Access to 3-aminobenzothiophenes and 3-aminothiophenes fused to 5-membered heteroaromatic rings through 6 pi-electrocyclization reaction of keteniminium salts

We described a general approach to 3-aminobenzothiophenes and 3-aminothiophenes fused to 5-membered heteroaromatic rings as thiophenes, furans and pyrroles through a 6 pi-elctrocyclization reaction of keteniminium salts. We investigated various substituents not only on the aromatic rings, but also at C-2 and on the nitrogen atom of the keteniminium salt. In particular, we have determined the electronic requirements of the nitrogen substitution to secure the efficient formation of the corresponding keteniminium salt. A clear relation between the pK(a) of the amine leading to the formation of the keteniminium salt and the yield obtained for benzothiophene is established and should find broad application to other reactions involving these intermediates. Additional insight on the ease of this 6 pi-electrocyclization reaction was gained through competition reactions and DFT calculations. (C) 2018 Elsevier Ltd. All rights reserved.