Access to ring-fused homo- and heteroaromatic derivatives via an initial ring-opening of 3-nitro-4-(phenylsulfonyl)thiophene

Article abstract of DOI:10.1021/jo034264q

Within an overall ring-opening/ring-forming protocol, the (e,e)-4-methylthio-2-nitro-3-phenylsul fonyl-1-pyrrolidino-1,3-butadiene (7) [derived from the initial opening of 3-nitro-4-(phenylsulfonyl)-thiophene (6) with pyrrolidine and silver nitrate in EtOH] is revealed to be an excellent precursor of nitro(phenylsulfonyl) derivatives of ring-fused aromatic (naphthalene, phenanthrene) or heteroaromatic (benzothiophene) compounds whose substitution pattern cannot be easily achieved by conventional methods. The key step is represented by a thermal electrocyclic rearrangement of (E,E)-1-aryl-4-methylsulfonyl-2-nitro-3-phenylsulfonyl-1,3-butadienes (9), which, thanks to proper geometric and electronic factors, occurs in unprecedentedly mild conditions and is followed by an irreversible, concerted syn β-elimination of methanesulfinic acid to aromatize the newly formed cyclohexadienic ring.