9-(4-chlorobenzoyl)-6-methoxy-1,2,3,4-tetrahydrocarbazole-3-carboxylic acid 9-[(4-chlorophenyl)-oxomethyl]-6-methoxy-1,2,3,4-tetrahydrocarbazole-3-carboxylic acid 9-(4-chlorophenyl)carbonyl-6-methoxy-1,2,3,4-tetrahydrocarbazole-3-carboxylic acid