Photoinduced [4 + 4], [4 + 2], and [2 + 2] cycloadditions of o-quinones with oxazoles: Chemo-, regio-, and diastereoselectivity

Article abstract of DOI:10.1021/jo101764f

Photoinduced reactions of the 1,2-dicarbonyl compounds phenanthrenequinone (PQ), 1-acetylisatin (IS), and benzil (BZ) with the oxazoles 1a-j have been investigated. In photoreactions of PQ with the oxazoles, in addition to the 1,4-dioxins derived from [4 + 2] cycloaddition and the oxetanes from the Paterno-Buechi [2 + 2] reactions, [4 + 4] cycloaddition products are formed in the reactions with 1a, 1c, 1g, 1i, and 1j, with the quinone's dicarbonyl unit (O=C-C=O) and the oxazole's C=N-C=C moiety as two 4π addends. Photoreactions of IS with the oxazoles 1f and 1g give the [4 + 4] cycloaddition products exclusively, while in photoreactions of IS with 1a, 1c, 1e, 1h, and 1i, [4 + 4] products are formed together with the [2 + 2] products. Reaction pathway partitioning in these photocycloaddtions strongly depends on the substitution pattern on the oxazole ring. The presence of a substituent at the oxazole's C2 atom hampers the [4 + 4] pathway by causing steric hindrance to radical pair recombination in the corresponding 1,7-diradical intermediate to form the [4 + 4] cycloaddition products. A substituent at the C4 atom results in steric hindrance for ring closure of the 1,4-diradicals in the [2 + 2] cycloaddition pathway, therefore favoring the [4 + 4] and [4 + 2] cycloaddition pathways. Regio- and diastereoselectivity in the [2 + 2] and [4 + 4] cycloadditions have been discussed based on the thermodynamic stability of the relevant triplet diradical intermediates and the conformations of these diradicals suitable for the intersystem crossing process. Photoreactions of BZ with the oxazoles afford only [2 + 2] cycloaddition products.

Full text of DOI:10.1021/jo101764f

pubs.acs.org/joc
Photoinduced [4 þ 4], [4 þ 2], and [2 þ 2] Cycloadditions of o-Quinones
with Oxazoles: Chemo-, Regio-, and Diastereoselectivity
Lei Wang,† Yu-Cheng Huang,† Yang Liu,† Hoong-Kun Fun,‡ Yan Zhang,*,† and
Jian-Hua Xu*,†
†School of Chemistry and Chemical Engineering, Key Lab of Analytical Chemistry for Life Science,
Ministry of Education of China, Nanjing University, Nanjing, 210093, People’s Republic of China, and
‡X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM Penang, Malaysia
njuzy@nju.edu.cn; xujh@nju.edu.cn
Received September 8, 2010
Photoinduced reactions of the 1,2-dicarbonyl compounds phenanthrenequinone (PQ), 1-acetylisatin (IS),
and benzil (BZ) with the oxazoles 1a-j have been investigated. In photoreactions of PQ with the oxazoles,
ꢀ
€
in addition to the 1,4-dioxins derived from [4 þ 2] cycloaddition and the oxetanes from the Paterno-Buchi
[2 þ 2] reactions, [4 þ 4] cycloaddition products are formed in the reactions with 1a, 1c, 1g, 1i, and 1j, with
the quinone’s dicarbonyl unit (OdC;CdO) and the oxazole’s CdN;CdC moiety as two 4π addends.
Photoreactions of IS with the oxazoles 1f and 1g give the [4 þ 4] cycloaddition products exclusively, while
in photoreactions of IS with 1a, 1c, 1e, 1h, and 1i, [4 þ 4] products are formed together with the [2 þ 2]
products. Reaction pathway partitioning in these photocycloaddtions strongly depends on the substitution
pattern on the oxazole ring. The presence of a substituent at the oxazole’s C2 atom hampers the [4 þ 4]
pathway by causing steric hindrance to radical pair recombination in the corresponding 1,7-diradical
intermediate to form the [4 þ 4] cycloaddition products. A substituent at the C4 atom results in steric
hindrance for ring closure of the 1,4-diradicals in the [2 þ 2] cycloaddition pathway, therefore favoring the
[4 þ 4] and [4 þ 2] cycloaddition pathways. Regio- and diastereoselectivity in the [2 þ 2] and [4 þ 4]
cycloadditions have been discussed based on the thermodynamic stability of the relevant triplet diradical
intermediates and the conformations of these diradicals suitable for the intersystem crossing process.
Photoreactions of BZ with the oxazoles afford only [2 þ 2] cycloaddition products.
Introduction
of diradicaloid intermediates involved in these reactions. The
most widely investigated photocycloadditions are the [2 þ 2]
Photocycloaddition has been a research focus in organic
photochemistry and is considered as one of the fundamental
methods to build complex frameworks which are otherwise
difficult to make.1 Study of the chemo-, regio-, and stereoselec-
tivity in the photocycloadditions has contributed greatly to
understanding the mechanisms of concerted and nonconcerted
reactions as well as to elucidating the structure and reactivity
(1) For reviews, see: (a) Cycloaddition Reactions in Organic Synthesis;
Kobayashi, S., Jørgensen, K. A., Eds.; Wiley-VCH: Weinham, Germany,
€
2002. (b) Muller, F.; Mattay, J. Chem. Rev. 1993, 93, 99. (c) Crimmins, M. T.
In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Paquette, L. A.,
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DOI: 10.1021/jo101764f
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Published on Web 10/28/2010
J. Org. Chem. 2010, 75, 7757–7768 7757
2010 American Chemical Society

Wang et al.
JOCArticle
photocycloadditions, including the ones between two CdC
bonds leading to cyclobutanes2,3 and the ones between a CdO
N-(ω-iminoalkyl)maleimide derivatives8 have also been re-
ported, which provided efficient approaches to N-containing
seven-membered rings and their annulated derivatives.
Photoinduced [4 þ 4] reactions represent the highest order
photocycloadditions known up to now. [4 þ 4] photodimer-
izations and [4 þ 4] photocycloadditions with dienes have
been found for naphthalene,9 anthracene,10 2-pyridones,11,12
and 2-pyrones13,14 derivatives, some of which have been
applied in the synthesis of natural products.11-14
1,2-Dicarbonyl compounds constitute another interesting
class of substrates in photocycloadditions. They may serve as
either 2π or 4π addends to take part in [2 þ 2] or [4 þ 2]
cycloadditions with alkenes. It has been reported that photo-
excited benzil,15 biacetyl,16 and phenylglyoxylates17 could
react with alkenes to give R-ketooxetanes via [2 þ 2]
cycloaddition and, less commonly, dihydrodioxins via [4 þ
2] cycloaddition. Photoreactions of 1-acetylisatin (IS) with
monoalkenes18 or cyclic dienes19 (such as furans, indoles)
proceed exclusively via [2 þ 2] cycloaddition pathway to give
R-ketooxetanes. Similarly, photoreactions of 2-methyliso-
quinoline-1,3,4-trione with alkynes gives products derived
from [2 þ 2] photocycloaddition.20 Photoreactions of phe-
nanthrenequinone (PQ) with alkenes afford R-ketooxetanes
by [2 þ 2] reaction and dihydrodioxins by [4 þ 2] reaction.21
€
bond and a CdC bond leading to oxetanes (the Paterno-Buchi
reaction).4,5 Therefore, exploration on novel photocycloaddi-
tions along with detailed study on the mechanisms involved is of
great interest both synthetically and theoretically.
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in the construction of carbo- and heterocycles with various
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in [3 þ 2] (meta) or [4 þ 2] (para) manner.6 The [3 þ 2]
photocycloaddition was able to create a three-ring system
with six newly formed chiral carbon atoms and has been
successfully applied in the synthesis of many polycyclic
natural products. Besides, [5 þ 2] photocycloadditions have
been found between phathalimide and alkenes.7 Intramole-
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CHART 1
CHART 2
A few years ago, we reported in a preliminary communica-
tion the photoinduced [4 þ 4] cycloaddition of the o-qui-
nones PQ and IS with oxazoles, in which the OdC;CdO
moiety in PQ and IS and the CdN;CdC unit in the oxazole
serve as the 4π addends.22 These reactions stand for a new
type of [4 þ 4] photocycloadditions between two hetero-
dienes. To our knowledge, this is also the first report on
photocycloaddition reactions with any azadiene (CdN;
CdC) as a 4π addend. Here we report our further investiga-
tion on these photoreactions by expanding the oxazoles used
to cover a wide range of ring-substitution pattern in search of
the reaction scope and the factors affecting reactions path-
way ([2 þ 2], [4 þ 2], and [4 þ 4]) competition. Reaction
mechanism, regio- and stereoselectivity in these pathways
are also discussed based on unambiguous determination of
the steric structures of the corresponding products.
are given in Chart 2 and Table 1. Irradiation of a benzene
solution of IS with 1a for 24 h resulted in complete conversion of
IS and gave the [4 þ 4]/[2 þ 2] product 2a in 12% yield and a
trans-spirooxetane product 3a (73%). Product 2a is formed by
initial [4 þ 4] photocycloaddition of IS with 1a followed by
further [2 þ 2] cycloaddition of another excited IS with the
primary [4 þ 4] product. A small amount of two other
diastereoisomeric [4 þ 4]/[2 þ 2] products and a cis-spiroox-
etane product 4a were also isolated with a total yield of 4%.
Comparison of the spectral data indicated that 2a has the same
configuration as the product 2g formed from IS and 1g, whose
steric structure was established by an X-ray crystallographic
analysis.22 Similar photolysis of IS with 2,4-diphenyloxazole 1b,
on the other hand, afforded only three spirooxetane products:
the trans-3b (27%), the cis-4b (41%), and an N-deacetylated
oxetane 5b (22%). In the photoreactions of IS with 4,5-diphe-
nyloxazoles 1c, thetrans-oxetane product 3c (48%)22 and its cis-
isomer 4c (20%) were formed along with a [4 þ 4]/[2 þ 2]
product 2c (22%). Photoreactions of IS with 2,5-diphenylox-
azole 1d gave two oxetane products: the trans-3d (22%) and the
cis- 4d (48%).22 For 2-methyl-5-ethoxyoxazole 1e, reaction with
photoexcited IS furnished a [4 þ 4]/[2 þ 2] product 2e (12%)
and two secondary products 6e (22%) and 7e (31%) derived
from the hydrolysis of the oxetane products. Obviously, 7e is the
primary hydrolysis product, while 6e is formed by intramole-
cular condensation of 7e. Similar to 4-phenyloxazole 1g, photo-
reaction of 4-(4-methoxyphenyl)oxazole 1f with IS gave a [4 þ 4]/
[2 þ 2] product 2f exclusively in 97% yield.
Results
Photoinduced cycloadditions of the 1,2-dicarbonyl com-
pounds IS, PQ, and benzil (BZ) with oxazoles have been
investigated. The oxazoles used in the photocycloadditions
include 1a-j (Chart 1).
1. Photoinduced Reactions of IS with Oxazoles 1a-j. The
results of photoinduced reactions of IS with the oxazoles 1a-j
(22) Zhang, Y.; Wang, L.; Zhang, M.; Fun, H.-K.; Xu, J.-H. Org. Lett.
2004, 6, 4893.
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TABLE 1. Photoreactions of IS with Oxazoles 1a-ja
products and yield (%)b
entry
oxazole
irrd. time (h)
conversion (%)
[4 þ 4]/[2 þ 2]
2a (12)
[2 þ 2]trans
[2 þ 2]cis
others
1
2
3
4
5
6
7
8
9
10
1a
1b
1c
1d
1e
24
24
24
24
12
48
24
48
24
24
100
38
3a (73)
3b (27)
3c (48)
3d (22)
4b (41)
4c (20)
4d (48)
5b (22)
100
100
100
100
100
100
100
100
2c (22)
2e (12)
2f (97)
2g (97)
2h (64)
2i (9)
6e (22) ,7e (31)
1f
1gc
1hc
1ic
1jc
3h (8)
3i (77)
3j (54)
4h (13)
4j (41)
aAll the reactions were run in PhH. bIsolated yield. cResults for 1g-j have been reported in ref 22.
CHART 3
The cis- and trans-spirooxetanes have similar spectral
2. Photoinduced Reactions of PQ with Oxazoles. The
results of photoinduced reactions of PQ with the oxazoles
are given in Chart 3 and Table 2. Photoreactions of PQ
with 2-ethyl-4-phenyloxazole 1a in MeCN gave the [4 þ4]
product 8a (20%) and an oxetane product 9a (69%). The
crystal structure of 8a is shown in Chart 3. Similar reactions
of PQ with 2,4-diphenyloxazole 1b gave a [4 þ 2] product 10b
and a secondary product 11 (55%) derived from further
reaction of 10b with a second molecule of excited PQ. In the
photoreactions of PQ with 4,5-diphenyloxazole 1c, the [4 þ 4]
product 8c (43%) was formed together with a 2:1 product 12
(13%) derived from secondary photo [4 þ 2] cycloaddition
of the [4 þ 4] product with another molecule of PQ. Photol-
ysis of PQ with 2,5-diphenyloxazole 1d afforded only a trans-
oxetane product 9d22 (45%) and a [4 þ 2] product 10d22
(32%).
characteristics. X-ray crystallographic analysis of several
oxetane products with both trans (3a, 3d, and 3i) and cis
(4d) configurations has helped to unambiguously assign their
steric structures.23 On the basis of these crystal structures, we
systematically compared the spectral data of the cis- and
trans-oxetanes and found that the following features can be
applied as “fingerprint” to distinguish the two isomers. (1) In
1
the H NMR spectra, the methyl in the N-acetyl groups
resonates at significantly higher field at ∼2.2 ppm for the cis-
but at ∼2.7 ppm for the trans-isomers. (2) The aromatic
protons of the cis- and trans-isomers absorb in the 7.1-7.7
and 6.8-7.4 ppm region, respectively. (3) In the 13C NMR,
the imine carbon for the cis- and trans-isomers resonates at
174-175 and 171-172 ppm, respectively.
(23) 3a and 4d: Usman, A.; Fun, H.-K; Wang, L; Zhang, Y.; Xu, J.-H.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2003, C59, o305. 3d: Li,
X.-M; Wang, L.; Xu, J.-H.; Zhang, S.-S.; Fun, H.-K. Acta Crystallogr., Sect.
E: Struct. Rep. Online 2003, E59, o1883. 3i: Usman, A.; Razak, I. A.; Fun,
H.-K.; Chantrapromma, S.; Zhang, Y.; Xu, J.-H. Acta Crystallogr. E: Struct.
Rep. Online 2002, E58, o145.
On the basis of the X-ray crystallographic analysis of the
[4 þ 4] products 8a, 8c, and 8i, and of the [4 þ 2] products 10d
and 10i, we compared the spectral data of the [4 þ 4] and the
[4 þ 2] products and found several main features that help to
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TABLE 2. Photoreactions of PQ with Oxazolesa
CHART 4
products and yield (100%)b
irrd.
entry oxazole time (h)
conversion
(%)
[4 þ 4] [2 þ 2] [4 þ 2] others
8a (20) 9a (69)
10b (25) 11 (55)
1
2
3
4
5
6
7
8
1a
1b
1c
24
24
24
24
12
24
24
24
100
80
100
100
100
100
100
90
8c (43)
10c (39) 12 (13)
9d (45) 10d (32)
13
1d
1e
1gc
1ic
1jc
8g (61) 9g (30)
8i (89) 9i (8)
8j (89) 9j (8)
TABLE 3. Photoreactions of BZ with Oxazoles
aAll the reactions were run in MeCN except for entry 3, which was
carried out in PhH. bIsolated yield. cReference 22.
irrd. time
(h)
conversion
(%)
products and
yield (%)a
entry
oxazole
1
2
1i
1j
48
48
80
70
15a (16), 16a(81)
15b (69)
distinguish these two types of products. (1) In the IR, the
C-O-C asymmetric stretching band occurs at ∼1150 cm-1
and is the strongest absorption band for the [4 þ 4] product,
while the CdN stretching band is at 1628-1659 cm-1 and
weaker than the 1150 cm-1 C-O-C band. In contrast, in the
[4 þ 2] products, the C-O-C band at 1150 cm-1 is never
the strongest band in IR, while the CdN stretching band is at
∼1657 cm-1 with a similar strength as the C-O-C 1150 cm-1
band. (2) In MS, the oxazole cation radical is always the base
peak for the [4 þ 4] products, while other fragmentations gave
much weaker peaks. On the other hand, for the [4 þ 2] product,
the fragment with a m/z180 derived from the decarbonylation of
PQ is always the base peak, while the oxazole cation radical peak
is rather weak. (3) In the 13C NMR, the imine carbon atom
(CdN) in the [4 þ 4] products absorbs at 171-174 ppm, at a
field strength 6-9 ppm higher than that of the [4 þ 2] product,
which resonates at ∼165 ppm.
aReactions were run in PhH.
Reactions of triplet excited IS takes place by initial attack
of the C3 carbonyl oxygen to the substrate.18,19,24 The [2 þ 2]
and [4 þ 4] photocycloadditions of IS with the oxazoles
proceed via the triplet 1,4-biradical A and the 1,7-biradical B,
respectively. Intersystem crossing (ISC) to the corresponding
singlet biradicals followed by radical pair recombination
gives the [2 þ 2] and [4 þ 4] products. The [4 þ 4] cycloadduct
ꢀ
then acts as an active alkene to undergo a further Paterno-
Buchi reaction with a second excited IS to give the [4 þ 4]/
[2 þ 2] product. Similarly, triplet biradicals G, H, and I in
Scheme 2 are the precursors of the [2 þ 2], [4 þ 2], and [4 þ 4]
cyclization products in the photoreactions of PQ with the
oxazoles. After ISC and radical pair recombination, they
afforded the corresponding cycloadducts 9, 10, and 8, re-
spectively. As a result, substituents at different positions of
the oxazole ring (R1, R2, and R3) may present different
steric hindrance to the radical pair recombination process in
A, B, G, H, and I, which leads to the formation of the
corresponding [2 þ 2], [4 þ 4], and [4 þ 2] products.
3. Photoinduced Reactions of BZ with Oxazoles. Com-
pared with IS and PQ, BZ showed lower photoreactivity with
oxazoles and took a much longer time to reach a similar
conversion as in the reactions of IS and PQ. Besides, in contrast
to the cyclic IS and PQ, BZ behave only as a 2πaddend and gave
the [2 þ 2] cycloaddition products in photoreactions with the
oxazoles 1i and 1j (Chart 4 and Table 3).
The competition of the different pathways is found to be
strongly dependent on the structures of the oxazoles. A
general trend can be perceived by inspection of the results
shown in Tables 1 and 2. That is, oxazoles without substit-
uents at C2 and C5 showed a strong preference for the
formation of the [4 þ 4] product over the [2 þ 2] and [4 þ 2]
products. Therefore, photoreactions of IS with the 4-arylox-
azoles 1f, 1g, and 1h gave the [4 þ 4] product exclusively or
predominantly. In the photocycloaddition of PQ with 4-phe-
nyloxazole, the [4 þ 4] cycloadduct is also the main product
(61%). Introduction of a substituent at C2, or even worse, at
both C2 and C5, impedes the formation of [4 þ 4] product
and favors the formation of [2 þ 2] and [4 þ 2] products. As a
result, in photoreactions of IS with 2-ethyl-4-phenyloxazole
1a (entry 1 in Table 1), 2-methyl-4-phenyloxazole 1i (entry 9
in Table 1), and 2-methyl-5-ethoxyoxazole 1e (entry 5 in
Table 1), the [4 þ 4] products are minor products with the
oxetanes being the major products. Photoreactions of IS
with 2,4-diphenyloxazole 1b, 2,5-diphenyloxazole 1d, and
2,4,5-trimethyloxazole 1j gave the oxetane products exclu-
sively. The results of photoreactions of PQ with these 2- and
2,4- or 2,5-substituted oxazoles are parallel with that of IS.
Therefore, reactions with 1b and 1d gave only [4 þ 2] and/or
[2 þ 2] products without [4 þ 4] products. The only exception
Discussion
1. Chemoselectivity in the Photocycloadditions. Photocy-
cloadditions of IS and PQ with the oxazoles proceed via the
nπ* triplet excited state of the IS18,19,24 and PQ.21 With the
oxazoles examined, IS takes part in [2 þ 2] and [4 þ 4]
photocycloadditions, while PQ undergoes [2 þ 2], [4 þ 2],
and [4 þ 4] cycloadditions. These reactions are proposed to
take place via mechanisms shown in Schemes 1 and 2. A
different type of biradicals were involved in these photo-
cycloaddition pathways, which was reminiscent of the in-
tramolecular cyclizations reported before.25 It is rational
that no cycloaddition was observed on the CdN bond of
the oxazole since the relative stability of the corresponding
biradical intermediates is much lower than that of other
biradical intermediates as shown in Schemes 1 and 2.
(24) (a) Wang, L.; Zhang, Y.; Hu, H.-Y.; Fun, H.-K; Xu, J.-H. J. Org.
Chem. 2005, 70, 3850. (b) Zhang, Y.; Wang, L.; Zhu, Y.; Xu, J.-H. Eur. J.
Org. Chem. 2004, 3, 527. (c) Xue, J.; Zhang, Y.; Wang, X.-L.; Fun, H.-K.; Xu,
J.-H. Org. Lett. 2000, 2, 2583.
(25) (a) Lewis, F.; Reddy, G. D.; Bassani, D. M. J. Am. Chem. Soc. 1993,
115, 6468. (b) Lewis, F.; Bassani, D. M.; Reddy, G. D. J. Org. Chem. 1993,
58, 6390.
J. Org. Chem. Vol. 75, No. 22, 2010 7761

Wang et al.
JOCArticle
SCHEME 1
SCHEME 2
to this trend is in the reaction of PQ with 2,4,5-trimethyloxazole
1j, which gave the [4 þ 4] product in 89% yield together with the
oxetane 9j (8%).
The reaction mechanisms shown in Schemes 1 and 2 reveal
that this trend in reaction pathway partitioning is mainly
influenced by the steric hindrance toward the intramolecular
radical pair recombination. The structures of the biradical
intermediates A, B, G, H, and I indicated that a sterically
demanding group at C2 would hamper the [4 þ 4] product
formation via biradical intermediates B and I by exerting
large steric hindrance for radical pair recombination after
the ISC process, leading to the collapse of the singlet
7762 J. Org. Chem. Vol. 75, No. 22, 2010

Wang et al.
JOCArticle
biradical to the starting o-quinone and the oxazole because
of a large interradicaloid distance (IRD).26 For the same
reason, a bulky group at C4 retards the formation of both
[2 þ 2] and [4 þ 2] products via diradicals A, G, and H at the
advantage of [4 þ 4] product. The results in Tables 1 and 2 are
in perfect agreement with this analysis. The preferential
formation of the [4 þ 4] product in the photoreaction of
PQ with 1j is probably the result of a balance between the
unfavorable steric influence for both the [4 þ 4] product and
the [2 þ 2] (as well as [4 þ 2]) product formation.
SCHEME 3. ISC Conformations of the Triplet 1,4-Biradical of
IS with 2-Alkyl-4-phenyloxazole
2. Regio- and Diastereoselectivity in the Photocycloaddi-
tions. The [2 þ 2] photocycloadditions of IS and PQ with the
oxazoles are highly regioselective to give spirooxetanes with
an acetal structure. This regioselectivity is similar to that in
the [2 þ 2] photocycloadditions of the n-π* excited carbonyl
compounds with furans,19 benzofurans,19 and oxazoles,27
and is believed to be the result of the intervening of the most
selectivity in the ISC and radical pair recombination of
various triplet 1,4-,30 1,5-,31 1,6-,32 1,7-,33 and 1,8-34 biradi-
cals. In the case of photoreactions of IS with the oxazoles, for
the 2-alkyl-4-phenyloxazoles (1a and 1i), in the ISC confor-
mation for the formation of the trans-oxetane (J in Scheme
3), there is no significant steric hindrance between the isatin
framework and the oxazole’s substituents either in the static
conformer or in the ISC process by inward rotation of the p
orbital at the oxazole’s C5 atom. This process, on the
contrary, may benefit from the π-π stacking interaction
emerging by the approaching of the phenyl at C4 of the
oxazole toward the isatin’s benzene ring. In sharp contrast,
in the ISC conformation leading to the cis-oxetane (K in
Scheme 3), there is significant steric hindrance between the
C4 phenyl of oxazole and the C3 carbonyl of IS. Further-
more, the ISC process would bring the alkyl group to
approach the isatin’s benzene ring, causing unfavorable
steric interaction between them. As a result, the trans-
oxetanes 3a and 3i predominate in the [2 þ 2] products.
For 2,4-diphenyloxazole, the steric hindrance between the
alkyl and the isatin benzene ring during the ISC process is
lifted and replaced by a favorable π-π stacking effect
between the two phenyls. This leads to an increased yield
of the cis-4c. For 2,5-diphenyloxazole, the steric hindrance in
the ISC conformation and during the ISC process for cis-
oxetane (see K in Scheme 3, with the Ph moved to C5) is
removed, allowing the cis-isomer to be the major product.
The [4 þ 4] cycloadducts of IS with the oxazoles are
formed with high regio- and diastereoselectivity. The crystal
structure of 2g and 2i22 and the parallelism in the spectral
data of the other [4 þ 4] products indicated that 2a, 2e, 2f,
and 2h have the same steric configuration as 2g and 2i. In all
the primary [4 þ 4] products (C in Scheme 1) the C2 carbonyl
oxygen atom in IS is bonded to the oxazole’s C2 atom, while
the C3 carbonyl oxygen atom is bonded to the C5 atom of the
oxazole, and the oxazole ring is syn to the isatin framework.
To search for the origin of this regio- and diastereoselec-
tivity, with the [4 þ 4] product of IS with 4-phenyloxazole 1i
as an example, calculations on the possible ISC conforma-
tions of the two regioisomeric triplet biradicals B and E
(Scheme 1) leading to the two regioisomers were carried out
with the DFT method at the UB3LYP-6-31G* level.35 For
biradical B leading to the experimentally found product C,
stable 1,4-diradical intermediate (the 2-oxabutane-1,4-diyl)
4,5
ꢀ
€
in the Paterno-Buchi reaction because an allylic type
radical as at the oxazole’s C4 atom in diradical A
(Scheme 1) is ∼7 kcal/mol more stable than the alternative
R-oxomethylene radical at oxazole’s C5 as in D (Scheme 1).28
The [4 þ 4] reactions of IS with the oxazoles are also
regioselective to give products where the C2 and C5 atoms
in the oxazole are linked to the C2 and C3 carbonyl oxygen
atoms, respectively.
The secondary [2 þ 2] photocycloadditions of IS with the
primary [4 þ 4] product C (Scheme 1) take place with the C3
carbonyl oxygen and carbon atoms of the IS linking to the C2
and C3 atoms of the isatin unit in the [4 þ 4] product,
respectively. This regioselectivity is the result of the forma-
tion of the most stable biradical intermediate E, which has
two benzylic radical centers.
The diastereoselectivity for the oxetane formation in the
photoreactions of IS with the oxazoles is manifested by the
relative stability of the triplet diradical conformations sui-
table for ISC.29,30 In these active ISC conformers with the
two spin-bearing orbitals orthogonal to each other within
˚
bond forming distance (∼2-3 A), steric effect of the oxa-
zole’s substituents significantly affects their thermodynamic
stability and the ISC process energy barrier. This concept has
been successfully applied in the rationalization of the stereo-
(26) (a) Ihmels, H.; Scheffer, J. R. Tetrahedron 1999, 55, 885. (b)
Leibovitch, M.; Olovsson, G.; Scheffer, J. R.; Trotter, J. J. Am. Chem.
Soc. 1998, 120, 12755. (c) Cheung, E.; Netherton, M. R.; Scheffer, J. R.;
Trotter, J. Org. Lett. 2000, 2, 77. (d) Schmidt-Mende, L.; Kroeze, J. E.;
€
Durrant, J. R.; Nazeeruddin, M. K.; Gratzel, M. Org. Lett. 2005, 7, 1315. (e)
Yang, C.; Xia, W.; Scheffer, J. R.; Botoshansky, M.; Kaftory, M. Angew.
Chem., Int. Ed. 2005, 44, 5087.
(27) (a) Griesbeck, A. G.; Fiege, M.; Lex, J. Chem. Commun. 2000, 589.
(b) Griesbeck, A. G.; Bondock, S. Can. J. Chem. 2003, 81, 555. (c) Griesbeck,
A. G.; Bondock, S.; Lex, J. J. Org. Chem. 2003, 68, 9899. (d) Bondock, S.;
Griesbeck, A. G. Monatsh. Chem. 2006, 137, 765. (e) Griesbeck, A. G.;
Bondock, S. Aust. J. Chem. 2008, 61, 573.
(28) (a) Bordwell, F. G.; Zhang, X. M. Acc. Chem. Res. 1993, 26, 510 and
references cited therein. (b) Leroy, G.; Sana, M.; Wilante, C.; Nemba, R. M.
J. Mol. Struct. 1989, 198, 159.
(29) (a) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. 1972, 11, 92. (b)
Wirz, J. Pure Appl. Chem. 1984, 56, 1289. (c) Carlacci, L.; Doubleday, C., Jr.;
Furlani, T. R.; King, H. F.; McIver, J. W., Jr. J. Am. Chem. Soc. 1987, 109,
5323. (d) Adam, W.; Grabowski, S.; Wilson, R. M. Acc. Chem. Res. 1990, 23,
165. (e) Kutateladze, A. J. Am. Chem. Soc. 2001, 123, 9279.
(30) (a) Griesbeck, A. G.; Mauder, H.; Stadtmueller, S. Acc. Chem. Res.
1994, 27, 70. (b) Griesbeck, A. G.; Abe, M.; Bondock, S. Acc. Chem. Res.
2004, 37, 919.
(32) Griesbeck, A. G.; Heckroth, H.; Schmickler, H. Tetrahedron Lett.
1999, 40, 3137.
(33) Griesbeck, A. G.; Kramer, W.; Lex, J. Angew. Chem., Int. Ed. 2001,
40, 577.
(31) Griesbeck, A. G.; Kramer, W.; Bartoschek, A.; Schmickler, H. Org.
Lett. 2001, 3, 537.
(34) Shen, Y.-M.; Yang, X.-L.; Chen, D.-Q.; Xue, J.; Zhu, L.; Fun, H.-K.;
Hu, H.-W.; Xu, J.-H. Chem.;Eur. J. 2010, 16, 2873.
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SCHEME 4. ISC Conformations of the 1,7-Biradical of IS with
4-Phenyloxazole
Conclusion
In summary, a new type of [4 þ 4] photocycloadditions
between two heterodienes have been investigated in the photo-
cycloadditions of o-quinones (IS and PQ) with oxazoles. The
OdC;CdO unit in o-quinone and the CdN;CdCpartinthe
oxazole serve as 4π addends in the photocycloadditions, leading
to the formation of the [4 þ 4] products 2 and 8 with regio- and
diastereoselectivity. The [4 þ 2] and [2 þ 2] photocycloadditions
also occurred in competition with the [4 þ 4] reaction for some
oxazoles. The partitioning between these photocycloaddition
pathways is determined by the difference in steric hindrances for
radical pair recombination of the biradical intermediates leading
to the [4 þ 4], [4 þ 2], and [2 þ 2] products. A substituent at the
C4 of oxazole disfavors the [2 þ 2] cycloaddition while it favors
the [4 þ 4] cycloaddition; a substituent at the C2 of oxazole,
on the other hand, disfavors the [4 þ 4] cycloaddition while it
favors the alternative [4 þ 2] and [2 þ 2] cycloaddition path-
ways. All the photocycloadditions are highly regioselective.
The regioselectivity can be rationalized by the intervening of
most stable biradical intermediates in each reaction pathway.
Diastereoselectivity in the [4 þ 4] and [2 þ 2] reactions is
dependent on the active conformations suitable for ISC and
the ensuing bond formation of the triplet biradical intermediates.
two gauche conformations L and M (Scheme 4) with geo-
metry suitable for ISC were found to be energy minima, with
L representing the global minimum and M as a local mini-
mum with an energy 4 kcal/mol higher than that of L. In
conformation L, the dihedral angle C1-C2-O3-C4 is -37°
and that between C2-O3-C4-C5 is 154°. In this geometry,
the phenyl group at C5 pointing downward and the two
radical centers (O6 and C7) are far apart from each other
˚
with an IRD of 5.17 A. In conformation M, the two dihedral
angles C1-C2-O3-C4 and C2-O3-C4-C5 are -36° and
-41°, respectively, with the phenyl at C5 pointing upward
and the two radical centers close to each other (IRD 2.81 A).
˚
Experimental Section
In both L and M, the isatin framework and the oxazole ring
are nearly perpendicular to each other, and as a result, the
two spin-bearing p orbitals are nearly orthogonal to each
other. In these two conformations, one of the 2p orbitals at
the O3 atom is parallel with and close to the C4-O8 σ bond,
enabling an efficient 2p(O3)-σ*(C4-O8) overlap. This
anomeric effect provides stereoelectronic stabilization to
the two conformations.
General Procedures for the Preparative Photolysis of o-Quinones
with Oxazoles. The light source was a medium-pressure mercury
lamp (500 W) in a cooling water jacket that was further surrounded
by a layer of filter solution (saturated NaNO2) to cut off light of
wavelength shorter than 400 nm. The solution of o-quinone and
oxazole in dry benzene or acetonitrile was purged with N2 for
10 min and then irradiated under continuous nitrogen purging. The
reaction course was monitored by TLC. At the end of the reaction,
the solvent was removed under reduced pressure and the residue
was separated by flash chromatography.
Clearly, the large IRD between the two radical centers
˚
(5.17 A) in L means that ISC in this conformation would
result in the O3-C4 bond cleavage to give the starting
material IS and 1i. ISC in M with a small IRD by inward
rotation of the p orbital at C7, on the other hand, leads to the
O6-C7 bond formation to give the [4 þ 4] product with the
actually found syn-configuration between the oxazole and
the isatin moieties. This ISC process is further benefited by
the π-π stacking interaction between the two benzene rings
of the oxazole and the isatin. Meanwhile, the high regios-
electivity in the [4 þ 4] product formation is accounted for by
the fact that in the conformation of M, suitable for ISC and
bond formation, the regioisomeric biradical E (Scheme 1) is
4.3 kcal/mol higher than M, resulting in the absence of the
regioisomeric [4 þ 4] product.
Photolysis of IS with 1a. A solution of IS (756 mg, 4 mmol)
and 1a (830 m g, 4.8 mmol) in benzene (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue was
separated by flash chromatography to give 2a (133 mg, 12%), 3a
(1.06 g, 73%), and 4a (58 mg, 4%).
2a, (2R,30S,5R,6aS,11aR)-10,11-diacetyl-2,5-dihydro-2-ethyl-4-
phenylspiro[2,5-epoxy-11a,6a-(epoxymethano)-11H-1,6,3-dioxazo-
cino[8,7-b]indole-13,30-[3H]indol]-20(10H)-one: colorless crystals
from acetone-petroleum ether, mp 188-190 °C; 1H NMR (300
MHz, CDCl3) δ 8.23 (td, 2H, J = 7.9, 1.0 Hz), 8.16 (d, 1H, J = 8.3
Hz), 7.56-7.47 (m, 2H), 7.39 (dd, 2H, J = 7.8, 1.3 Hz), 7.35-7.30
(m, 1H), 7.16 (t, 2H, J = 7.8 Hz), 7.02 (td, 1H, J = 8.0, 1.4 Hz),
6.51 (dd, 1H, J = 7.6, 0.9 Hz), 6.40-6.35 (m, 2H), 2.53 (s, 3H),
2.41 (q, 2H, J = 6.9 Hz), 2.23 (s, 3H), 1.15 (t, 3H, J = 7.4 Hz); 13
C
NMR (75 MHz, CDCl3) δ 171.68, 171.64, 169.92, 169.24, 143.64,
140.50, 132.40, 131.76, 131.36, 128,95, 128.42, 128.33, 128.17,
127.93, 125.44, 123.52, 123.17, 123.08, 122.83, 116.42, 101.46,
29.95, 26.13, 24.96, 7.63; IR (KBr) νmax/cm-1 2982, 2937, 1779,
1717, 1613, 1470, 1377, 1071, 1004, 757; MS m/z (%) 507 (0.1), 405
(0.3), 346 (3), 278 (6), 173 (88), 146 (100), 130 (38), 90 (49); EA
foundC, 67.24;H, 4.56;N, 7.62, C31H25N3O7 requiresC, 67.51;H,
4.54; N, 7.62.
3a, (1R,30R,5R)-10-acetyl-3-ethyl-1-phenylspiro[4,6-dioxa-2-
azabicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: color-
less crystals from acetone-petroleum ether, mp 118-120 °C;
1H NMR (300 MHz, CDCl3) δ 8.15 (d, 1H, J = 8.2 Hz),
7.24-7.19 (m, 4H), 7.10-7.06 (m, 2H), 6.96 (dd, 1H, J = 7.2,
1.5 Hz), 6.87 (s, 1H), 6.67 (dd, 1H, J = 7.5, 1.1 Hz), 2.71 (s, 3H),
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
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Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
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7764 J. Org. Chem. Vol. 75, No. 22, 2010

Wang et al.
JOCArticle
2.67-2.58 (m, 2H), 1.36 (t, 3H, J = 7.6 Hz); 13C NMR (300
MHz, CDCl3) δ 173.02, 172.30, 170.31, 139.56, 133.78, 130.92,
128.75, 128.65, 126.72, 125,67, 124.59, 124.38, 116.32, 94.53,
88.89, 26.43, 22.11, 9.99; IR (KBr) νmax/cm-1 2993, 2947,
1778, 1712, 1658, 1604, 1466, 1277, 950, 752; MS m/z (%) 236
(1), 189 (1), 146 (100), 130 (8), 90 (35), 43 (79); EA found C,
69.52; H, 4.95; N, 7.88, C21H18N2O4 requires C, 69.61; H, 4.97;
N, 7.73.
4a, (1S,30R,5S)-10-acetyl-3-ethyl-1-phenylspiro[4,6-dioxa-2-
azabicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless
crystals from acetone-petroleum ether, mp 124-126 °C; 1H
NMR (300 MHz, CDCl3) δ 8.20 (d, 1H, J = 8.2 Hz), 7.57 (dd,
1H, J = 7.4, 1.2 Hz), 7.47 (td, 1H, J = 7.9, 1.4 Hz), 7.34-7.29 (m,
4H), 7.20-7.17 (m, 2H), 7.04 (s, 1H), 2.65 (q, 2H, J = 7.6 Hz),
2.17 (s, 3H), 1.37 (t, 3H, J = 7.5 Hz); 13C NMR (300 MHz,
CDCl3) δ 174.95, 171.01, 169.79, 141.59, 134.29, 131.86, 128.85,
128.78, 126.30, 126.08, 125.25, 121.92, 116.75, 105.40, 94.17,
88.61, 25.64, 22.15, 10.04; IR(KBr) νmax/cm-1 2984, 1776, 1703,
1657, 1610, 1458, 1288, 1169, 939, 751; MS m/z (%) 362 (Mþ, 0.6),
334 (3), 278 (5), 236 (21), 189 (10), 173 (96), 146 (70), 130 (55), 90
(42), 56 (100); EA found C, 69.63; H, 5.02; N, 7.64, C21H18N2O4
requires C, 69.61; H, 4.97; N, 7.73.
Photolysis of IS with 1b. A solution of IS (400 mg, 2.1 mmol)
and 1b (550 mg, 2.5 mmol) in benzene (40 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 24 h
to reach a 38% conversion of IS. Solvent was removed under
reduced pressure and the residue was separated by flash chro-
matography to give 3b (100 mg, 27%), 4b (150 mg, 41%), 5b (80
mg, 22%), and recovered IS (230 mg).
3b, (1R,30R,5R)-10-acetyl-1,3-diphenylspiro[4,6-dioxa-2-aza-
bicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless
crystals from acetone-petroleum ether, mp 126-128 °C; 1H
NMR (300 MHz, CDCl3) δ 8.16 (d, 1H, J = 8.1 Hz), 8.12 (dt, 2H,
J = 7.0, 1.5 Hz), 7.57 (t, 1H, J = 7.2 Hz), 7.48 (t, 2H, J = 7.5 Hz),
7.29-7.15 (m, 6H), 7.06 (s, 1H), 7.02 (d, 1H, J = 7.5 Hz), 6.90 (t,
1H, J = 7.5 Hz), 2.66 (s, 3H); 13C NMR (300 MHz, CDCl3) δ
172.06, 170.41, 167.24, 139.65, 133.84, 132.48, 131.02, 129.03,
128.77, 128.72, 128.56, 126.83, 126.65, 126.22, 125.74, 124.65,
124.40, 116.37, 107.18, 94.49, 89.60, 28.47; IR(KBr) νmax/cm-1
1775, 1712, 1638, 1493, 1372, 1339, 1274, 1171, 957, 747, 697; m/z
(%) 382 (1), 221 (100), 193 (61), 165 (10), 146 (49), 105 (13), 90
(46), 63 (16), 43 (30); EA found C, 73.08; H, 4.29; N, 6.76,
C25H18N2O4 requires C, 73.17; H, 4.39; N, 6.83.
4b, (1S,30R,5S)-10-acetyl-1,3-diphenylspiro[4,6-dioxa-2-aza-
bicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless
blocks from acetone-petroleum ether, mp 114-116 °C; 1H
NMR (300 MHz, CDCl3) δ 8.23 (d, 1H, J = 8.4 Hz), 8.15 (dt,
2H, J = 7.0, 1.5 Hz), 7.61 (tt, 1H, J = 7.2, 1.2 Hz), 7.53-7.43
(m, 4H), 7.36-7.27 (m, 5H), 7.22 (s, 1H), 7.20 (td, 1H, J = 7.5,
1.0 Hz), 2.22 (s, 3H); 13C NMR (300 MHz, CDCl3) δ 174.87,
169.84, 165.48, 140.29, 134.40, 132.84, 131.88, 129.32, 129.14,
128.93, 128.82, 128.79, 126.54, 126.40, 126.28, 125.26, 122.04,
116.74, 105.87, 94.59, 89.08, 25.78; IR(KBr) νmax/cm-1 1768,
1711, 1632, 1494, 1374, 1334, 1306, 1281, 1206, 1177, 1015, 938,
780, 693; m/z (%) 221 (19), 193 (13), 189 (16), 146 (100), 119 (7),
90 (32), 76 (2), 63 (10), 43 (50); EA found C, 73.05; H, 4.16; N,
6.97, C25H18N2O4 requires C, 73.17; H, 4.39; N, 6.83.
5b, (1R,30R,5R)-1,3-diphenylspiro[4,6-dioxa-2-azabicyclo[3.2.0]-
hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless needles from
acetone-petroleum ether, mp 128-130 °C; 1H NMR (300
MHz, CDCl3) δ 8.21 (d, 2H, J = 7.5 Hz), 7.62 (t, 1H, J = 7.5
Hz), 7.53 (t, 3H, J = 7.5Hz), 7.32-7.35 (m, 6H), 7.23 (s, 1H), 7.15
(s, 1H), 7.05 (t, 1H, J = 7.5 Hz), 6.83 (d, 1H, J = 7.8 Hz); IR
(KBr) νmax/cm-1 3208, 1725, 1641, 1621, 1468, 1333, 1276, 1209,
1118, 1048, 1016, 963, 751, 696; m/z (%) 368 (0.1), 340 (2), 221
(100), 193(66), 165 (11), 147 (26), 119(43), 105 (8), 89(48), 77 (10),
62 (26), 51 (10); EA found C, 75.12; H, 4.20; N, 7.48, C23H16N2O3
requires C, 75.00; H, 4.35; N, 7.61.
Photolysis of IS with 1c. A solution of IS (756 mg, 4 mmol)
and 1c (1.768 g, 8 mmol) in benzene (80 mL) was irradiated with
light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue
was separated by flash chromatography to give 2c (261 mg,
22%), 3c (787 mg, 48%), and 4c (326 mg, 20%).
2c, (2R,30S,5R,6aS,11aR)-10,11-diacetyl-2,5-dihydro-4,5-diphenyl-
spiro[2,5-epoxy-11a,6a-(epoxymethano)-11H-1,6,3-dioxazocino-
[8,7-b]indole-13,30-[3H]indol]-20(10H)-one: colorless crystals from
acetone-petroleum ether, mp 224-226 °C; 1H NMR (300 MHz,
CDCl3) δ 8.29 (d, 1H, J = 7.8 Hz), 8.21 (d, 1H, J = 6.9 Hz), 8.11
(d, 1H, J = 8.4 Hz), 7.52 (t, 1H, J = 7.8 Hz), 7.42-7.30 (m, 4H),
7.18-7.08 (m, 6H), 7.01-6.90 (m, 3H), 6.61 (d, 1H, J = 7.5 Hz),
6.37 (t, 1H, J = 7.8 Hz), 2.57 (s, 3H), 2.30 (s, 3H); 13C NMR (300
MHz, CDCl3) δ 173.70, 171.65, 170.08, 169.29, 13.49, 140.40,
135.18, 132.09, 132.79, 131.49, 129.92, 129.43, 129.25, 128.82,
128.70, 128.45, 127.89, 127.79, 125.99, 125.47, 123.54, 123.27,
116.76, 116.41, 116.32, 115.86, 115.66, 114.36, 110.81, 26.16,
25.11; IR (KBr) νmax/cm-1 3060, 2974, 1779, 1711, 1690, 1607,
1468, 1371, 1341, 1312, 1279, 1193, 1174, 1079, 785; m/z (%) 555
(0.8), 514 (0.2), 334 (6), 278 (13), 221 (82), 193 (40), 165 (39), 146
(82), 105 (33), 90 (48), 77 (31), 43 (100); EA found C, 70.29; H,
4.08; N, 7.04, C35H25N3O7 requires C, 70.12; H, 4.17; N, 7.01.
3c, (1R,30R,5R)-10-acetyl-1,5-diphenylspiro[4,6-dioxa-2-azabicyclo-
[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless crystals
from acetone-petroleum ether, mp 122-124 °C; 1H NMR (300
MHz, CDCl3) δ 8.14 (d, 1H, J = 8.2 Hz), 7.80-7.76 (m, 2H), 7.68
(s, 1H), 7.63-7.57 (m, 3H), 7.21 (td, 1H, J = 7.9, 1.4 Hz), 7.11 (dd,
1H, J = 7.6, 0.9 Hz), 7.05-6.87 (m, 4H), 6.73-6.70 (m, 2H), 2.75
(s, 3H); 13C NMR (300 MHz, CDCl3) δ 172.43, 170.29, 157.85,
139.91, 134.25, 133.65, 131.01, 130.50, 128.95, 128.16, 128.02,
127.57, 127.37, 126.63, 124.50, 124.34, 116.56, 116.38, 92.80,
89.10, 26.46, IR (KBr) νmax/cm-1 3055, 1777, 1709, 1619, 1497,
1467, 1450, 1372, 1340, 1309, 1272, 1170, 754, 698; m/z (%) 410
(Mþ, 0.4), 382 (1), 340 (0.6), 263 (0.4), 221 (94), 193 (77), 165 (84),
146 (100), 90 (60), 77 (27), 43 (84); EA found C, 73.20; H, 4.58; N,
6.71, C25H18N2O4 requires C, 73.17; H, 4.39; N, 6.83.
4c (1S,30R,5S)-10-acetyl-1,5-diphenylspiro[4,6-dioxa-2-aza-
bicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless
1
crystals from acetone-petroleum ether, mp 128-130 °C; H
NMR (300 MHz, CDCl3) δ 8.19 (d, 1H, J = 8.2 Hz), 7.96-7.93
(m, 2H), 7.67-7.64 (m, 2H), 7.59-7.54 (m, 3H), 7.48 (td, 1H,
J = 7.8, 1.5 Hz), 7.34 (td, 1H, J = 7.5, 0.9 Hz), 7.18-7.07 (m,
3H), 6.96-6.92 (m, 2H), 2.05 (s, 3H); 13C NMR (300 MHz,
CDCl3) δ 174.32, 169.63, 156.41, 139.84, 133.80, 133.21, 131.77,
130.33, 129.65, 128.58, 128.50, 128.12, 127.24, 126.40, 125.16,
122.22, 117.03, 116.66, 94.53, 87.45, 25.42; IR (KBr) νmax/cm-1
3092, 1784, 1700, 1626, 1492, 1466, 1450, 1344, 1282, 1172, 1127,
940, 698; m/z (%) 410 (Mþ, 0.4), 382 (6), 340 (2), 263 (1), 221
(100), 193 (80), 165 (78), 146 (64), 105 (61), 90 (52), 77 (45), 44
(60); EA found C, 72.92; H, 4.44; N, 6.92, C25H18N2O4 requires
C, 73.17; H, 4.39; N, 6.83.
Photolysis of IS with 1d. A solution of IS (756 mg, 4 mmol)
and 1d (1.768 g, 8 mmol) in benzene (80 mL) was irradiated with
light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue
was separated by flash chromatography to give 3d (359 mg,
22%) and 4d (787 mg, 48%).
3d, (1R,30R,5R)-10-acetyl-3,5-diphenylspiro[4,6-dioxa-2-azabicy-
clo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless crys-
tals from acetone-petroleum ether, mp 218-220 °C; 1H NMR
(300 MHz, CDCl3) δ 8.28 (d, 1H, J = 8.1 Hz), 8.14 (td, 2H, J =
6.9, 1.5 Hz), 7.74-7.71 (m, 2H), 7.64-7.41 (m, 8H), 7.25 (td, 1H,
J = 7.5, 0.9 Hz), 5.18 (s, 1H), 2.63 (s, 3H); 13C NMR (300 MHz,
CDCl3) δ 171.32, 170.35, 169.80, 139.81, 135.55, 132.64, 131.48,
130.07, 129.03, 128.87, 128.69, 126.63, 126.25, 126.03, 125.78,
124.56, 117.05, 114.76, 86.65, 81.17, 26.54; IR (KBr) νmax/cm-1
3020, 1775, 1711, 1635, 1463, 1336, 1273, 1169, 975, 696; m/z (%)
J. Org. Chem. Vol. 75, No. 22, 2010 7765

Wang et al.
JOCArticle
410 (Mþ, 4), 368 (1), 306 (6), 264 (5), 221 (42), 165 (10), 146 (12),
105 (100), 77 (56); EA found C, 73.27; H, 4.63; N, 6.73,
C25H18N2O4 requires C, 73.17; H, 4.39; N, 6.83.
(d, 1H, J = 6.8 Hz), 6.45 (t, 1H, J = 7.4 Hz), 6.33 (s, 1H), 3.81 (s,
3H), 2.54 (s, 3H), 2.29 (s, 3H); IR (KBr) ν 3023, 1780, 1707, 1687,
1608, 1570, 1518, 1264, 1172, 757 cm-1;MSm/z(%) 364(1),278(1),
189 (20), 175 (59), 146 (100), 90 (59), 63 (27), 43 (95); EA found
C, 65.13; H, 4.28; N, 7.57, C30H23N3O8 requires C, 65.10; H, 4.16;
N, 7.59.
Photolysis of PQ with 1a. A solution of PQ (832 mg, 4 mmol)
and 1a (1.384 g, 8 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue was
separated by flash chromatography to give 8a (305 mg, 20%) and 9a
(1.052 g, 69%).
4d, (1S,30R,5S)-10-acetyl-3,5-diphenylspiro[4,6-dioxa-2-aza-
bicyclo[3.2.0]hept-2-ene-7,30-[3H]indol]-20(10H)-one: colorless
needles from acetone-petroleum ether, mp 152-154 °C; 1H
NMR (300 MHz, CDCl3) δ 8.26 (d, 1H, J = 8.1 Hz), 8.16 (td,
2H, J = 7.2, 1.5 Hz), 7.90-7.81 (m, 2H), 7.69-7.63 (m, 1H),
7.58-7.50 (m, 5H), 7.46-7.37 (m, 2H), 7.13 (td, 1H, J = 7.8, 0.9
Hz), 5.32 (s, 1H), 2.71 (s, 3H); 13C NMR (300 MHz, CDCl3) δ
174.67, 170.15, 168.95, 140.61, 135.05, 133.03, 131.84, 129.97,
129.06, 128.96, 128.47, 126.89, 126.44, 126.31, 125.39, 122.52,
116.85, 115.79, 87.60, 78.20, 26.62; IR (KBr) νmax/cm-1 3063,
1769, 1700, 1632, 1463, 1265, 1172, 759; m/z (%) 410 (Mþ, 1),
306 (4), 264 (2), 221 (100), 165 (29), 146 (34), 105 (79), 77 (41);
EA found C, 73.26; H, 4.67; N, 6.81, C25H18N2O4 requires C,
73.17; H, 4.39; N, 6.83.
Photolysis of IS with 1e. A solution of IS (756 mg, 4 mmol)
and 1e (1.016 g, 8 mmol) in benzene (80 mL) was irradiated with
light of wavelength >400 nm under N2 atmosphere for 12 h.
Solvent was removed under reduced pressure and the residue
was separated by flash chromatography to give 2e (120 mg,
12%), 6e (273 mg, 22%), and 7e (414 mg, 31%).
2e, (2R,30S,5R,6aS,11aR)-10,11-Diacetyl-2,5-dihydro-2-methyl-
5-ethoxyspiro[2,5-epoxy-11a,6a-(epoxymethano)-11H-1,6,3-dio-
xazocino[8,7-b]indole-13,30-[3H]indol]-20(10H)-one: yellow powder
from petroleum ether-acetone, mp 254-256 °C; 1H NMR
(300 MHz, CDCl3) δ 8.42 (d, 1H, J = 8.3 Hz), 8.27 (d, 1H, J =
8.1 Hz), 7.96 (d, 1H, J = 7.6 Hz), 7.56 (td, 1H, J = 8.1, 1.4 Hz),
7.52 (td, 1H, J = 7.4, 1.7 Hz), 7.40 (td, 1H, J = 7.1, 0.9 Hz), 7.19
(d, 1H, J = 7.5 Hz), 7.14 (td, 1H, J = 6.5, 1.3 Hz), 5.65 (d, 1H,
J = 1.8 Hz), 4.36 (ddd, 2H, J = 14.4, 7.8, 0.9 Hz), 2.47 (s, 3H),
2.40 (s, 3H), 1.89 (s, 3H), 1.39 (t, 3H, J = 7.1 Hz); IR (KBr) ν
1761, 1715, 1695, 1607, 1377, 1196, 1010, 759 cm-1; MS m/z (%)
461 (0.1), 334 (9), 278 (24), 201 (25), 146 (34), 99 (33), 43(100);
EA found C, 61.71; H, 4.39; N, 8.43, C26H23N3O8 requires C,
61.78; H, 4.55; N, 8.32.
6e, ethyl 1-acetyl-20-methyl-2-oxo-40H-spiro[indoline-3,50-
oxazole]-40-carboxylate: colorless crystals from petroleum
ether-acetone, mp 146-148 °C; 1H NMR (300 MHz, CDCl3)
δ 8.25 (d, 1H, J = 8.2 Hz), 7.42 (td, 1H, J = 7.8, 1.7 Hz),
7.28-7.19 (m, 2H), 5.17 (s, 1H), 3.87-3.70 (m, 2H), 2.73 (s, 3H),
2.22 (s, 3H), 0.75 (t, 3H, J = 7.1 Hz); IR (KBr) ν 2987, 1747,
1711, 1665, 1608, 1537, 1265, 1017, 766 cm-1; MS m/z (%) 316
(Mþ, 12), 274 (39), 243 (16), 201 (75), 158 (20), 127 (56), 99 (93),
43 (100); EA found C, 60.81; H, 5.01; N, 8.99, C16H16N2O5
requires C, 60.76; H, 5.06; N, 8.86.
7e, ethyl 2-acetamido-2-(1-acetyl-3-hydroxy-2-oxoindolin-3-yl)-
acetate: colorless crystals from petroleum ether-acetone, mp
184-186 °C; 1H NMR (300 MHz, CDCl3) δ 8.25 (d, 1H, J = 8.1
Hz), 7.47 (d, 1H, J = 7.4 Hz), 7.41 (td, 1H, J = 7.3, 1.3 Hz), 7.71
(t, 1H, J = 7.5 Hz), 6.73 (d, 1H, J = 5.7 Hz), 5.36 (d, 1H, J = 6.4
Hz), 3.98-3.86 (m, 2H), 2.73 (s, 3H), 2.22 (s, 3H), 0.97 (t, 3H,
J = 7.1 Hz); IR (KBr) ν 2987, 1746, 1710, 1665, 1537, 1469,
1261, 1166, 1131, 1015, 765 cm-1; MS m/z (%) 334 (Mþ, 0.04),
219 (2), 191 (16), 146 (40), 102 (30), 43 (100); EA found C, 57.56;
H, 5.56; N, 8.37, C16H18N2O6 requires C, 57.49; H, 5.39; N, 8.38.
Photolysis of IS with 1f. A solution of IS (378 mg, 2 mmol) and
1f (700 mg, 4 mmol) in benzene (40 mL) was irradiated with light
of wavelength >400 nm under N2 atmosphere for 48 h. Solvent
was removed under reduced pressure and the residue was
separated by flash chromatography to give 2f (536 mg, 97%).
2f, (2R,30S,5R,6aS,11aR)-10,11-diacetyl-2,5-dihydro-4-phenyl-
spiro[2,5-epoxy-11a,6a-(epoxymethano)-11H-1,6,3-dioxazocino-
[8,7-b]indole-13,30-[3H]indol]-20(10H)-one: yellow crystals from
petroleum ether-acetone, mp 246-248 °C; 1H NMR (300 MHz,
CDCl3) δ 8.20 (m, 3H), 7.50 (m, 2H), 7.35 (d, 2H, J = 8.9 Hz), 7.06
(t, 1H, J = 7.3 Hz), 6.98 (s, 1H), 6.66 (d, 2H, J = 8.9 Hz), 6.54
8a, 2,5-dihydro-2-ethyl-4-phenyl-2,5-epoxyphenanthro[9,10-g]-
1,6,3-dioxazocine: yellow crystals from acetone-petroleum
ether, mp 152-154 °C; H NMR (300 MHz, CDCl3) δ 8.50
1
(d, 2H, J = 7.3 Hz), 8.39 (dt, 1H, J = 7.5, 1.2 Hz), 8.30 (dt, 1H,
J = 7.5, 1.2 Hz), 7.93 (dd, 2H, J = 7.4, 1.3 Hz), 7.62-7.48 (m,
4H), 7.43-7.32 (m, 3H), 6.83 (s, 1H), 2.78-2.63 (m, 2H), 1.41 (t,
3H, J = 7.5 Hz); 13C NMR (75 MHz, CDCl3) δ 170.97, 133.94,
132.71, 132.35, 128.89, 128.29, 128.26, 128.22, 128.16, 128.06,
127.77, 126.97, 126.85, 125.97, 125.75, 123.24, 122.36, 122.24,
122.14, 100.49, 29.57, 8.31; IR (KBr) νmax/cm-1 2975, 1630,
1597, 1454, 1319, 1236, 1118, 966, 757; m/z (%) 381 (Mþ, 1), 297
(0.9), 249 (0.6), 208 (9), 180 (43), 173 (100), 130 (60), 90 (31), 63
(12); EA found C, 78.52; H, 5.04; N, 3.68, C25H19NO3 requires
C,78.74; H, 4.99; N, 3.67.
9a, 3-ethyl-1-phenylspiro[4,6-dioxa-2-azabicyclo[3.2.0]hept-
2-ene-7,90(100H)-phenanthren]-100-one: yellow crystals from
acetone-petroleum ether, mp 168-170 °C; 1H NMR (300
MHz, CDCl3) δ 8.80-8.76 (m, 2H), 8.24 (d, 1H, J = 7.7 Hz),
7.79-7.72 (m, 2H), 7.63-7.59 (m, 2H), 7.53-7.47 (m, 3H),
7.41-7.34 (m, 3H), 6.68 (s, 1H), 2.29 (qd, 2H, J = 16.0, 7.7 Hz),
1.06 (t, 3H, J = 7.5 Hz); 13C NMR (75 MHz, DMSO-d6) δ
174.33, 173.99, 148.41, 135.91, 131.81, 129.78, 129.48, 129.01,
128.57, 128.29, 128.26, 127.61, 127.16, 126.16, 124.81, 124.35,
124.12, 122.15, 119.90, 115.82, 64.66, 27.73, 9.97; IR (KBr) νmax
/
cm-1 2978, 1815, 1643, 1534, 1450, 1097, 1008, 755; m/z (%) 381
(Mþ, 42), 353 (5), 297 (100), 252 (10), 165 (14), 104 (5), 57 (14);
EA found C, 78.75; H, 5.09; N, 3.63, C25H19NO3 requires
C,78.74; H, 4.99; N, 3.67.
Photolysis of PQ with 1b. A solution of PQ (832 mg, 4 mmol)
and 1b (1.768 g, 8 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue was
separated by flash chromatography to give 10b (430 mg, 25%) and
11 (703 mg, 55%).
10b, 9a,12a-dihydro-11,12a-diphenylphenanthro[90,100:5,6]-
[1,4]dioxino[2,3-d]oxazole: yellow blocks from acetone-petroleum
ether, mp 148-150 °C; 1H NMR (300 MHz, CDCl3) δ 8.61 (d, 2H,
J = 6.3 Hz), 8.38 (d, 1H, J = 6.9 Hz), 8.28 (d, 1H, J = 7.5 Hz), 7.97
(d, 2H, J = 7.8 Hz), 7.84 (d, 2H, J = 6.6 Hz), 7.66-7.42 (m, 8H),
7.32 (t, 2H, J = 7.2 Hz), 6.49 (s, 1H); 13C NMR(75 MHz, CDCl3) δ
165.92, 140.26, 135.41, 133.56, 132.68, 129.38, 129.02, 128.94,
128.40, 127.99, 127.81, 127.55, 126.96, 126.47, 125.65, 125.87,
125.77, 125.64, 122.69, 122.49, 121.57, 120.75, 105.20, 103.53; IR
(KBr) νmax/cm-1 1646, 1450, 1350, 1246, 1140, 1063, 949, 756, 695;
m/z (%) 323 (5), 236 (2), 221 (100), 202 (24), 193 (65), 180 (93), 165
(11), 152 (32), 89 (51), 76 (22), 63 (26), 43 (38); EA found C, 81.19;
H, 4.42; N, 3.21, C29H19NO3 requires C, 81.12; H, 4.43; N, 3.26.
11 10-[[(9aR,12aS)-11,12a-diphenylphenanthro[90,100:5,6]-
[1,4]dioxino[2,3-d]oxazol-9a(12aH)-yl]oxy]-9-phenanthrenol: color-
1
less crystals from acetone-petroleum ether, mp 182-184 °C; H
NMR (300 MHz, CDCl3) δ 8.71-8.68 (m, 1H), 8.61-8.54 (m, 4H),
8.42-8.39 (m, 1H), 8.19 (td, 2H, J = 6.9, 1.5 Hz), 7.87 (dd, 1H, J =
7.8, 1.5 Hz), 7.76-7.73 (m, 2H), 7.67-7.49 (m, 10H), 7.43-7.39 (m,
2H), 7.36 (s, 1H), 7.28-7.23 (m, 1H), 7.19 (t, 2H, J = 7.5 Hz); 13
C
NMR (75 MHz, CDCl3) δ 165.37, 142.44, 136.42, 136.31, 133.09,
132.68, 129.94, 129.77, 129.43, 129.39, 129.03, 128.51, 128.36, 127.67,
7766 J. Org. Chem. Vol. 75, No. 22, 2010

Wang et al.
JOCArticle
127.42, 127.26, 127.22, 126.95, 126.82, 126.66, 126.47, 126.11, 125.97,
125.88, 25.24, 125.10, 124.50, 124.44, 123.44, 122.72, 122.66, 122.59,
122.43, 122.05, 121.57, 120.43, 105.69; IR (KBr) νmax/cm-1 3489,
1650, 1606, 1500, 1450, 1352, 1334, 1235, 1212, 1056, 925, 881, 723;
m/z (%) 429 (6), 401 (3), 281 (7), 208 (28), 194 (16), 180 (100), 165
(10), 152 (35), 105 (25), 105 (25), 76 (19); EA found C, 81.00; H, 4.09;
N, 2.19, C43H27NO5 requires C, 81.00; H, 4.24; N, 2.20.
Photolysis of PQ with 1c. A solution of PQ (624 mg, 3 mmol)
and 1c (1.33 g, 6 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 24 h.
Solvent was removed under reduced pressure and the residue was
separated by flash chromatography to give 8c (553mg,43%),12 (124
mg, 13%), and 10c (502 mg, 39%).
8c, 2,5-dihydro-4,5-diphenyl-2,5-epoxyphenanthro[9,10-g]-
1,6,3-dioxazocine: colorless crystals from acetone-petroleum
ether, mp 206-208 °C; 1H NMR (300 MHz, CDCl3) δ
8.57-8.45 (m, 4H), 7.84 (dd, 2H, J = 7.9, 1.7 Hz), 7.70-7.48
(m, 10H), 7.22 (dt, 1H, J = 7.4, 2.2 Hz), 7.13 (t, 2H, J = 7.5 Hz);
13C NMR (75 MHz, CDCl3) δ 173.96, 136.04, 133.97, 132.53,
132.50, 130.15, 129.29, 129.08, 128.82, 128.77, 128.72, 128.34,
128.19, 128.18, 127.51, 127.45, 127.06, 126.62, 126.34, 123.59,
122.86, 122.53, 113.62, 109.17; IR (KBr) νmax/cm-1 3070, 1670,
1443, 1315, 1231, 1110, 1012, 753, 688; m/z (%) 429 (Mþ, 4), 297
(4), 221 (100), 193 (45), 180 (44), 165 (51), 152 (26), 105 (16), 77
(24), 63 (9), 44 (2); EA found C,81.08; H, 4.40; N, 3.47,
C29H19NO3 requires C, 81.12; H, 4.43; N, 3.26.
132.98, 132.47, 130.61, 129.75, 129.42, 128.68, 128.66, 128.54,
128.51, 128.03, 127.94, 126.92, 126.46, 124.37, 123.66, 114.84,
94.47, 78.43; IR (KBr) νmax/cm-1 3052, 1706, 1449, 1292, 1020,
960, 699; m/z (%) 429 (Mþ, 2), 264 (1), 221 (100), 180 (62), 165
(29), 152 (32), 105 (49), 77 (49); EA found C, 81.10; H, 4.46;
N,3.18, C29H19NO3 requires C, 81.12; H, 4.43; N, 3.26.
10d, 9a,12a-dihydro-9a,11-diphenylphenanthro[90,100:5,6][1,4]-
dioxino[2,3-d]oxazole: colorless crystals from acetone-petroleum
ether, mp 144-146 °C; 1H NMR (300 MHz, CDCl3) δ 8.64 (d, 2H,
J = 7.7 Hz), 8.38-8.35 (m, 1H), 8.31-8.27 (m, 1H), 7.98-7.95
(m, 2H), 7.84-7.82 (m, 2H), 7.67-7.43 (m, 8H), 7.34 (t, 2H, J =
7.5 Hz), 6.57 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 166.19,
138.47, 134.89, 133.81, 132.75, 129.86, 129.03, 128.96, 128.47,
127.72, 127.59, 127.06, 126.93, 126.43, 125.96, 125.90, 125.84,
125.71, 125.47, 122.68, 122.58, 121.48, 120.84, 108.40, 100.77; IR
(KBr) νmax/cm-1 3064, 1648, 1452, 1330, 1140, 990, 760; m/z (%)
429 (Mþ, 0.04), 306 (1), 264 (0.6), 221 (100), 180 (67), 165 (34), 105
(19), 77 (39); EA found C, 81.15; H, 4.34; N,3.41, C29H19NO3
requires C, 81.12; H, 4.43; N, 3.26.
Photolysis of PQ with 1e. A solution of PQ (832 mg, 4 mmol)
and 1e (1.016 g, 8 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for 12 h.
Solvent was removed under reduced pressure and the residue was
separated by flash chromatography to give 13 (102 mg).
13: colorless crystals from petroleum ether-acetone, mp
162-164 °C; 1H NMR (300 MHz, CDCl3) δ 8.06 (dd, 1H,
J = 7.7, 1.2 Hz), 7.97 (d, 1H, J = 7.9 Hz), 7.89 (d, 1H, J = 7.8
Hz), 7.75 (td, 1H, J = 8.0, 1.4 Hz), 7.52-7.40 (m, 3H), 7.36 (td,
1H, J = 7.0, 1.1 Hz), 4.84 (d, 1H, J = 1.2 Hz), 3.70-3.59 (m,
1H), 3.49-3.38 (m, 1H), 2.41 (s, 3H), 0.77 (t, 3H, J = 7.1 Hz); IR
(KBr) ν 3358, 2975, 1736, 1706, 1657, 1528, 1206, 1019, 774, 735
cm-1; MS m/z (%) 335 (15), 294 (54), 235 (20), 221 (100), 180
(47), 152 (29), 99 (51), 71 (29); EA found C, 67.96; H, 5.64; N,
3.84, C20H19NO5 requires C, 67.99; H, 5.38; N, 3.97.
Photolysis of BZ with 1i. A solution of BZ (800 mg, 3.8 mmol)
and 1i (1.2 g, 7.5 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >330 nm under N2 atmosphere for 48 h
to reach a 80% conversation of BZ. Solvent was removed under
reduced pressure and the residue was separated by flash chro-
matography to give 15a (180 mg, 16%) and 16a (910 mg, 81%).
15a, (1R,5R,7R)-7-benzoyl-1,7-diphenyl-3-methyl-4,6-dioxa-2-
azabicyclo[3.2.0]hept-2-ene: white powders from acetone-
petroleum ether, mp 149-150 °C; 1H NMR (300 MHz, CDCl3)
δ 7.93 (dd, 2H, J = 7.5, 1.5 Hz), 7.53-7.44 (m, 3H), 7.40 (t, 2H,
J = 7.5 Hz), 7.30-7.25 (m, 2H), 7.21-7.16 (m, 6H), 6.60 (s, 1H),
2.08 (s, 3H); IR (KBr) ν 3059, 1667, 1597, 1446, 1268, 1013, 959,
860, 739, 694 cm-1; MS m/z (%) 340 (0.4), 236 (4),167 (4), 159
(100), 131 (29), 105 (79), 77 (60), 51 (18); EA found C, 78.20; H,
5.12; N, 3.50, C24H19NO3 requires C, 78.05; H, 5.15; N, 3.79.
16a, (1S,5S,7R)-7-benzoyl-1,7-diphenyl-3-methyl-4,6-dioxa-
2-azabicyclo[3.2.0]hept-2-ene: colorless crystals from acetone-
petroleum ether, mp 147-149 °C; 1H NMR (300 MHz, CDCl3)
δ 7.72 (4H, dd, J = 8.1, 1.5 Hz), 7.53-7.33 (m, 11H), 6.48 (s,
1H), 1.91 (s, 3H); IR (KBr) ν 3054, 1682, 1668, 1598, 1490, 1383,
1259, 1071, 930, 857, 797, 697, 657 cm-1; MS m/z (%) 298 (1),
236 (4), 178 (2), 159 (100), 105 (82), 77 (69); EA found C,
78.12; H, 5.07; N, 3.52, C24H19NO3 requires C, 78.05; H, 5.15;
N, 3.79.
Photolysis of BZ with 1j. A solution of BZ (420 mg, 2.0 mmol)
and 1j (0.888 g, 8.0 mmol) in acetonitrile (40 mL) was irradiated
with light of wavelength >330 nm under N2 atmosphere for 48 h
to reach a 70% conversation of BZ. Solvent was removed under
reduced pressure and the residue was separated by flash chro-
matography to give 15b (310 mg, 69%).
15b, (1R,5R,7R)-1,3,5-trimethyl-4,6-dioxa-2-azabicyclo-
[3.2.0]hept-2-ene: yellow oil from acetone-petroleum ether;
1H NMR (300 MHz, CDCl3) δ 7.81 (t, 4H, J = 9.0 Hz),
7.43-7.38 (m, 3H), 7.34-7.29 (m, 3H), 1.89 (s, 3H), 1.61 (s,
12: colorless crystals from acetone-petroleum ether, mp
216-218 °C; 1H NMR (300 MHz, CDCl3) δ 8.65 (d, 1H, J =
7.9 Hz), 8.58 (d, 1H, J = 8.1 Hz), 8.49 (d, 1H, J = 7.5 Hz), 8.24
(d, 1H, J = 7.2 Hz), 7.95 (d, 1H, J = 8.4 Hz), 7.83 (dd, 1H, J =
7.7, 1.0 Hz), 7.73-7.62 (m, 5H), 7.51-7.44 (m, 7H), 7.34 (td, 1H,
J = 7.8, 1.2 Hz), 7.27-7.16 (m, 6H), 7.02 (t, 2H, J = 7.6 Hz); 13
C
NMR (75 MHz, CDCl3) δ 172.28, 136.66, 135.20, 132.84,
132.52, 132.49, 131.90, 131.46, 131.22, 129.90, 129.83, 129.57,
129.24, 128.77, 128.56, 128.29, 127.77, 127.68, 127.60, 127.48,
127.00, 126.80, 126.68, 125.75, 125.46, 125.11, 125.05, 124.56,
122.63, 122.44, 122.36, 121.06, 120.89, 114.91, 109.95; IR (KBr)
ν
max/cm-1 1709, 1644, 1620, 1451, 1317, 1110, 1061, 1036, 760;
m/z (%) 637 (Mþ, 9), 429 (13), 400 (78), 316 (19), 296 (9), 221
(100), 193 (49), 180 (44), 165 (41), 152 (23), 105 (35), 77 (19), 63
(12), 41 (26); EA found C, 81.24; H, 4.31; N, 2.12, C43H27NO5
requires C, 81.00; H, 4.24; N, 2.20.
10c, 9a,12a-dihydro-9a,12a-diphenylphenanthro[90,100:5,6]-
[1,4]dioxino[2,3-d]oxazole: colorless crystals from acetone-
petroleum ether, mp 190-192 °C; 1H NMR (300 MHz, CDCl3)
δ 8.72-8.71 (m, 2H), 7.68-7.61 (m, 2H), 7.62-7.67 (m, 4H),
7.45-7.26 (m, 4H), 7.36 (s, 1H), 7.19-7.14 (m, 6H); 13C NMR
(75 MHz, DMSO-d6) δ 159.11, 137.67, 135.97, 135.53, 135.50,
129.49, 129.01, 128.16, 128.03, 127.90, 127.83, 127.65, 127.05,
126.91, 126.86, 126.21, 125.56, 123.84, 123.76, 121.17, 120.70,
112.79, 108.25; IR (KBr) νmax/cm-1 3068, 1709, 1634, 1606,
1499, 1450, 1353, 1178, 1027, 761; m/z (%) 429 (Mþ, 0.1), 296
(0.4), 221 (85), 208 (26), 193 (51), 180 (100), 165 (60), 152 (41), 77
(24), 51 (14), 43 (10); EA found C, 80.99; H, 4.30; N, 3.15,
C29H19NO3 requires C, 81.12; H, 4.43; N, 3.26.
Photolysis of PQ with 1d. A solution of PQ (832 mg, 4 mmol)
and 1d (1.768 g, 8 mmol) in acetonitrile (80 mL) was irradiated
with light of wavelength >400 nm under N2 atmosphere for
24 h. Solvent was removed under reduced pressure and the
residue was separated by flash chromatography to give 9d (776
mg, 45%) and 10d (541 mg, 32%).
9d, 3,5-diphenylspiro[4,6-dioxa-2-azabicyclo[3.2.0]hept-
2-ene-7,90(100H)-phenanthren]-100-one: yellow crystals from
acetone-petroleum ether, mp 192-194 °C; 1H NMR (300
MHz, CDCl3) δ 8.01-7.94 (m, 4H), 7.87 (dd, 3H, J = 7.0, 1.4
Hz), 7.77 (d, 1H, J = 7.7 Hz), 7.69 (t, 1H, J = 7.4 Hz), 7.56-7.36
(m, 8H), 7.18 (t, 1H, J = 7.5 Hz), 5.13 (s, 1H); 13C NMR (75
MHz, CDCl3) δ 196.72, 168.81, 137.53, 136.06, 135.63, 135.17,
J. Org. Chem. Vol. 75, No. 22, 2010 7767

Wang et al.
JOCArticle
3H), 1.10 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 198.47, 164.92,
136.61, 135.72, 132.64, 129.31,128.58, 128.35, 128.01, 125.73,
113.61, 98.93, 81.96, 18.92, 17.19, 14.28.
(NCET-08-0271), and the Austria-China Cooperation pro-
ject (2007DFA41590, WTZ-Project Nr. CN 08/2010).
Supporting Information Available: General experimental
information; calculation results on the biradical intermediates L
and M; copies of 1H and 13C NMR spectra for new compounds;
and crystal structure and CIF of compound 8a and 12. Thismaterial
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. The authors would like to acknowledge
financial support from the National Natural Science Foun-
dation of China (20702024, 20972067, 90813035), the Pro-
gram for New Century Excellent Talents in University
7768 J. Org. Chem. Vol. 75, No. 22, 2010