ABSTRACT
Phenols are one of the largest groups of secondary plant constituents. They are aromatic alcohols since the hydroxyl group is always attached to a benzene ring. Simple phenols consist of an aromatic ring in which a hydrogen is replaced by a hydroxyl group. Salicylic acid is a carboxylated phenol, that is, carboxylic acid and a hydroxyl group added to a benzene ring. It is rarely found freely in plants, but usually occurs as glycosides, esters and salts. Lignans are dimeric compounds in which phenylpropane units are linked between their side chains at the C-8 positions to form three-dimensional networks. Quinones are polycyclic aromatic compounds in which one hexane ring contains two opposite carbonyl groups. In 1968 lapachol was identified as an antitumour agent, showing significant activity against Walker 256 carcinosarcoma in vivo, particularly following twice daily oral administration. The isopentenyl side chain in lapachol is thought to play a pivotal role in this activity.
