Terpenes - Lecture 2 PDF

TERPENES CHEM 354 – LECTURE 2 Lawrence S. Borquaye (Ph.D.)  TERPENES & TERPENOIDS Terpenes - class of >50,000 compounds containing carbon atoms in multiples of five Terpenoids - oxygen-containing terpenes (alcohols, ketones, aldehydes) The name "terpene" is derived from th...

TERPENES CHEM 354 – LECTURE 2 Lawrence S. Borquaye (Ph.D.)  TERPENES & TERPENOIDS Terpenes - class of >50,000 compounds containing carbon atoms in multiples of five Terpenoids - oxygen-containing terpenes (alcohols, ketones, aldehydes) The name "terpene" is derived from the word "turpentine“ Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Terpenoids (& terpenes) have been isolated from both terrestrial and marine plants, and fungi. In nature, terpenes occur predominantly as hydrocarbons, alcohols and their glycosides, ethers, aldehydes, ketones, carboxylic acids and esters. TERPENES & TERPENOIDS  BRIEF HISTORY O. Wallich (1887) Conjectured that terpenes must be constructed from isoprene units Ruzicka (1953) Turned this hypothesis into a general rule This principle has since been confirmed experimentally (the Isoprene Rule) Each group of terpenes arises from the head-to-tail condensation of a variable number of isoprene units  TERPENES Terpenes are natural products that are structurally related to isoprene. CH3 or H2C C CH CH2 Isoprene (2-methyl-1,3-butadiene)  THE ISOPRENE UNIT An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds) Myrcene contains two isoprene units.  THE ISOPRENE UNIT The isoprene units of myrcene are joined "head-to-tail." head tail tail head  CLASSIFICATION OF TERPENES Class Number of Number of Isoprene Units Carbon Atoms Hemiterpenes 1 5 Monoterpenes 2 10 Sesquiterpenes 3 15 Diterpenes 4 20 Sesterpenes 5 25 Triterpenes 6 30 Tetraterpenes 8 40 Polyterpenes >8 >40  CLASSIFICATION OF TERPENES 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. 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.  MONOTERPENES Monoterpenes consist of two isoprene units with molecular formula C10H16. Examples are menthol and citral. OH O H a-Phellandrene Menthol Citral (eucalyptus) (peppermint) (lemon grass)  REPRESENTATIVE MONOTERPENES OH O H a-Phellandrene Menthol Citral (eucalyptus) (peppermint) (lemon grass)  REPRESENTATIVE MONOTERPENES a-Phellandrene Menthol Citral (eucalyptus) (peppermint) (lemon grass)  MONOTERPENES Acyclic Monocyclic 13  MONOTERPENES Bicyclic monoterpenes: These are further divided into three classes. b) Containing - 6+4- membered rings. a) Containing -6+3- c) Containing -6+5- membered rings membered rings. 14  SESQUITERPENES Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples: H humulene, farnesenes, farnesol. a-Selinene (celery) SESQUITERPENES a-Selinene (celery)  SESQUITERPENES Monocyclic Acyclic Bicyclic 17  DITERPENES Diterpenes are composed of four isoprene units with molecular formula C20H32. Examples: cafestol, kahweol, cembrene and taxadiene (precursor of taxol). Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol. They are known to be antimicrobial and antiinflammatory. DITERPENES OH Vitamin A DITERPENES OH Vitamin A DITERPENES Vitamin A  DITERPENES Acycliclic diterpene Monocyclic diterpene 22  TRITERPENES Triterpenes consist of six isoprene units and have the molecular formula C30H48. The linear triterpene squalene, the major constituent of shark liver oil. tail-to-tail linkage of isoprene units Squalene (shark liver oil)  TETRATERPENES Tetraterpenes contain eight isoprene units and have the molecular formula C40H64. Biologically important tetraterpenes include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.  POLYTERPENES Polyterpenes consist of long chains of many isoprene units. Natural rubber consists of polyisoprene in which the double bonds are cis.  NON-ISOPRENOID TERPENES O Valerane Eremophilone -Vetivone CLASSIFY Artemisinin (antimalarial) TERPENE BIOSYNTHESIS  TERPENE BIOSYNTHESIS  The five-carbon building blocks of all terpenes, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), are derived from two independent pathways localized in different cellular compartments.  The cytosol localized Mevalonate pathway provides C5 units for sesquiterpene and triterpene biosynthesis.  The methylerythritol phosphate (MEP or nonmevalonate) pathway, localized in the plastids, is thought to provide IPP and dimethylallyl diphosphate for hemiterpene, monoterpene, and diterpene and tetraterpene biosynthesis.  TERPENE BIOSYNTHESIS CYTOSOL PLASTID Acetyl CoA G3P + Pyruvate Mevalonic Acid MethylErythritol Phosphate (MEP) Sesquiterpenes Hemiterpenes, monoterpenes, and Triterpenes diterpenes, and tetraterpene  TERPENE BIOSYNTHESIS Mevalonic acid Three molecules of acetyl-coenzyme A are used to form mevalonic acid. Two molecules combine initially in a Claisen condensation to give acetoacetyl-CoA, and a third is incorporated via a stereospecific aldol addition giving the branched-chain ester β-Hydroxy-β- MethylGlutaryl-CoA (HMG-CoA).  O O O CoA H 2C S CoA CoA H2C S H 2C S H Claisen B Enz The mevalonate pathway does not use malonyl derivatives and it thus diverges from the O O acetate pathway at the CoA very first step. S Acetoacetyl CoA  Enz-SH CoA Enz H 3C S H 3C S O Stereospecific aldol reaction Enz H2C S In the second step, it should be noted S that, on purely chemical grounds, CoA acetoacetyl-CoA is the more acidic O O substrate, and might be expected to H-B-Enz act as the nucleophile rather than the third acetyl-CoA molecule. The enzyme thus achieves what is a less favourable reaction. β-Hydroxy-β-MethylGlutaryl-CoA (HMG-CoA)  HMG-COA HMG-CoA 2 NADPH Reductase H2O The conversion of HMGCoA into (3R)- MVA involves a two- step reduction of the thioester group to a (3R)-Mevalonic Acid primary alcohol.  (3R)-Mevalonic acid The 6-carbon compound, MVA, is transformed into the five- carbon phosphorylated isoprene units in a series of reactions, beginning with phosphorylation of the primary alcohol group. Decarboxylation/ dehydration then give IPP. H+ Isopentenyl pyrophosphate 3,3-Dimethylallyl (IPP) Pyrophosphate (DMAPP) Mevalonic Acid or Elucidate mechanistically The formation of IPP and Methylerythritol Phosphate (MEP) DMAPP via the Non- Mevalonate pathway H+ Isopentenyl pyrophosphate 3,3-Dimethylallyl pyrophosphate IPP DMAPP IPP is isomerized to the other isoprene unit, DMAPP, by an isomerase enzyme which stereospecifically removes the pro-R proton (HR) from C-2, and incorporates a proton from water on to C-4. Whilst the isomerization is reversible, the equilibrium lies heavily on the side of DMAPP (WHY ???). OPP DMAPP Allylic Carbocation DMAPP possesses a good leaving group, the diphosphate, and can yield via an SN1 process an allylic carbocation which is stabilized by charge delocalization. This generates a reactive electrophile and therefore a good alkylating agent. DMAPP reacts as an electrophile.  In contrast, IPP with its terminal double bond is more likely to act as a nucleophile, especially towards the electrophilic DMAPP. DMAPP SN1 IPP Electrophile Nucleophile (Electron deficient) (Electron rich) These differing reactivities are the basis of terpene biosynthesis, and carbocations feature strongly in mechanistic rationalizations of the pathways. Therefore, terpenoids are synthesized by joining IPP (a nucleophile) and DMAPP (an electrophile) in a head to tail manner.  Head DMAPP C. OPP SN1 C C Head C C Tail. C Head C C IPP OPP C C Tail Tail Head OPP  JOINING ISOPRENE UNITS Head-to-Tail Tail to Middle Tail-to-Tail Larger terpenoid units dimerize tail-to-tail. CARBON-CARBON BOND FORMATION IN TERPENE BIOSYNTHESIS OPP + OPP The key process involves the double bond of isopentenyl pyrophosphate (IPP) acting as a nucleophile toward the allylic carbon of dimethylallyl pyrophosphate (DMAPP). CARBON-CARBON BOND FORMATION OPP + OPP – OPP + OPP  AFTER C—C BOND FORMATION... The carbocation can lose a proton to give a double bond. + OPP  AFTER C—C BOND FORMATION... OPP The carbocation can lose a proton to + –H give a double bond. + OPP  AFTER C—C BOND FORMATION... OPP This compound is called geranyl pyrophosphate. It can undergo hydrolysis of its pyrophosphate to give geraniol (rose oil). AFTER C—C BOND FORMATION... OPP H2O OH Geraniol  FROM 10 CARBONS TO 15 OPP + OPP Geranyl pyrophosphate + OPP FROM 10 CARBONS TO 15 OPP + –H + OPP  FROM 10 CARBONS TO 15 OPP This compound is called farnesyl pyrophosphate. Hydrolysis of the pyrophosphate ester gives the alcohol farnesol.  FROM 15 CARBONS TO 20 OPP OPP Farnesyl pyrophosphate is extended by another isoprene unit by reaction with isopentenyl pyrophosphate.  CYCLIZATION Rings form by intramolecular carbon- carbon bond formation. + OPP OPP E double Z double bond bond Limonene + –H + OH H2O a-Terpineol  BICYCLIC TERPENES + + + + a-Pinene -Pinene  ISOLATION OF MONO- AND SESQUITERPENES Both mono- and sesquiterpenes have a common source - essential oils. Their isolation is carried out in two steps: Isolation of essential oils from plant parts: The plants having essential oils generally have the highest concentration at some particular time. Therefore better yield of essential oil plant material have to be collected at this particular time. e.g. From jasmine at sunset. There are four general methods of extractions of oils. Expression method Steam distillation method Extraction by means of volatile solvents Adsorption in purified fats 55  ISOLATION OF MONO- AND SESQUITERPENES Steam distillation is most widely used method. In this method, macerated plant material is steam distilled to get essential oils into the distillate form and extracted by using pure organic volatile solvents (hexanes, pet ether). If compound decomposes during steam distillation, it may be extracted with petrol at 50 ˚C. After extraction solvent is removed under reduced pressure. 56  ISOLATION OF MONO- AND SESQUITERPENES Separation of Terpenes from Essential Oil: A number of terpenoids are present in essential oils obtained from the extraction. Definite physical and 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 terpenoids.  SEPARATION AND ISOLATION OF TERPENOIDS FROM VOLATILE OIL Chemical method: 1) Separation of terpene hydrocarbon: These are separated by using Tilden’s reagent [Nitrosyl chloride (NOCl) in chloroform.] The terpene hydrocarbons on treatment with Tilden reagent forms crystalline adduct having sharp m.p., which is separated from volatile oil followed by hydrolysis or decomposed to get back the terpenoid hydrocarbon. 2) Separation of terpene alcohol: Terpene alcohols on reaction with Pthallic anhydride forms di-ester, which precipitate out from volatile oil. These di-esters on treatment with NaHCO3 in presence KOH, yields back terpene alcohol and Pthallic acid 3) Separation of terpene aldehyde and ketone: Terpene aldehydes and ketones forms crystalline adduct on reaction with NaHSO3 and phenyl 58 hydrazines etc. These crystalline adducts can be hydrolyzed to get back carbonyl compounds.  SEPARATION AND ISOLATION OF TERPENOIDS FROM VOLATILE OIL Chemical method: 1) Separation of terpene hydrocarbon: These are separated by using Tilden’s reagent [Nitrosyl chloride (NOCl) in chloroform.] The terpene hydrocarbons on treatment with Tilden reagent forms crystalline adduct having sharp m.p., which is separated from volatile oil followed by hydrolysis or decomposed to get back the terpenoid hydrocarbon. 59  SEPARATION AND ISOLATION OF TERPENOIDS FROM VOLATILE OIL Chemical method: 2) Separation of terpene alcohol: Terpene alcohols on reaction with Pthallic anhydride forms di-ester, which precipitate out from volatile oil. These di-esters on treatment with NaHCO3 in presence KOH, yields back terpene alcohol and Pthallic acid 60  SEPARATION AND ISOLATION OF TERPENOIDS FROM VOLATILE OIL Chemical method: 2) Separation of terpene alcohol: 61  SEPARATION AND ISOLATION OF TERPENOIDS FROM VOLATILE OIL Chemical method: 3) Separation of terpene aldehyde and ketone: Terpene aldehydes and ketones forms crystalline adduct on reaction with NaHSO3 and phenyl hydrazines etc. These crystalline adducts can be hydrolyzed to get back carbonyl compounds. 62  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Molecular formula: Molecular formula is determined by usual quantitative analysis and MW determination methods and by means of mass spectrometry. If terpene is optically active, its specific rotation can be measured. 63  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Nature of oxygen atom present: If oxygen is present in terpenoids its functional nature is generally as alcohol, aledhyde, ketone or carboxylic groups. Presence of oxygen atom present: presence of –OH group can be determined by the formation of acetates with acetic anhydride and benzoates with 3,5-dinitirobenzoyl chloride. Acetate formation 3,5-DNB derivative formation  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Presence of >C=O group: Terpenes containing carbonyl function form crystalline addition products like oxime, phenyl hydrazone and bisulphite etc. CHO + NH2OH Citral (a) Geranial (trans) CH N OH Oxim  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Unsaturation: The presence of olefinic double bond is confirmed by means of bromine, and number of double bond determination by analysis of the bromide or by quantitative hydrogenation or by titration with monoperpthalic acid. Presence of double bond also confirmed by means of catalytic hydrogenation or addition of halogen acids. Number of moles of HX absorbed by one molecule is equal to number of double bonds present. 66  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Unsaturation: Addition of nitrosyl chloride (NOCl) (Tilden’s reagent) and epoxide formation with peracid also gives idea about double bonds present in terpenoid molecule.  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Dehydrogenation: On dehydrogenation with sulphur or selenium, terpenes are converted to aromatic compounds. On examination of these products, the skeleton structure and position of side chains in the original terpene can be determined. For example α-terpineol on Se-dehydrogenation yields para-cymene. 68 α-terpineol p-Cymene  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Oxidative degradation: Oxidative degradation has been a parallel tool for elucidating the structure of terpenes. Reagents for degradative oxidation are ozone, acid, neutral or alkaline potassium permanganate, chromic acid, sodium hypobromide, osmium tetroxide, nitric acid, lead tetra acetate and peroxy acids. Since oxidizing agents are selective, depending on a particular group to be oxidized, the oxidizing agent is chosen with the help of structure of degradation products. Myrcene  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Relation between general formula of compound and type of compounds: For example limonene (mol. formula. C10H16) absorbs 2 moles of hydrogen to give tetrahydrolimonene (mol. Formula C10H20) corresponding to the general formula. CnH2n. It means limonene has monocyclic structure. limonene 70  GENERAL METHODS OF STRUCTURE ELUCIDATION: REACTIONS Spectroscopic studies: UV Spectroscopy: In terpenes containing conjugated dienes or α,β-unsaturated ketones, UV spectroscopy is very useful tool IR Spectroscopy: IR spectroscopy is useful in detecting group such as hydroxyl group (~3400cm-1) or an oxo group (saturated 1750-1700cm-1) NMR Spectroscopy Mass Spectroscopy X-ray analysis  GENERAL PROPERTIES OF TERPENES 1. Most of the terpenes are colourless, 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 undergo addition reaction with hydrogen, halogen, acids, etc. A number of addition products have antiseptic properties. 3. They undergo polymerization and dehydrogenation 4. They are easily oxidized nearly by all the oxidizing agents. On thermal decomposition, most of the terpenes yields isoprene 72 as one of the product.  What are terpenes/terpenoids/isoprenoids used for?