A New, Simple, and High-Yielding Synthesis of 2,9-Dihydro-1H-pyrido[3,4-b]indol-1-ones

A new, simple, and high-yielding method was developed to prepare 2,9-dihydro-1 H- pyrido[3,4- b ]indol-1-ones by selective cyclization of the appropriate N -(2,2-dimethoxyethyl)-1 H -indole-2-carboxamide in polyphosphoric acid at 110 °C for 30 minutes. The reaction yield was strongly dependent on the acid used. The method was less affected by the presence of electron-donating and -withdrawing substituents at 5-position of the indole nucleus.