Terpenoids Lecture: Isoprenoids: What Are Terpenoids PDF

Terpenoids  They are organic naturally occurring compounds, they are the most widely distributed natural compounds, they are biosynthesized by almost all higher green plants. The lower plants e.g. bacteria and fungi, produce a number of interested terpenoids. Marine organisms produce some a...

Terpenoids  They are organic naturally occurring compounds, they are the most widely distributed natural compounds, they are biosynthesized by almost all higher green plants. The lower plants e.g. bacteria and fungi, produce a number of interested terpenoids. Marine organisms produce some abnormal terpenoid compounds. In the animal kingdom terpenoids are detected in defence secretion of some insects and termed “insect pheromones”. Therefore, the term terpenoid cover a wide range of secondary active constituents. Nearly more than 50000 terpenoid compounds have been detected, isolated and their chemical structure have been elucidated, this number is higher than the number of any other naturally occurring compounds. The term terpenoids means that these compounds are of common biosynthetic origin, because they are formed from repeated carbon units. Each unit characterized by: - Containing 5-carbon atoms - branched chain - Unsaturated  C-C-C-C-C C C-C-C-C C-C-C C C  These units are termed isoprene units and hence the term isoprenoid compounds are some time used to designate these compounds. ISOPRENOIDS TERPENOIDS The chemical structure of terpenoids are so diverse and numerous, start from simple C5 units and then multiplied to form higher terpenoids compounds.  Hemiterpenoids C5 X2 Monoterpenoid C10 X2 X2 Tetraterpenoids C40 Xn Polyterpenoid nC5  If you add to this , the diversity of the type, number and position of the functional groups e.g. OH, C=O, NH2 – etc., then a huge number of compounds will be formed. CHO OCH3 OH  Isoprene Rule :‫قاعذة االيزوبرين‬ The most important structural feature of nearly all terpenoid compounds are their derivation from Monomeric structural units called isoprene units. This observation was first reported by scientist called O.Wallach at 1887  Later confirmed experimentally by chemical analysis of thermal decomposition products of a number of terpenoid compounds ranging in molecular weight from : low molecular weight e.g. volatile oils, medium molecular weight, e.g. steroids and high molecular weight e.g. rubber. These analysis showed that terpenoids are constructed from a whole number of 5 carbon units derived from 2 methyl-butadiene.  Further studies by Prof. Ignold in 1925 showed that, the isoprene units are linked together in systematic fashion known as Head to Tail linkage or fashion. OPP IPP - C5 OPP GPP - C10 OPP FPP - C15 OPP GGPP - C20  This was finally crowned by prof. Ruzcka in 1953 into what is known as “isoprene Rule” which stated that : Terpenoid are formed from multiple of isoprene units according to head to tail linkage However, there are always an exceptions, some compounds, although of terpenoid origin, but did not obey the rule such as 1-Artemisiaketone 2-cholesterol.  (1) In Artemisia ketone, the arrangement of isoprene units did not follow head to tail fashion. (2) In cholesterol, the number of carbon (27) did not give whole number of C5 units. This deviation from the rule can be rationalized as follow : Formation of terpenoid from isoprene units follow two type of modes, 1-General mode and 2-exceptional mode. (1) General mode : This also known as linear combination mode )‫ (موديل األرتباط الخطى‬in which the isoprene units condensed together in head to tail fashion forming straight chains compounds)‫(مركباث راث سالسل خطيت‬. OPP IPP - C5 OPP GPP - C10 OPP FPP - C15 OPP GGPP - C20 (II) Exceptional mode The terpenoid compounds in this mode are formed from the linear combination compounds via chemical or mechanical modification e.g. : 1. Rearrangement. 2. Cleavage. 3. Cyclization. 4. Degradation. 5. Dimerization and Polymerization.