6 Heterocyclic Chemistry derivatives is shown in Box 1.7.It includes four natural products(nico- tine,pyridoxine,cocaine and morphine)and two synthetic compounds (nifedipine and paraquat). Box 1.7 Biologically Active Pyridine and Piperidine Heterocycles NO Nifedipine (addictive drug (vitamin B) (cardiovascular drug) and an insecticide HO Me CO-M OCOPh Cocaine Morphine tate of mophine) There are many thousands of other heterocyclic compounds,both nat- ural and synthetic,of major importance,not only in medicine but also in most other activities known to man.Small wonder then that their chemistry forms a major part of both undergraduate and postgraduate curricula. 1.4 General Principles 1.4.1 Aromaticity Many fully unsaturated heterocyclic compounds are described as aro matic,and s some have a close similarity to benzene and its derivatives For example,pyridine(azabenzene)is formally derived from benzene through the replacement of one CH unit by N.As a result,the consti- 6 Heterocyclic Chemistry derivatives is shown in Box 1.7. It includes four natural products (nico￾tine, pyridoxine, cocaine and morphine) and two synthetic compounds (nifedipine and paraquat). There are many thousands of other heterocyclic compounds, both nat￾ural and synthetic, of major importance, not only in medicine but also in most other activities known to man. Small wonder then that their chemistry forms a major part of both undergraduate and postgraduate curricula. 1.4 General Principles I =4=1 Aromaticity Many fully unsaturated heterocyclic compounds are described as aro￾matic, and some have a close similarity to benzene and its derivatives. For example, pyridine (azabenzene) is formally derived from benzene through the replacement of one CH unit by N. As a result, the consti-