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:

  • derived from plants

  • with a basic character (hence the term alkaloid from alkali)

  • 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:

  1. Monocyclic alkaloids - which contain only one ring.  A typical monocyclic alkaloid is nicotine:

  1. 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

  1. Polycyclic alkaloids such as strychnine, lysergic acid, cannabinol, morphine, heroin and codeine. 

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