From β-Nitrothiophenes to Ring-Fused Nitrobenzenes:  An Overall Ring-Enlargement Process via a Facile, Aromatization-Driven, Thermal 6π Electrocyclization¹

Abstract

[reaction: see text] 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 6pi electrocyclizations as a tool for the construction of the benzene ring is furthermore provided.

5‐Substituted Benzothiophenes: Synthesis, Mechanism, and Kinetic Studies

“…The same protocol was repeated to prepare 5‐nitrobenzothiophene 3b which is a known compound synthesized by different methodologies; in particular, it was easily obtained by ring‐opening/ring‐closing procedure described in several papers by Dell'Erba et al . .…”

Conjugated Nitrodienes. Synthesis and Reactivity

“…The reaction of 40 with aryl Grignard reagents gave 2-nitrodienes 44 , which by sulfur oxidation with MCPBA yielded (1 E ,3 Z )-1-aryl-4-(methylsulfonyl)-2-nitro-1,3-butadienes 45 . These versatile compounds undergo 6π-electrocyclization in refluxing xylene to give 72–92% yields of nitrobenzene ring-fused derivatives 46 , whereas their reaction with LHMDS resulted in an intramolecular Michael addition to afford thiopyran S , S -dioxides 47 (Scheme ).…”

De Novo Synthesis of Multisubstituted Aryl Amines Using Alkene Cross Metathesis

“…We can conceive of two pathways by which this bis-enamine can form the aryl amine product 3a , originating from (i) Mannich type cyclization, followed by elimination of two molecules of pyrrolidine, or (ii) elimination of pyrrolidine and 6π-electrocyclization, followed by another elimination. We are not presently able to distinguish between these two pathways, but note that the substituted intermediate F bears a strong similarity to systems in which a facile 6π-electrocyclization is supported by calculation and other experimental observations . However, if the electrocyclization pathway is followed, then it is not clear what drives the formation of the cis -alkene that must be present in F for cyclization to occur.…”

“…We are not presently able to distinguish between these two pathways, but note that the substituted intermediate F bears a strong similarity to systems in which a facile 6π-electrocyclization is supported by calculation 13 and other experimental observations. 14 However, if the electrocyclization pathway is followed, then it is not clear what drives the formation of the cis -alkene that must be present in F for cyclization to occur.…”