Chapter 12 

 ALKALOIDS 



The alkaloids do not represent a chemically homogeneous group, so that any gen- 

 eralizations about them are subject to many exceptions. They all contain nitrogen, fre- 

 quently in a heterocyclic ring, and many but not all, are basic as their name indicates. 

 Simple, aliphatic amines are not included here although the distinction is not sharp — 

 ephedrine and mescaline are often placed with the alkaloids although their nitrogens are 

 aliphatic: 



OCH3 OH 



CHjO-^^CHjCHgNHj ^^CHCHCHg 



OCH 



HN 

 3 CH3 



mescaline ephedrine 



Classification of alkaloids is done on the basis of the ring system present, e. g. 



Q CD 00 



pyridine piperidine tropane isoquinoline 



The purines and pyrimidines are conveniently considered separately because of their bi- 

 ochemical relation to the nucleic acids. The purines, caffeine and theobromine, are fre- 

 quently placed with the alkaloids, though; and the distinction appears to be a physiological 

 rather than a chemical one. Some alkaloid structures with their ring-numbering systems 

 are given in Figure 12-1. 



The alkaloids as a group are distinguished from most other plant components by 

 their basic (cationic) nature. Therefore, they normally exist in plants as the salts of 

 various organic acids and are frequently handled in the laboratory as salts of hydrochloric 

 or sulfuric acid. These salts, and frequently the free alkaloids, are colorless crystalline 

 compounds. A few alkaloids are liquids, and colored ones are even more rare (berberine 

 and serpentine are yellow). Alkaloids are frequently optically active; and normally only 

 one of the optical isomers is found naturally, although in a few cases racemic mixtures 

 are known. 



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