Synthesis of 2,3-Disubstituted Carbazoles, Benzo[c]carbazoles, and Phenanthrenes Through FeCl3-Mediated Cyclization of Triene Frameworks

Abstract A facile synthesis of 2,3-disubstituted carbazoles through electrocyclization of 2,3-divinylindoles using FeCl3 in DMF at reflux is reported. The methodology was found to be applicable for smooth transformation of 3-aryl-2-vinylindole as well as 2-styrylbiphenyl into the respective benzo[c]carbazole and phenanthrene.

Over the years, our research group has exploited electrocyclization of 1-phenylsulfonyl-2,3-divinylindoles as a key step for the syntheses of quinocarbazoles, 1a staurosporine aglycone, 1b,1c and also for accessing a wide variety of substituted carbazoles. 2 We have also accomplished a Lewis acid mediated electrocyclization strategy for accessing calothrixin B and its derivatives. 3 In all these reports, the thermal electrocyclization followed by aromatization of 2,3-divinylindoles could be performed using 10 % Pd/C in xylenes at reflux to give the respective carbazoles in good yields. However, the inconsistent quality of 10 % Pd-C, difficulty in the aromatization of intermediate dihydrocarbazole, coupled with prolonged reaction time at elevated temperature, makes this protocol unsuitable for performing the reaction on a multi-gram scale. We sought to develop an alternative procedure that avoids the use Pd/C and also overcomes the disadvantages noted above. Hence, in a further continua-tion of our work on the synthesis of carbazoles, 1 -3 we report herein the synthesis of a wide variety of carbazole derivatives 2 through FeCl 3 -mediated 4,5 electrocyclization as a key step. The synthesis of benzo[c]carbazoles 3 could also be achieved from the respective 2-vinyl-3-arylindoles (Scheme 1).
To our delight, FeCl 3 -mediated electrocyclization of a wide variety of 2,3-divinylindoles could be smoothly performed to afford the respective carbazoles.

Letter Syn Open
anhydrous CH 2 Cl 2 at room temperature, led to 3-veratryl-2vinylindole 9 in 93 % yield. The 2-vinyl ester 9, upon cyclization using 50 mol% FeCl 3 in anhydrous 1,2-DCE reflux, furnished benzo[c]carbazole 10 in a moderate yield (Scheme 5). Attempts to improve the yield of the benzo[c]carbazole 10 either by increasing the number of equivalents of FeCl 3 or by prolonging the reaction time was not found to be useful.
In summary, we have achieved the syntheses of 2,3disubstituted carbazoles, benzo[c]carbazoles, and phenanthrene derivatives by employing FeCl 3 -mediated cyclization of the corresponding triene frameworks. For the first time, the FeCl 3 -mediated cyclization of two vinylic carbons as well as phenyl and vinylic carbons could be achieved in acceptable yields.

Acknowledgment
We thank the CSIR, New Delhi for financial support. P.R. and T.M thank the University Grants Commission (UGC), New Delhi for fellowships. For NMR facilities, the authors thank the Department of Science and Technology Funds for the Improvement of Science and Technology (DST-FIST).

Supporting Information
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