Benzotriazole-Mediated Conversions of para-H-Substituted Pyrylium, Benzo[b]pyrylium, and Xanthylium Salts into para-Position Functionalized Derivatives (An Indirect Electrophilic Substitution of Electron-Deficient Heteroaromatics)

Abstract

4H-Substituted pyrylium 9, benzo[5]pyrylium 15, and xanthylium salts 22 react with benzotriazole to give the corresponding 4H-(benzotriazol-1-yl)pyrans 10, benzo[b]pyrans 16, or xanthenes 23. Novel anion precursors 10, 16, and 23 undergo smooth lithiations at the positions α to the benzotriazol-1-yl function, i.e., at the para-position of the O-heterocycle. Subsequent trapping with different alkyl halides gives intermediates 14, 21, and 25, which are converted by mineral acid into para-functionalized pyrylium 11-13, benzo[b]pyrylium 17-20, and the xanthylium 24 salts in good to excellent yields.