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  Types of Compounds Found in Plants, Chemistry tutorial

Introduction

Online medical dictionary describes natural products as naturally occurring compounds that are end products of secondary metabolism; they are exclusive compounds for particular organism or classes of organisms. Harshly speaking, any biological molecule is a natural product, but the expression is generally preserved for secondary metabolites, tiny molecules (mol wt up to 1500 amu approx) generated via an organism but that aren't strictly needed for the survival of the organism. They are dissimilar from the more prevalent macromolecules such as proteins, nucleic acids, and polysaccharides that make up the basic machinery for the more elemental procedures of life.

Natural products are organic compounds, which are shaped via living systems. The elucidation of their structures and chemistry, synthesis and biosynthesis are major areas of organic chemistry. Naturally occurring compounds may be divided into three broad categories. Firstly, there are those compounds which take place in all cells and play a central role in the metabolism and reproduction of those cells. These compounds include the nucleic acids and the common amino acids and sugars. They are recognized as primary metabolites. Secondly, there are the high-molecular-weight polymeric materials these as cellulose, the lignins and the proteins which form the cellular structures. Finally, there are those compounds that are feature of a bounded range of species; such are the secondary metabolites. Most primary metabolites exert their biological effect within the cell or organism that is responsible for their production. Secondary metabolites, on the other hand, have often attracted interest because of their biological effect on other organisms. The biologically active constituents of medicinal, commercial and poisonous plants have been studied throughout the development of organic chemistry. Many of these compounds are secondary metabolites. It has been estimated that over 40% of medicines have their origins in these natural products. A number of screening programmes for bioactive compounds exist and have led to new drugs, for example taxol, which is used for the treatment of various cancers. Natural products often have an ecological role in regulating the interactions between plants, microorganisms, insects and animals. They can be defensive substances, antifeedants, attractants and pheromones. 

History and Background of Natural Products

Natural products (and their derivatives and analogs) represent over 50% of all drugs in clinical use. Man has been using plant based medicines in the form of crude drugs such as tinctures, teas, poultices, powders, and other herbal formulations since centuries. The plant based indigenous knowledge was passed down from generation to generation in various parts of the world throughout its history and has significantly contributed to the development of different traditional systems of medicine. The use of plants as medicines has involved the isolation of active compounds, beginning with the isolation of morphine from opium in the early 19th century and subsequently led to the isolation of early drugs such as cocaine, codeine, digitoxin and quinine. 

Many natural products were known to mankind for thousands of years. Ethanol produced by fermentation has its origin in pre-history, natural dyes have been known for long, sucrose the most common and well known natural product has its history of use since the time of Alexander the Great. With the discovery of salicin from willow tree extracts and the development of aspirin in 1899, the art of exploiting natural products became a molecular science. The discovery of penicillin in 1928 and its subsequent development as an anti-infective agent represents another milestone in the history of natural products, and marked the beginning of a new era in drug discovery, in which bacteria and fungi were added to the plant kingdom as sources for biologically active compounds. Isolation of active compounds, beginning with the isolation of morphine from opium in the early 19th century subsequently led to the isolation of early drugs such as cocaine, codeine, digitoxin and quinine, etc. 

Before the middle of the 20th century, chemists in this field were intent merely on isolating and determining the structure of natural products by classical methods. However, after 1945 and over the next several decades, scientists developed a host of analytical tools that greatly improved the study of these natural materials. Advancement like new separation techniques, spectral methods of identification and new synthetic tools led to major advances in the analysis of natural products. This resulted in isolation and synthesis of thousands of these products. Natural products also inspired chemists and physicians and their rich structural diversity and complexity prompted synthetic chemists to produce them in the laboratory, often with therapeutic applications in mind, and many drugs used today are natural products or natural-product derivatives. Recent years have seen considerable advances in our understanding of natural-product biosynthesis. Coupled with improvements in approaches for natural-product isolation, characterization and synthesis, these could be opening the door to a new era in the investigation of natural products in the academia and industry.

Classification of Natural Products

There is no rigid scheme for classifying natural products, their immense diversity in structure, function, and biosynthesis is too great to allow them to fit neatly into a few simple categories. In practice, however, workers in the field often speak of five main classes of natural products:

  • terpenoids and steroids
  • alkaloids
  • polyketides and fatty acids
  • phenylpropanoids
  • Specialized carbohydrates.

Terpenoids and steroids

Such are an enormous group of materials, more than 35,000 are known, they are obtained biosynthetically from isopentenyl diphosphate. Terpenoids have an enormous variety of apparently unrelated structures, while steroids have a ordinary tetracyclic carbon skeleton and are modified terpenoids, which are biosynthesised from the triterpene lanosterol.   

Alkaloids

Like terpenoids, alkaloids constitute a huge and different class of compounds, through more than 12,000 instances recognized at present. They enclose a basic amine group in their structure and are derived biosynthetically from amino acids.  

Fatty acid and polyketides

It is a class of that more than 10,000 are recognized. They are biosynthesised from easy acyl precursors these as acetyl CoA, propionyl CoA, and methylmalonyl. Non-ribosomal polypeptides are peptide-like compounds that are biosynthesised from amino acids via amultifunctional enzyme compound with no direct RNA transcription. The penicillins are fine instances. 

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