A π-extension Of Pyrrole Or Indole Ring Systems To Indole Or Carbazole Via[2+2+2] Benzannulation

Indole and carbazole ring systems are typical biocative structural units which are widely used in dyes,pharmaceuticals and optical functional materials.In recent years,numerous cases of[4+2]benzannulation reaction to synthesize indoles or carbazoles from pyrroles or indoles and 4-carbon synthons have been developed.Theπ-extension of pyrrole or indole via benzannulation is one of the effective way to synthesize indoles or carbazoles.Most of the recently developed[4+2]benzannulation methods could not avoid a use of precious metal catalyst,harsh conditions and complexity of 1,4-biselectrophiles which limited the application of the method in synthetic chemistry.There are two types of[2+2+2]benzannulation patterns.The first is[2+2′+2′]benzannulation involves a reaction of pyrrole or indole with two molecules of dicarbon synthon.The second one is[2+2′+2′′]benzannulation involves a reaction of pyrrole or indole with two different molecules.In this work,a three-component reaction ofα-hydroxyketone and 1,3-dicarbonyl compounds and a pyrrole or an indole has been developed by using Sc（OTf）₃ or ZrCl₄ as catalyst,ethanol as solvent in synthesis of indoles or carbazole respectively.38 indole derivatives and 9 carbazole derivatives were successfully synthesized.The three-component reaction ofα-hydroxyketone,1,3-dicarbonyl compounds and pyrroles（indoles）is carried out in following steps:The first nucleophilic attack of pyrrole to the scandium-activated carbonyl carbon ofα-hydroxyketone leads to the formation of adduct.After an elimination of water,product gives an enol tautomer then the second nucleophilic attack to scandium-activated 1,3-dicarbonyl compounds’carbonyl carbon leads to the formation of second adduct.A dehydrative cyclization then produces the indole and carbazole product.A facile way to realize all-carbon annulativeπ-extension of pyrrole and indole ring systems via[2+2+2]benzannulation has been provided in the reaction.