Definition, typology, and occurrence of alkaloids

Chapter Outline

1.1 Definition

The definition of the term alkaloid is not simple and, in many cases, is a source of academic controversy. This controversy was especially active at the beginning of 20th century, when the progress in chemical research on alkaloids occurred. Many new achievements in the field have been opposed and very critically discussed. Moreover, scientific discussions around the alkaloids continue.8,23 Alkaloids were fascinating subject in the past, and nowadays, they are an object of strong scientific and economic interest, especially in medicine and the pharmaceutical industry. Difficulties with the definition of such a group of secondary, natural molecules as alkaloids stem, also nowadays, from the similarities between alkaloids and other secondary compounds, despite their diversity and occurrence in complexes with different moieties, sometimes classified as different biochemical types, such as peptides, proteins, and sugars. Moreover, alkaloids were used for long time before their discovery as chemical molecules with a unique nomenclature. For example, the alkaloid quinine was use in herbal form by native people of America many hundred years before it was given that name. Awareness of the possibility of using this herbal medicine in the cure of malaria came in the year 1638, when some aristocrats were officially cured of malaria by this mean.87 Attempts to define the term alkaloid originated at the time of the discovery of these compounds. Friedrich Sertürner, an apothecary’s assistant from Westphalia, first isolated morphine (Figure 1.1), one of the most important alkaloids in the applied sense.43 This was in 1805 and proved a significant step forward in chemistry and pharmacology.10,70,366 Using the method developed by Friedrich Sertürner, the pharmaceutical researchers Pierre Joseph Pelletier and Joseph Benaimé Caventou isolated, from 1817 to 1821, a remarkable range of other alkaloids (Figure 1.2), such as brucine (a close relative of strychnine), febrifuge, quinine, caffeine, and veratrine.42–44 After that, the first attempts to discover alkaloids by synthetic means took place in the middle of 1800s. One of them was an attempt to produce synthetic quinine in the laboratory. However, many other chemical compounds have been made synthetically before quinine, which had been done only in 1900s. Nowadays, the synthetic production of some important alkaloid is more common than their isolation from natural sources. The progress in both theoretical and applied chemistry led to the synthesis such new compounds, which do not exist in the nature and can only be manufactured.

The term alkaloid was first mentioned in 1819 by W. Meißner, an apothecary from Halle. He observed that these compounds appeared “like alkali” and so named them alkaloids.67

For the biologist, the alkaloid is a pure and perfect natural product. From the biological point of view, the alkaloid is any biologically active and heterocyclic chemical compound that contains nitrogen and may have some pharmacological activity and, in many cases, medicinal or ecological use.11 This definition, as a relatively wide one based on application, can be criticized as inexact. However, it presents a general picture of what kinds of compound are under consideration. The biological and chemical nature of this group of compounds leads to the conclusion that each definition of alkaloids...