Natural product to modern discovery of their active ingredients and eventually to the development of new drugs. A large number of currently prescribed drugs have been either directly derived from or inspired by natural products. A few representative examples are listed below. Some of the oldest natural product based drugs are analgesics. The bark of the willow tree has been known from antiquity to have pain relieving properties. This is due to presence of the natural product salicin which in turn may be hydrolyzed into salicylic acid. A synthetic derivative acetylsalicylic acid better known as aspirin is a widely used pain reliever.
Phenylarsonic acid was first prepared by Michaelis and Loenser. X-ray crystallography indicates that the molecules are connected by hydrogen-bonds consistent with short distance of 2.5 Å separating the oxygen atoms. The arsenic center is tetrahedral. Several derivatives of phenylarsonic acid have been used as additives for animal feeds. These include 4-hydroxy-3-nitrobenzenearsonic acid (3-NHPAA, or Roxarsone), "p"-arsanilic acid ("p"-ASA), 4-nitrophenylarsonic acid (4-NPAA), and "p"-ureidophenylarsonic acid ("p"-UPAA). Phenylarsonic acid Phenylarsonic acid is the chemical compound with the formula CHAsO(OH), commonly abbreviated PhAsOH. This colourless solid is an organic derivative of arsenic acid, AsO(OH), where one OH group has been replaced by a phenyl group.