Nitrogen Heterocycles Except Alkaloids

C. Specialized Heterocycles 1. Nitrogen Heterocycles (Except Alkaloids)... 

Most alkaloids have basic properties coimected with a heterocyclic tertiary nitrogen. Notable exceptions are colchicine, caffeine, and paclitaxel. Most alkaloids are biosynthetically derived from amino acids such as phenylalanine, tyrosine, tryptophan, ornithine, and lysine. Alkaloids represent a wide variety of chemical structures. About 20000 alkaloids are known, most being isolated from plants. But alkaloids have also been found in microorganisms, marine organisms such as algae, dinoflagellates, and puffer fish, and terrestrial animals such as insects, salamanders, and toads. 

True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non-nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as L-ornithine, L-lysine, L-phenylalanine/L-tyrosine, L-tryptophan and L-histidine . Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensine (Figure 4). A fuller list of examples appears in Table 1. 

Alkaloids are basic plant natural products that typically have a nitrogen atom as part of a heterocyclic ring system and indeed are classified on this basis. Thus major classes of alkaloids are based on indole, isoquinoline, pyrrolidine, piperidine, pyrrolizidine, quinoline, tropane, quinolizidine or other heterocyclic ring systems. Other alkaloids are basic monoterpenoid, sesquiterpenoid, diterpenoid, steroid, purine, pyrimidine or peptide entities. Some of these compounds are exceptionally toxic [1,6, 7-12]. 

Chemistry provides a definition of alkaloids in purely chemical terms. Chemists stress that alkaloids are any group of complex heterocyclic nitrogen compounds, which have strong physiological activity, are often toxic, and retain their own basic chemical properties. It is also stated that there are a few exceptions to this definition.Another chemical definition states only that alkaloids are nitrogen-containing compounds derived from plants and animals. Later, chemists stressed that alkaloids were bio-... 

True alkaloids derive from amino acid and share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine, which is a brown liquid. True alkaloids... 

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