ORGANIC
LETTERS
2006
Vol. 8, No. 7
1339-1342
Highly Efficient Construction of Benzene
Ring in Carbazoles by
Palladium-Catalyzed endo-Mode
Oxidative Cyclization of
3-(3′-Alkenyl)indoles
Aidi Kong, Xiuling Han, and Xiyan Lu*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
Received January 6, 2006
ABSTRACT
A highly efficient construction of the benzene ring in carbazoles by palladium-catalyzed endo-mode oxidative cyclization of 3-(3
′-alkenyl)-
indoles was developed. The reaction utilizes benzoquinone as the stoichiometric oxidant and is conducted under mild conditions.
The transition-metal-catalyzed intramolecular cyclization of
indole derivatives represents an attractive route for the
synthesis of polycyclic compounds.1 For example, Stoltz
reported the formation of cyclopentane-fused indoles from
the 3-nonterminal-alkenyl indoles under the catalysis of Pd-
(II).1h Widenhoefer used the platinum-catalyzed cyclization
of 2-alkenylindoles to obtain the cyclohexane-fused indoles.1i
On the other hand, carbazole alkaloids are a class of natural
products that exhibit a variety of biological activities,2 of
which indolocarbazoles represent an important class of
antitumor antibiotics, such as the protein kinase C (PKC)
inhibitor.3 Carbazole derivatives are also widely used as
organic materials, due to their photorefractive, photoconduc-
tive, and light-emitting properties.4 Thus, the synthesis of
substituted carbazoles has attracted considerable attention.2c,5
Among the many methods available, the synthetic strategies
which utilize indole derivatives as the starting materials are
often very fruitful.6 However, the palladium-mediated reac-
tions available often need severe conditions,7 multiple steps,8
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Chem. Soc. 1993, 115, 9323. (d) Baran, P. S.; Corey, E. J. J. Am. Chem.
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C. J. Org. Chem. 2003, 68, 7625. (h) Ferreira, E. M.; Stoltz, B. M. J. Am.
Chem. Soc. 2003, 125, 9578. (i) Liu, C.; Han, X.; Wang, X.; Widenhoefer,
R. A. J. Am. Chem. Soc. 2004, 126, 3700. (j) Liu, C.; Widenhoefer, R. A.
J. Am. Chem. Soc. 2004, 126, 10250.
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Press: New York, 1993; Vol. 44, p 257. (b) Chakraborty, D. P.; Roy, S.
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10.1021/ol060039u CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/07/2006