First synthesis of 1-(indol-2-yl)azulenes by the Vilsmeier–Haack type arylation with triflic anhydride as an activating reagent

Azulene derivatives reacted with 2-indolinones in the presence of triflic anhydride (Tf2O) to afford 1-(indol-2-yl)azulenes in good yields. In the cases of the reaction of 6-tert-butyl-1-(methylthio)azulene (11) and 1-(1,4-dihydropyridin-4-yl)azulene 14, 1,1′-biazulene derivative 24 and 1-(indol-2-yl)azulene (2) were obtained under the similar reaction conditions, respectively, instead of the presumed electrophilic substitution products.