From -Nitrothiophenes to Ring-Fused Nitrobenzenes: An Overall Ring-Enlargement Process via a Facile, Aromatization-Driven, Thermal 6π Electrocyclization 1 Lara Bianchi, † Carlo Dell’Erba, † Massimo Maccagno, † Giovanni Petrillo,* ,† Egon Rizzato, ‡ Fernando Sancassan, † Elda Severi, † and Cinzia Tavani † Dipartimento di Chimica e Chimica Industriale, Universita ` di Genova, Via Dodecaneso 31, I-16146 Genova, Italy, and Dipartimento di Chimica Organica “A. Mangini”, Universita ` di Bologna, Via San Giacomo 11, I-40126 Bologna, Italy petrillo@chimica.unige.it Received May 27, 2005 In prosecution of previous work on the thermal cyclization of 1-aryl-4-methanesulfonyl-2-nitro-3- phenylsulfonyl-1,3-butadienes (7), the 3-unsubstituted derivatives 8, deriving from the initial ring opening of 3-nitrothiophene (2), have been likewise found herein to undergo cyclization, followed by aromatization, in analogous mild experimental conditions, leading to the ring-fused homo- or heteroaromatic nitro derivatives 10. The concerted electrocyclic nature of the process is strongly supported by the outcome of tests based on the variation of the polarity of the solvent or of the electron density on the aryl of 8. Thus, the successful application of the process to the non-phenylsulfonyl-activated 8 significantly widens the scope of a synthetically valuable overall ring-opening/ring-closing procedure from nitrothiophenes. Support to the recently renewed interest in thermal 6π electrocyclizations as a tool for the construction of the benzene ring is furthermore provided. Introduction As compared to the photochemical counterpart, 2 not- withstanding an undeniable theoretical significance the thermal 6π electrocyclization has reportedly 3 received little attention as far as its potentialities in organic synthesis are concerned. More recently, though, the interest in the thermal electrocyclization of conjugated trienes for the construction of the benzene ring has found, in particular, renewed strength: this is due to the possibility to exploit the presence of a suitable leaving group (Y) capable of driving the reaction toward aroma- tization after cyclization of the triene to a cyclohexadiene intermediate (Scheme 1). 4 In this line, within a long-standing research project on the synthetic exploitation of the versatile butadiene building-blocks deriving from the ring-opening of ni- * Corresponding author. Phone: +39-10-353-6121. Fax: +39-10- 353-6118. † Universita ` di Genova. ‡ Universita ` di Bologna. (1) Synthetic Exploitation of the Ring-Opening of Nitrothiophenes, Part XVII. For part XVI, see ref 5a. (2) For the photochemical cyclization of stilbenes to phenanthrenes see: Lewis, F. D.; Kurth, T. L.; Kalgutkar, R. S. Chem. Commun. 2001, 1372-1373. Mallory, F. B.; Mallory, C. W. In Organic Reactions; John Wiley & Sons: New York, 1984. Doyle, T. D.; Benson, W. R.; Filipescu, N. J. Am. Chem. Soc. 1976, 98, 3262-3267. Liu, L.; Yang, B.; Katz, T. J.; Poindexter, M. K. J. Org. Chem. 1991, 56, 3769-3775. Rodier, J.- M.; Myers, A. B. J. Am. Chem. Soc. 1993, 115, 10791-10795. Caldwell, R. A.; Mizuno, K.; Hansen, P. E.; Vo, L. P.; Frentrup, M.; Ho, C. D. J. Am. Chem. Soc. 1981, 103, 7263-7269. Laarhoven, W. H. Pure Appl. Chem. 1984, 56, 1225. Mallory, F. B.; Mallory, C. W. Org. React. 1984, 30, 1. Floyd, A. J.; Dyke, S. F.; Ward, S. E. Chem. Rev. 1976, 76, 509. For the photochemical heterostilbene to heterophenanthrene cycliza- tion see: Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am. Chem. Soc. 2001, 123, 3878-3884. Irie, M. Chem. Rev. 2000, 100, 1685-1716. Rawal, V. H.; Jones, R. J.; Cava, M. P. Tetrahedron Lett. 1985, 26, 2423-2426. (3) Okamura, W. H.; De Lera, A. R. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: New York, 1991; Vol. 5, pp 699-750. (4) (a) Ogura, K.; Takeda, M.; Xie, J. R.; Akazome, M.; Matsumoto, S. Tetrahedron Lett. 2001, 42, 1923-1925. (b) Guner, V. A.; Houk, K. N.; Davies, I. W. J. Org. Chem. 2004, 69, 8024-8028. (c) Davies, I. W.; Marcoux, J. F.; Kuete, T. F.; Lankshear, M. D.; Taylor, J. D. O.; Tsou, N.; Dormer, P. G.; Hughes, D. L.; Houk, K. N.; Guner, V. J. Org. Chem. 2004, 69, 1298-1308. (d) de Koning, C. B.; Rousseau, A. L.; van Otterlo, W. A. L. Tetrahedron 2003, 59,7-36 and references therein. 8734 J. Org. Chem. 2005, 70, 8734-8738 10.1021/jo051059x CCC: $30.25 © 2005 American Chemical Society Published on Web 09/30/2005