Heterocycles in Nature 165Heterocyclic secondary metabolites
An enormous structural variety of secondary metabolites, that is substances not involved in primary metabolism, have
been isolated from many types of organisms. Often, these compounds have no obvious purpose and do not appear to
convey any benefi t to the organism. However, many do have very considerable biological effects on mammals, notably on
man, and plant extracts containing them were used in ancient medicine. We give here just a small selection to illustrate
the enormous range and variety of structures.
Plants of many genera produce alkaloids – many thousands are known. The name ‘alkaloid’ was coined because these natural
substances are basic (amines), that is alkali-like, although the name is often used now to refer to nitrogenous natural com-
pounds generally, whether basic or not. A structurally simple example is nicotine, a highly toxic substance, the major active
component in tobacco (Nicotiana sp.). Coniine, the active ingredient of hemlock (Conium maculatum), is another toxic
example, famously used to put Socrates to death. Alkaloids that have been of benefi t to mankind include morphine from the
opium poppy (Papaver somniferum) and quinine (Cinchona offi cinalis). Although not immediately obvious, morphine is a
1-benzyl-isoquinoline alkaloid (of which there are many); the modifi ed isoquinoline unit is highlighted in red.
Alkaloids derived biosynthetically from tryptophan/tryptamine are amongst the most prolifi c: in the structures
shown, for ergotamine (the ergot fungus, Claviceps purpurea), physostigmine (Physostigma venenosum) and strychnine
(Strychnos nux vomica), the residual tryptamine unit is coloured.
(These three compounds are also highly toxic and all have been used in both medicine and in less benign circumstances.
Ergotamine is a toxic contaminant found in certain foods (page 192) and a medicine for migraine; physostigmine is the
active component of Calabar beans, which were used in ‘trial by ordeal’ in West Africa, and is a medicine for glaucoma
and other diseases; strychnine was used as a vermin poison and for murder (possibly many undetected, as the symptoms
of poisoning resemble those of tetanus), but was also used (in low doses!) as a stimulant.)
1-Benzopyryliums (chromyliums) occur as pigments in fl ower petals, in substances known as anthocyanins – most
blue, purple and red fl ower colours are due to anthocyanins. The actual colour shown by a particular anthocyanin
depends on pH and upon the presence of co-pigments – the pH is important because deprotonation of phenolic