2-ETHOXY-5-[(4-METHYLPIPERAZIN-1-YL)SULFONYL]BENZOIC ACID;2-Ethoxy-5-[(4-methyl-1-piperazinyl;Benzoic acid, 2-ethoxy-5-[(4-methyl-1-piperazinyl)sulfonyl]-;Sildenafil Impurity 61;Sildenafil intermediate 2