From β-nitrothiophenes to ring-fused nitrobenzenes:: An overall ring-enlargement process via a facile, aromatization-driven, thermal 6π electrocyclization

[GRAPHICS] In prosecution of previous work on the thermal cyclization of 1-aryl-4-rnethanesulfonyl-2-nitro-3phenylsulfonyl-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 6,7 electrocyclizations as a tool for the construction of the benzene ring is furthermore provided.