Cascade radical reaction of 2-alkynyl-substituted aryl radicals with aryl isothiocyanates: A novel entry to benzothieno[2,3-b] quinolines through alpha-(arylsulfanyl)imidoyl radicals

The novel cascade radical reaction of 2-(phenylalkynyl)aryl radicals with 4 -Y-phenyl isothiocyanates (Y = H, OMe, Me, Cl, CN) provides a useful one-po t protocol for the production of 8-Y-substituted (12) and/or 9-Y-substitute d benzothieno[2,3-b]quinolines (11). The whole process entails primary; for mation of an alpha-(2-alkynylarylsulfanyl)imidoyl radical that undergoes sm ooth 5-exe-dig cyclization onto the alkynyl triple bond. The derived 1-phen ylvinyl radical then exhibits six-membered cyclization cyclization onto the isothiocyanate ring, to give 11, and/or five-membered ipso-cyclization to an azaspiro intermediate, whose eventual rearrangement affords 12. The over all findings clearly showed that the relative proportion of the outcoming i someric benzothienoquinolines ii and 12 can be markedly affected by the nat ure of the original isothiocyanate substituent. Moreover, the findings also furnished the first chemical evidence that enhancing the eledrophilic powe r of the employed radical can properly enhance the reactivity of aryl radic als toward isothiocyanates.