A plethora of naturally occurring plant products have been found to be related wherein they are comprised of one or more units of isoprene (C5H8)-a hydrocarbon:

In general, terpenoids, may be defined as natural products whose structures are considered to be divided into several isoprene units; therefore, these compounds are invariably termed as isoprenoids. Besides, this particular group of compounds is sometimes collectively referred to as the terpenes in relatively older texts. Logically, the–oid suffix seems to be more acceptable and
convincing, as it is in the same vein for steroids, alkaloids, flavonoids, etc., However, the-ene suffix must be solely confined to the unsaturated hydrocarbon belonging to this specific class of compounds.
It has now been established experimentally that the isoprene units come into being through the biogenetic means starting from acetate via mevalonic acid. Each such unit essentially consists of five-carbons having two unsaturated bonds and possesses a branched chain. The terpenoids usually have a number of such isoprene units joined together in a head to tail manner, as exemplified below:
Terpenoids are broadly classified on the basis of the number of isoprene units incorporated into a specific unsaturated hydrocarbon terpenoid molecule, such as:
(a) Monoterpenoids: These are built up of two isoprene units and have the molecular formula C10H16;
(b) Sesquiterpenoids: These are composed of three isoprene units and have the molecular formula C15H24;
(c) Diterpenoids: These are comprised of four isoprene units and have the molecular formula C20H32;

(d) Triterpenoids: These contain six isoprene units and have the molecular formula C30H48; and
(e) Tetraterpenoids These are made up of eight isoprene units and have the molecular (or Carotenoids): formula C40H64.
Biogenetic Isoprene Rule The very idea and basic concept that terpenoids are essentially built up of several isoprene units is commonly termed as the biogenetic isoprene rule as could be observed from the various typical examples cited earlier.
Meroterpenoids It has been observed that a good number of other natural products do exist which
essentially belong to mixed biosynthetic origin and are mostly made up from isoprene as well as
nonisoprenoid entities.

ergotamine, quinine, cannabinol and vitamin-E
Examples A few typical examples are: ergotamine, quinine, cannabinol and vitamin-E.
More than 20, 000 naturally occurring large variety of terpenoids have been duly isolated and characterized, and thus constitute a major congregation of such products when compared to any other individual class of natural products. In fact, the chemical ecology rests heavily and predominantly on the occurrence of profusely distributed plant terpenoids, and hence, the latter play a broad-spectrum of highly specific and characteristic roles in the plant kingdom, such as:
insect propellents and antifeedants, phytoalexins, attractants for pollingranes, pheromones, defensive substances against herbivorous animals, allelochemicals, signal molecules and above all the plant growth hormones. Terpenoids usually engage in a variety of probable interactions, for instance: plant and plant, plant and microorganism, and plant and animal.
The International Union of Pure and Applied Chemistry (IUPAC) recommends a systematic mode of nomenclature of terpenoids; however, the names suggested by it are not only lengthy but also quite cumbersome. Therefore, the old and the trivial names of most terpenoids are used most frequently even today for naming the relatively common substances: examples:

Trivial Name

3, 7- Dimethyl-2, 6-octadien-1-ol;
1-Methyl-4-(1-methylethynyl)- cyclohexene;
7-Methyl-3-methylene-1, 6-octadiene;
Carbon-Skeleton in Terpenoids A comparative study of carbon-skeleton in terpenoids has revealed that a great majority of monocyclic terpenes essentially possess a para menthane carbon skeleton; besides, derivatives of cyclopentane and methylated cyclohexanes also exists invariably.
Generally, two different methods of tackling the structural problems normally encountered in terpenoids are adopted, namely:
(i) Dehydrogenation: Mostly the terpene hydrocarbon, dienes, upon dehydrogenation give rise to p-cymene. Having identified the prevailing carbon-skeleton in it, the exact location of the double bonds in the existing framework may be established by oxidative degradation to the corresponding simple aliphatic acids, and
(ii) Oxidation: Tilden was pioneer for the strategic incorporation of nitrosyl chloride (O=N–Cl) function by the help of a specific reagent (Tilden Reagent) so as to characterize and ensure the purity of the starting material via formation of definite crystalline derivatives of the corresponding terpenes under investigation. It has been established by Tilden’s study that the olefenic linkage reacted specifically with this reagent to yield the respective nitrosochloride adduct, as shown below:

 nitrosochloride adduct
In a situation, where the C-atom possesses both nitrosomoiety and a H-atom, the former undergoes isomerization readily to yield isonitrochloride an oxime. However, in the absence of this specific characteristic feature the nitrosochloride is fairly stable.
It has been observed that when the substance is monomeric* the corresponding nitrosochloride
provides a distinct blue colouration, which also ascertains the presence of tetrasubstituted ethylenes.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar

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