General and efficient synthesis of 2,3-unsubstituted indoles catalyzed by acidic mesoporous molecular sieves

A general and efficient method for the synthesis of 2,3-unsubstituted indoles has been established by the intramolecular cyclization of N-benzyl 2-anilinoacetals. Acidic mesoporous molecular sieve (MCM-41-SO3H) has shown excellent catalytic activity on this transformation, and the 2,3-unsubstituted indoles bearing 7-substituent or strong electron-withdrawing substituents also could be achieved by this protocol. Moreover, the heterogeneous catalyst, MCM-41-SO3H, could be conveniently recovered and reused without obvious loss of the catalytic activity. This work will provide an economic and environmental-benign method for the construction of various indole derivatives.