The Alkaloids
It is quite difficult to give a precise definition for the class of compounds referred to as alkaloids [1]. However it is probably fair to say that the definition proposed by Ladenburg in the late 1880s is probably reasonable and still valid today. Ladenburg suggested that alkaloids were compounds:
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derived from plants
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with a basic character (hence the term alkaloid from alkali)
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contained a nitrogen based heterocylic ring within their molecules
In general alkaloids are physiologically very active and often extremely poisonous - they are the "bad guys" of the natural products world. Having said this, many find applications in medicine when administered in small doses.
Alkaloids can be sub-categorised [2] according to:
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Monocyclic alkaloids - which contain a single, unfused ring. A typical monocyclic alkaloid is nicotine:
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Bicyclic alkaloids - these may be illustrated by the tropane alkaloids which consist of molecules with a 1,4 nitrogen bridged cycloheptane structure. Atropine and cocaine are examples
