ORGANIC
LETTERS
2010
Vol. 12, No. 20
4435-4437
Exclusive Photodimerization Reactions
of Chromone-2-carboxylic Esters
Depending on Reaction Media
Masami Sakamoto,* Fumitoshi Yagishita, Masayuki Kanehiro, Yoshio Kasashima,
Takashi Mino, and Tsutomu Fujita
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
Chiba UniVersity, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received August 2, 2010
ABSTRACT
The irradiation of chromone-2-carboxylic esters resulted in the stereo- and regioselective formation of C2 chiral anti-HH dimers from the triplet
excited state. On the contrary, photolysis in the solid-state gave anti-HT dimers exclusively controlled by molecular arrangement in the crystal.
Stereoselective photochemical reactions have become key
topics in organic photochemistry.1 Many research efforts
have concentrated on obtaining stereoselective photodimers
using such intermolecular interactions as hydrogen-bonding,
π-π, and cation-π interactions. Solid-state photoreactions
have also been reported for stereoselective dimerization.2 In
most cases, since several types of photodimers were pro-
duced, controlling the selectivity of the products is very hard.
In rare cases, it was reported that alkyl 2-naphthoate
selectively gave C2 chiral cubane-like photodimers.3 In
another case, coumarin and thiocoumarin in an inclusion
complex with a host-guest complex also gave anti-head-
to-head dimers by the solid-state photoreaction.4 These very
rare reactions selectively lead to C2 chiral photodimers. C2
symmetry materials are very useful and widely used for
ligands for catalytic asymmetric synthesis and synthetic
materials. The development of a new reaction leading to C2
chiral materials is strongly required.5
We have now found that the photolysis of 2-chromone-
carboxylic esters in solution effectively gave C2 symmetry
chiral dimers exclusively in specific chemical yields; fur-
thermore, solid-state photolysis selectively gave another type
of anti-head-to-tail photodimers. Chromone benzo-γ-pyrone
is the parent of a large number of naturally occurring
compounds such as flavonoids and plant pigments. Excited
simple chromone forms a triplet excited state, and most of
(1) (a) Inoue, Y. Chem. ReV. 1992, 92, 741–770. (b) Inoue, Y. In Chiral
Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker: New
York, 2004; Vol. 11, pp 129-177. (c) Grosch, B.; Bach, T. In Chiral
Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker: New
York, 2004; Vol. 11, pp 315-340. (d) Sakamoto, M. In Chiral Photo-
chemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker: New York,
2004; Vol. 11, pp 415-461.
(2) (a) Horspool, W. M. Synthetic Organic Photochemistry; Plenum
Press: New York, 1984. (b) Coyle, J. C. Photochemistry in Organic
Synthesis; The Royal Society of Chemistry: London, 1986; pp 163-188.
(c) Griesbeck, A. G.; Mattay, J. Synthetic Organic Photochemistry; Marcel
Dekker: New York, 2005; pp 141-160. (d) Yamada, S.; Uematsu, N.;
Yamashita, K. J. Am. Chem. Soc. 2007, 129, 12100–12101.
(3) (a) Collin, P. J.; Roberts, D. B.; Sugowdz, G.; Wells, D.; Sasse,
W. H. F. Tetrahedron Lett. 1972, 321–324. (b) Tung, C.-H.; Wu, L.-Z.;
Zhang, L.-P.; Chen, B. Acc. Chem. Res. 2003, 36, 39–47. (c) Lei, L.; Wu,
L.-Z.; Wu, X.-L.; Liao, G.-H.; Zhang, L.-P.; Tung, C.-H.; Ding, K.-L.
Tetrahedron Lett. 2006, 47, 4725–4727.
(4) Tanaka, K.; Toda, F.; Mochizuki, E.; Yasui, N.; Kai, Y.; Miyahara,
I.; Hirotsu, K. Angew. Chem., Int. Ed. 1999, 38, 3523–3525.
(5) (a) Yoon, T. P.; Jacobsen, E. N. Science 2003, 299, 1691. (b) Pfaltz,
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10.1021/ol101734k 2010 American Chemical Society
Published on Web 09/24/2010