An alternative approach to the synthesis of functionalized pyrido[2,3-b]indoles

A new synthesis of α-carbolines was developed starting from the easily accessible 3-substituted indol-2(3H)-one derivatives 2 and enamines 3. The intermediates 4 afforded the α-carbolines 5 and 6 by thermal cyclization with ammonium acetate in glacial acetic acid by way of pyridine ring formation.