The sort of functional groups that are commonly found in lead compounds include alcohols, phenols, amines, aromatic/heteroaromatic rings, carboxylic acids, alkenes, and ketones. Knowledge of the typical reactions that can be carried out on such functional groups allows the synthesis of various analogs. A full synthesis involves synthesizing the lead compound and its analogs from simple starting materials. Therefore, the full synthesis of a complex natural product may prove too time-consuming, too costly or low yielding to be practical. Semi-synthetic syntheses involve the synthesis of the lead compound and/or its analogs from a naturally occurring compound. The product obtained can then be esterified with different acid chlorides to produce a large range of semi-synthetic penicillins. If the biosynthetic route to the natural compound is known, an analog of a biosynthetic intermediate can be synthesized and fed to the organism in the hope that it will be converted to an analog.