Facile synthetic approach for 5-aryl-9-hydroxypyrano [3,2-f] indole-2(8H)-one

An appropriate method for the synthesis of 5-aryl-9-hydroxypyrano[3,2-f]indole-2(8H)-one was described. The targeted compounds were obtained starting from vanillin via nine steps. Interestingly, in the final cyclization step, the intermediate 4-(2-halogeno phenyl)-7-methoxy-1H-indole-6-yl propiolate could convert directly into the final product in one step reaction using PtCl4 or Pd(PPh3)4/trifluoroacetic acid as catalysts. The possible catalytic mechanism for PtCl4 and Pd(PPh3)4/trifluoroacetic acid was discussed.