390 (m) cm-1; H NMR
1
(500 MHz, CDCl3) d 1.13 (t, J = 7.4 Hz, 3H), 1.93 (sext, J =
7.4 Hz, 2H), 3.85 (t, J = 7.4 Hz, 2H), 4.48 (s, 2H), 6.55 (dd, J =
7.6, 1.2 Hz, 2H), 6.75 (dd, J = 7.6, 1.2 Hz, 2H), 6.95 (td, J =
7.6, 1.2 Hz, 2H), 6.96 (td, J = 7.6, 1.2 Hz, 2H), 7.04 (td, J = 7.6,
1.4 Hz, 2H), 7.10 (td, J = 7.6, 1.4 Hz, 2H), 7.45 (d, J = 7.9 Hz,
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128.3, 128.4, 129.5, 130.0, 132.1, 133.9, 179.4; HRMS (FAB),
calcd for C33H25NO2 (M+) 467.1885, found 467.1872. Crystal
data for 2a: C33H25NO2, M = 467.54, Monoclinic, space group
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89, 4936; (c) R. Matlin, K. F. Brinton and B. T. Nivaggioli, J. Phys.
Org. Chem., 2007, 20, 83.
◦
3
˚
b = 100.23 , V = 2352.65(4) A , Z = 4, T = 150 K, Dc =
1.320 Mg m-3, m = 0.082 mm-1, 26400 reflections measured, 4812
unique (Rint = 0.0268) which were used in all calculations. The
final R1 and wR2 were 0.0487, wR2 = 0.0925 (all data). CCDC
606845.†
Cycloadduct 3a: Mp: 209–212 ◦C (colorless crystals from ethyl
acetate–hexane); UV-vis (CH3CN) 267 nm (26700); IR (KBr)
1
3064 (w), 2963 (w), 1740 (m), 1685 (s), 1447 (m) cm-1; H NMR
(500 MHz, CDCl3) d 1.05 (t, J = 7.4 Hz, 3H), 1.80 (sext, J =
7.4 Hz, 2H), 3.97 (dt, J = 13.2, 7.4 Hz, 1H), 4.05 (dt, J = 13.2,
7.4 Hz, 1H), 4.23 (s, 2H), 6.04 (d, J = 7.7 Hz, 2H), 6.59 (td, J = 7.5,
0.9 Hz, 2H), 6.86 (td, J = 7.6, 1.0 Hz, 2H), 7.29 (d, J = 7.7 Hz,
2H), 7.34 (td, J = 7.6, 1.6 Hz, 2H), 7.42 (td, J = 7.7, 1.2 Hz,
2H), 7.61 (d, J = 7.7 Hz, 2H), 8.03 (dd, J = 7.7, 1.2 Hz, 2H);13C
NMR (125 MHz, CDCl3) d 11.5, 21.4, 41.3, 52.4, 54.0, 122.2,
123.0, 126.5, 126.9, 127.0, 127.6, 128.4, 128.6, 128.7, 129.9, 132.2,
134.4, 175.1; HRMS (FAB), calcd for C33H25NO2 (M+) 467.1885,
found 467.1879. Crystal Data for 3a: C33H25NO2, M = 467.54,
Orthorhombic space group P212121 (no. 19), a = 12.473(1), b =
9 (a) D. J. Maradyn and A. C. Weedon, J. Am. Chem. Soc., 1995,
117, 5359; (b) N. Haddad and N. Galili, Tetrahedron Lett., 1997, 38,
6083.
3
˚
13.377(1), c = 13.888(1), V = 2317.3(4) A , Z = 4, T = 150 K,
Dc = 1.340 Mg m-3, m = 0.083 mm-1, 13053 reflections measured,
5198 unique (Rint = 0.0324) which were used in all calculations.
The final R1 and wR2 were 0.0437 and 0.0900 (all data). CCDC
606846.†
10 A. R. Matlin, C. F. George, S. Wolff and W. C. Agosta, W. C., J. Am.
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3.2 Analytical photocycloaddition of 1. Imide 1 was dissolved
in deuterated solvent (ca. 10-2 M) and was irradiated with a 450
W high-pressure Hg lamp. Prior to irradiation argon gas was
bubbled into the solution for 20 min. After appropriate duration
under irradiation, 1H-NMR spectra of the reaction mixtures were
recorded and the product ratios were determined based on their
integrals.
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