Heterogeneous Synthesis of Indole-2-Carboxylic Esters Via Bischler Cyclization Under the Catalysis of Sulfonic Acid-Functionalized SBA-15 Silicas

Phenylsulfonic acid-functionalized SBA-15 silica was found to be an efficient heterogeneous acid catalyst in Bischler cyclization for the synthesis of indole-2-carboxylic esters. Under the optimized conditions, a wide range of substituted 4-nitrobenzyl 3-hydroxy-2-(N-methyl-N-arylamino)-2-enoates could be smoothly transformed to the corresponding 4-nitrobenzyl indole-2-carboxylates in excellent yields (87-99 %). Moreover, the catalyst could be conveniently recovered and reused at least four times without significant loss of catalytic activity.