Int. J. Pharm. Sci. Rev. Res., 27(2), July – August 2014; Article No. 45, Pages: 272-278 ISSN 0976 – 044X Research Article Chemistry of Terpenoids Nita Yadava*, Rajesh Yadava, Anju Goyalb a*Department of Pharmacy, SRMS, College of Engg. and Tech, Bareilly, U.P, India. bDepartment of Pharmaceutical Sciences, B.N. College of Pharmacy, Udaipur, Rajasthan, India. *Corresponding author’s E-mail: [email protected] Accepted on: 25-05-2014; Finalized on: 30-06-2014. ABSTRACT Plant terpenoids are used extensively for their aromatic qualities. They play a role in traditional herbal remedies and are under investigation for antibacterial, antineoplastics, and other pharmaceutical functions. Plants do not only accumulate terpenes for herbivore defense, but also emit volatile blends in response to herbivory and many other biotic and abiotic stresses. These terpene- containing volatiles attract natural enemies of the attacking herbivores but due to the complexity of these volatile blends, it is difficult to attribute a specific function to a particular terpene. The importance of terpenes in both nature and human application is difficult to overstate. Basic knowledge of terpene and isoprene biosynthesis and chemistry has accelerated the pace at which scientists have come to understand many plant biochemical and metabolic processes. The abundance and diversity of terpene compounds in nature can have ecosystem-wide influences. Although terpenes have permeated human civilization since the Egyptians, terpene synthesis pathways are only now being understood in great detail. The use of bioinformatics and molecular databases has largely contributed to analyzing exactly how and when terpenes are synthesized. The steroids and sterols in animals are biologically produced from terpenoid precursors. Sometimes terpenoids are added to proteins, e.g., to enhance their attachment to the cell membrane; this is known as isoprenylation. Keywords: Isoprene, Isolation, Secondary metabolites, Spectroscopic methods and structure elucidation, Terpenes. INTRODUCTION Isoprene rule here are many different classes of naturally Thermal decomposition of terpenoids give isoprene as occurring compounds. Terpenoids also form a one of the product. Otto Wallach pointed out that Tgroup of naturally occurring compounds majority of terpenoids can be built up of isoprene unit.7 which occur in plants, a few of them have also been Isoprene rule states that the terpenoid molecules are obtained from other sources. Terpenoids are volatile constructed from two or more isoprene unit. substances which give plants and flowers their fragrance. They occur widely in the leaves and fruits of higher plants, conifers, citrus and eucalyptus.1 The term ‘terpene’ was given to the compounds isolated from terpentine, a volatile liquid isolated from pine trees. Isoprene unit The simpler mono and sesquiterpene is chief constituent of the essential oils obtained from sap and tissues of Further Ingold suggested that isoprene units are joined in certain plant and trees. The di and tri terpenoids are not the terpenoid via ‘head to tail’ fashion. steam volatile. They are obtained from plant and tree Special isoprene rule states that the terpenoid molecule is gums and resins. Tertraterpenoids form a separate group constructed of two or more isoprene units joined in a of compounds called ‘Carotenoids’. ‘head to tail’ fashion.8 The term ‘terpene’ was originally employed to describe a mixture of isomeric hydrocarbons of the molecular formula C10H16 occurring in the essential oils obtained from sap and tissue of plants, and trees. But there is a tendency to use more general term ‘terpenoids’ which include hydrocarbons and their oxygenated derivatives. But this rule can only be used as guiding principle and not However the term terpene is being used these days by 2-6 as a fixed rule. For example carotenoids are joined tail to some authors to represent terpenoids. tail at their central and there are also some terpenoids By the modern definition: “Terpenoids are the whose carbon content is not a multiple of five. hydrocarbons of plant origin of the general formula In applying isoprene rule we look only for the skeletal unit (C5H8)n as well as their oxygenated, hydrogenated and of carbon. The carbon skeletons of open chain dehydrogenated derivatives.” monoterpenoids and sesquiterpenoids are, International Journal of Pharmaceutical Sciences Review and Research Available online at www.globalresearchonline.net 272 © Copyright protected. Unauthorised republication, reproduction, distribution, dissemination and copying of this document in whole or in part is strictly prohibited. © Copyright protected. Unauthorised republication, reproduction, distribution, Int. J. Pharm. Sci. Rev. Res., 27(2), July – August 2014; Article No. 45, Pages: 272-278 ISSN 0976 – 044X Bicyclic Terpenoids: They contain two rings in the structure. Tricyclic Terpenoids: They contain three rings in the structure. Tetracyclic Terpenoids: They contain four rings in the structure. Examples Table 1: Classification of Terpenoids Number of carbon Value of n Class atoms 10 2 Monoterpenoids (C10H16) 15 3 Sesquiterpenoids (C15H24) Myrcene (monoterpene) Farnesol (Sesquiterpene) 20 4 Diterpenoids (C20H32) Ingold (1921) pointed that a gem alkyl group affects the 25 5 Sesterpenoids (C25H40) stability of terpenoids. He summarized these results in 30 6 Troterpenoids (C30H48) the form of a rule called ‘gem dialkyl rule’ which may be 40 8 Tetraterpenoids (C H ) stated as Gem dialkyl group tends to render the 40 64 cyclohexane ring unstable where as it stabilizes the three, >40 >8 Polyterpenoids (C5H8)n four and five member rings.” Some examples of mono, sesqui and di Terpenoids: This rule limits the number of possible structure in closing Mono Terpenoids the open chain to ring structure. Thus the monoterpenoid open chain give rise to only one possibility for a i) Acyclic Monoterpenoids monocyclic monoterpenoid i.e the p-cymene structure. Myrcene Citral Geraniol ii) Monocyclic monoterpenoids P-cymene structure Bicyclic monoterpenoids contain a six member and a three member ring. Thus closure of the ten carbon open chain monoterpenoid gives three possible bicyclic structures. Limonene α-Terpineol Menthol iii) Bicyclic monoterpenoids: These are further divided into three classes. a) Containing -6+3-membered rings Camphor(6+5) Pinane(6+4) Carane(6+3) system system System CLASSIFICATION OF TERPENOIDS Most natural terpenoids hydrocarbon have the general formula (C5H8)n. They can be classified on the basis of value of n or number of carbon atoms present in the Thujane Carane structure.9-11 (Table-1) b) Containing -6+4- membered rings Each class can be further subdivided into subclasses according to the number of rings present in the structure. Acyclic Terpenoids: They contain open structure. Monocyclic Terpenoids: They contain one ring in the structure. Pinane International Journal of Pharmaceutical Sciences Review and Research Available online at www.globalresearchonline.net 273 © Copyright protected. Unauthorised republication, reproduction, distribution, dissemination and copying of this document in whole or in part is strictly prohibited. © Copyright protected. Unauthorised republication, reproduction, distribution, Int. J. Pharm. Sci. Rev. Res., 27(2), July – August 2014; Article No. 45, Pages: 272-278 ISSN 0976 – 044X c) Containing -6+5-membered rings this particular time. e.g. From jasmine at sunset. There are four methods of extractions of oils. a) Expression method b) Steam distillation method c) Extraction by means of volatile solvents Bornane (Camphane) non bornane (iso camphane) d) Adsorption in purified fats Some bicyclic monoterpenes are Steam distillation is most widely used method. In this method macerated plant material is steam distilled to get essential oils into the distillate form these are extracted by using pure organic volatile solvents. If compound decomposes during steam distillation, it may be extracted with ether at 50oC. After extraction solvent is removed under reduced pressure. Camphor α-pinene ii) Separation of Terpenoids from essential oil Sesquiterpenoids A number of terpenoids are present in essential oil i) Acyclic sesquiterpenoids ii) Monocyclic obtained from the extraction. Definite physical and sesquiterpenoids iii) Bicyclic sesquiterpenoids chemical methods can be used for the separation of terpenoids. They are separated by fractional distillation. The terpenoid hydrocarbons distill over first followed by the oxygenated derivatives. More recently different chromatographic techniques have been used both for isolation and separation of Farnesol Zinziberene Cadinene terpenoids. 14-16 Diterpenoids General Properties of Terpenoids i) Acyclic diterpenoids 1. Most of the terpenoids are colorless, fragrant liquids which are lighter than water and volatile with steam. A few of them are solids e.g. camphor. All are soluble in organic solvent and usually insoluble in water. Most of them are optically active. 2. They are open chain or cyclic unsaturated compounds having one or more double bonds. Consequently they Phytol undergo addition reaction with hydrogen, halogen, acids, etc. A number of addition products have antiseptic ii) Mono cyclic diterpenoids properties. 3. They undergo polymerization and dehydrogenation 4. They are easily oxidized nearly by all the oxidizing agents. On thermal decomposition, most of the terpenoids yields isoprene as one of the product. General Methods of Structure Elucidation of 17-19 Vitamin A Terpenoids