A Novel and Facile Synthesis of N-Substituted 8-hydroxybenzo[g] chromeno[4, 3-b]indol-6(13H)-ones by a Nenitzescu Reaction

N-Substituted 8-hydroxybenzo[g]chromeno[4,3-b]indol-6(13H)-ones have been efficiently synthesized via a Nenitzescu reaction of naphthoquinone (or benzoquinone) and N-substituted 4-aminocoumarins in the presence of a catalytic amount of KHSO4 with a [bmim][BF4]/toluene system.