6. Terpenes: C5 to C20

RA Macahig

FM Dayrit HO CH3

3 _ 1 CO2 5 OH 3-(R)-MVA Introduction • Terpenes make up very prominent and characteristic group of plant secondary metabolites. Terpene metabolites range from volatile compounds with 10 carbons to colored polyenes with 40 carbons. • The word “terpene” comes from turpentine, the yellow to brown thick oleoresin which is obtained as an exudate from the terebinth tree (Pistacia terebinthus). • Terpenes are historically, culturally and economically important: • oleoresins, such as pine and eucalyptus oils; rubber (gutta percha) • distillates of the resin yield solvents and thinners • “essential oils” and perfumes, which are extracted from flowers and leaves by pressing, alcohol extraction or steam distillation • drugs and steroids

6. Terpenes: C5 to C20 (Dayrit) 2 Introduction Some characteristic terpenes: 10 AcO O OH CH3 H C CH3 1 O Ph O 3 2 O CH 9 7 8 3 Ph NH O CH3 CH3 CH3 H OH H 4 O Camphor: monoterpene from HO AcO PhCO Cinamomum camphora. 2 Taxol: antitumor diterpene from 21 24 20 23 26 Pacific yew, Taxus species 18 25 CH3 27 19 11 13 17 CH 3 14 1 9 10 8 3 5 6 HO Cholesterol: steroid originally Carotene: C40 terpene which is synthesized in isolated from gallstones; component of the chloroplast; important plant pigment; believed all cell membranes to be one of the important natural anti-oxidants.

6. Terpenes: C5 to C20 (Dayrit) 3 Introduction

• In the late 19th century, Otto Wallach noted that upon chemical degradation, many of the products obtained had chemical formulas which were in multiples of 5 carbons. In the 1860s, these C5 units were called “isoprene” units. This is the basis of the “isoprene rule” which was formulated by Leopold Ruzicka. Isoprene represents the basic skeletal structure of the C5 unit.

2 n

isoprene n natural rubber limonene The most prolific producer of isoprene-type polymers is the rubber tree, Hevea brasiliensis.

6. Terpenes: C5 to C20 (Dayrit) 4 Overview of terpene biogenesis • Isopentenyl diphosphate (IPP) is the C5 precursor of all isoprenoids. In plants, IPP is formed via two distinct OPP biosynthetic pathways: HO CH • The mevalonic acid (MVA) pathway 3 operates in the cytoplasm and is responsible for the smaller terpenes _ CO2 and the phytosterols OH • The methyl erythritol phosphate HO CH (MEP) pathway is responsible for the 3 OH chloroplast isoprenoids (-carotene, lutein, prenyl chains of chlorophylls HO and plastoquinone-9). OP

6. Terpenes: C5 to C20 (Dayrit) 5 The Mevalonic Acid (MVA) pathway • 3R-Mevalonic acid (MVA) is biosynthesized from three acetates.

O + H HOCH3 HOCH3 O H3CSCoAOO 3 3 _ 1 H3C SCoA H3C SCoA CO25 1 5 OH O OO CoAS SCoA 3-(R)-MVA - H2C SCoA Note: 3S-MVA is an unnatural stereoisomer. There is no evidence that it is incorporated into terpenes. • MVA is converted to isopentenyl diphosphate (IPP) which is converted to its isomer, dimethylallyl diphosphate (DMAPP).

HOCH3 HOCH 3 POCH3 3 -CO 2 ATP3 ATP 3 2 _ IPP _ 4.1.1.33 1CO5 1 1 OPP 2 CO2 5 5 OH OPP _ O O OPP 5.3.3.2 3-(R)-MVA5-diphospho-(R)-MVA

DMAPP 4.1.1.336. Terpenes: C5: to C20Diphosphomevalonate (Dayrit) OPP 6 decarboxylase 5.3.3.2 : Isopentyl-diphosphate--isomerase The Mevalonic Acid (MVA) pathway

MVA pathway for isoprenoid biosynthesis with labeling pattern from [1-13C]glucose metabolized via glycolysis. (Rohmer, Pure Appl Chem 2003)

6. Terpenes: C5 to C20 (Dayrit) 7 The Methyl Erythritol Phosphate (MEP) pathway

OH HOCH • Glyceraldehyde-3- 2 Glyceraldehyde- O PO 3-phosphate) (C phosphate (GAP) and CHO 3 OH OH O phosphoenolpyruvate (PEP) HO OH Phosphoenol are formed from glucose. Glucose H3C CO2H pyruvate3) (C

• GAP condenses with PEP to form MEP. MEP is converted to IPP which forms its isomer DMAPP. OH

PO CHO HOCH3 Glyceraldehyde-3-phosphate3) (C OH

O IPP HO OPP OP H3C CO2H Methylerythritol phosphate Phosphoenol3) pyruvate 5) (MEP) (C (C DMAPP OPP 6. Terpenes: C5 to C20 (Dayrit) 8 The Methyl Erythritol Phosphate (MEP) pathway

MEP pathway for the biosynthesis of isoprenoids with labeling pattern from [1-13C]glucose metabolized via glycolysis. (Rohmer, Pure Appl Chem 2003)

6. Terpenes: C5 to C20 (Dayrit) 9 Evolution of the MVA and MEP pathways • The MVA pathway was originally thought to be the obligatory intermediate for all terpenes. (This is the pathway assumed in pre-2000 literature.) • The MEP pathway was first found in eubacteria and green algae, and was later shown to operate in the plant’s chloroplast. It is hypothesized that the MEP evolved first, and was incorporated into plants from cyanobacteria. • Some fungi and yeasts have been shown to use the MVA pathway. Because the plant cytosol uses the MVA pathway, it is believed that the higher evolved organisms (fungi and yeast) may be the source of the plant’s nuclear DNA. • The co-occurrence of two distinct major metabolic pathways in plant cells is unique for isoprenoid formation in plant cells.

6. Terpenes: C5 to C20 (Dayrit) 10 “Isoprenoid biosynthesis: The evolution of two ancient and distinct pathways across genomes” (Lange et al., PNAS, 97(24): 13172–13177, Nov 21, 2000) (p 1) • IPP is “the central intermediate in the biosynthesis of isoprenoids, the most ancient* and diverse class of natural products. Two distinct routes of IPP biosynthesis occur in nature: the MVA pathway and the recently discovered DXP** pathway.” “The evolutionary history of the enzymes involved in both routes and the phylogenetic distribution of their genes across genomes suggest that:  the MVA pathway is germane to archaebacteria,  that the DXP pathway is germane to  eubacteria, and that eukaryotes have inherited MVA  their genes for IPP biosynthesis from DXP (MEP) prokaryotes.” * In evolutionary terms, the fats are6. Terpenes:probably C5 the to older C20 (Dayrit) group! 11 ** DXP (deoxyxylulose 5-phosphate) pathway = MEP pathway “Isoprenoid biosynthesis: The evolution of two ancient and distinct pathways across genomes” (Lange et al., PNAS, 97(24): 13172–13177, Nov 21, 2000) (p 2)

“The occurrence of genes specific to the DXP pathway is restricted to plastid-bearing eukaryotes, indicating that these genes were acquired from the cyanobacterial ancestor of plastids. “However, the individual phylogenies of these genes, with only one exception, do not provide evidence for a specific affinity between the plant genes and their cyanobacterial homologues. The results suggest that:  lateral gene transfer between eubacteria  subsequent to the origin of plastids has  played a major role in the evolution of this MVA  DXP (MEP) pathway.”

6. Terpenes: C5 to C20 (Dayrit) 12 The MVA and MEP pathways: taxonomic distribution

Organism Pathways Bacteria MVA or MEP Archaea MVA Green Algae MEP Fungi MVA Plants MVA and MEP Animals MVA

6. Terpenes: C5 to C20 (Dayrit) 13 The MVA and MEP pathways: practical implications

• The mevalonate-independent methylerythritol phosphate (MEP) pathway is present in many bacteria and in the chloroplasts of all phototrophic organisms. It represents an alternative to the well-known MVA pathway, which is present in animals, fungi, plant cytoplasm, archaebacteria, and some eubacteria. • The MEP pathway in these bacteria represents a novel selective target for antibacterial and antiparasitic drugs. • The MEP pathway is also present in nonphototrophic eukaryotes, but belonging to phyla related to phototrophic unicellular eukaryotes, such as the parasite responsible for malaria, Plasmodium falciparum. This presents a potential target for a new class of antibacterial and antiparasitic drugs.

6. Terpenes: C5 to C20 (Dayrit) 14 The MVA and MEP pathways:DXS, 1-deoxy- dpractical-xylulose-5-phosphate implications synthase DXR, 1-deoxy-d-xylulose-5-phosphate reductoisomerase HMGR, 3-hydroxy-3-methylglutaryl coenzyme A HDS, hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase reductase IDS, isopentenyl diphosphate dimethylallyl diphosphate synthase IDI, isopentenyl diphosphate isomerase

IDI, isopentenyl diphosphate isomerase Roberts, Nature Chemical Biology, 2007. Biology, Chemical Nature Roberts,

Compartmentalized biosynthesis of IPP and DMAPP via the cytosolic MVA and the plastidic MEP pathways. 6. Terpenes: C5 to C20 (Dayrit) 15 OPP Hemiterpenes (rare)

DMAPP, C5

OPP The terpene family is formed by Monoterpenes condensation of C5 OPP (IPP) units: geranyl pyrophosphate, C10

• C10, monoterpenes OPP • C15, sesquiterpenes

• C20, diterpenes. OPP Sesquiterpenes farnesyl pyrophosphate, C15

OPP

OPP Diterpenes geranylgeranyl pyrophosphate, C20

6. Terpenes: C5 to C20 (Dayrit) 16 Terpene chains are produced by condensation of DMAPP with IPP in “head-to-tail” manner. DMAPP is the “starter unit” while IPP is the nucleophile which lengthens the terpene chain.

:Base H OPP OPP -OPP _ OPP IPP X DMAPP X Enz Enz _ -Enz-X

OPP OPP farnesylpyrophosphate geranylpyrophosphate

OPP OPP _ IPP X etc. Enz

6. Terpenes: C5 to C20 (Dayrit) 17 C30 terpenes are formed by head-to-head dimerization of C15 sesquiterpenes. This leads to the triterpenes, steroids, and carotenes.

OPP + PPO (farnesyl pyrophosphate, C15) x 2

squalene, C30

Triterpenes, C30 Steroids

OPP + PPO (geranylgeranyl pyrophosphate, C20) x 2

C40

18 Carotenes, C40 HOCH2 Plastids Cytosol O Overview of OH OH HO OH OP Terpene CoA-SCOCH OH 3 Glucose PO CHO CO2H Biosynthesis GAP PEP

in Plants H3C OH HO CH3 OH MVA HOC MEP 2 HO OPP OP

R OPP OPP OPP OPP DMAPP, C5 Prenyl IPP, C5 DMAPP, C5 IPP, C5 side-chain

Monoterpenes OPP Monoterpenes OPP geranyl pyrophosphate, C10 geranyl pyrophosphate, C10

IPP, C5 IPP, C5

Sesquiterpenes Sesquiterpenes OPP OPP farnesyl pyrophosphate, C15 farnesyl pyrophosphate, C15

IPP, C5 H Polyprenyl R side-chain Head-to-head n Diterpenes dimerization OPP

geranylgeranyl pyrophosphate, C20 OPP Diterpenes geranylgeranyl pyrophosphate, C20

Head-to-head dimerization Triterpenes & Steroids squalene,6. Terpenes: C30 C5 to C20 (Dayrit) 19

Carotenoids, C40 Estimates of number of structural groups and compounds known for each of the major types of terpenes. (Devon and Scott, 1972; Dictionary of Terpenoids, 1991)

Main terpene group Number of structural types Approx. number of known compounds (1991)

Monoterpene 8 850 Sesquiterpene 88 2,800 Diterpene 53 2,500 Triterpene 25 1,500 Phytosteroid 19 800

6. Terpenes: C5 to C20 (Dayrit) 20 Phylogenetics and natural products: Evolution of MVA biosynthesis in plants Acetate

Mevalonate Scutellaria

C10 Iridoids Indole alkaloids Plumeria spp. (Labiatae) (Apocynaceae) Jasmine

C15 Sesquiterpenes Sesquiterpene lactones (Myrtaceae) (Compositae)

C20 Diterpenes Diterpene acids Daisy (Euphorbiaceae) (Leguminosae)

C30 Steroidal alkaloids Tomato (Solanaceae) Lima bean

6. Terpenes: C5 to C20 (Dayrit) 21 Common transformations of the terpenes.

1. Sn2-type attack of carbon nucleophile on a carbon electrophile with loss of PP ): (-O -OPP OPP OPP _ OPP IPP X DMAPPX Enz Enz

:Base 2. E2-type elimination reaction with H formation of double bond: OPP X OPP Enz

CH3OPP 3. 1,3-AllylPP): shift ofH3C OXPP group (-O R1 3 1 R1 R2 3 1 R2

CH3 CH3 4. Epoxidation[O] of olefin: R 1 R2 R1 R2 O

6. Terpenes: C5 to C20 (Dayrit) 22 Common transformations of the terpenes.

CH 3 CH3 5. Electrophilic attackE+ on double bondR or epoxideR 1 2 R1 R2 to produce carbocation + + (E + is usually H ): E

CH3 CH3 H+ R1 R2 R1 + R2 O OH

CH3 6. FormationCH 3ofH cyclopropyl E+ CH3 groupR from olefin:+ 1 3 1 3 -H 1 R R1 2 2 + R1 2R2 1 3 R2 E 2 E

6. Terpenes: C5 to C20 (Dayrit) 23 Common transformations of the terpenes.

7. Cationic rearrangements:

CH3 a. 1,2-Shift of methyl or CH R + 3 hydrogen: 2 + 2 R R1 2 2 1 R1 1

CH3 H b. 1,3-Shift of hydrogen: HC H + 3 + R 3 1 1 R2 R 3 1 1 R2

+ c. Acid-catalyzed cationic + H cyclization:

R + R R

6. Terpenes: C5 to C20 (Dayrit) 24 Common transformations of the terpenes.

OH OPP 8. Alkylation of alcohol and alkylO groups with isopentenyl group (prenylation); analogousO-alkylation to methylation:

OH OH

C-alkylation

PPO

6. Terpenes: C5 to C20 (Dayrit) 25 Hemiterpenes, C5 • The true C5 terpenes are known as hemiterpenes. However, there is a large number of C5 compounds that are degradation products of larger compounds and these are not considered as true terpenes. • Prenylation is an alkylation process which adds C5 units to a substrate by attack of a carbon nucleophile or alcohol

on the C5 diphosphate (C5H9-OPP). The reaction takes place by nucleophilic displacement of diphosphate. If the substrate is a non-terpene compound, a mixed metabolite compound is produced.

6. Terpenes: C5 to C20 (Dayrit) 26 A . Tru e he m it e rp e n e s

C H O C O 2 H is o va le ra ld e h y d e is o vale ric a c id F o un d in o ra n ge s , le m o n , p e p pe rm int , F ou n d in h o p s a n d t o b a c c o . e u c a ly p tu s an d m a n y o t h e r vo la t ile oils . Various aspects It is a c o m p o n e n t o f fla vo rs a n d p e rfum e s . of C5 terpene H O C B . F a ls e he m it e rp e n e s C O 2 H 2

chemistry: C O H 2 s in e c io ic a c id t ig lic ac id a ng e lic a c id A. true C . E x a m p le s o f p ren y la t io n hemiterpenes, 1. [O ] H O O O 2. O P P O O B. false O H O O H O O hemiterpenes, O O and fu s c in H + H +

C. examples of H O O O

O P P H O O prenylation. O

O O C H 3

O C H 3 O O C H a n is o fla vo n o id 3

O C H 3

O O O

ro t e no n e O O C H 3 27

O C H 3 Monoterpenes, C10 Monoterpenes are characteristic plant natural products, in particular, in the flowers. These are important materials for perfumes and food flavors. The monotepenes are classified structurally into the following: acyclic, monocyclic, bicyclic and tricyclic. Biosynthetically, the monoterpenes can be grouped into the following: acyclic, cyclic, iridoid, and irregular.

Geranyl diphosphate (C10H17-OPP) is the starting point for the monoterpenes. OPP

6. Terpenes: C5 to C20 (Dayrit) 28 CHO CHO Acyclic OPP OH monoterpenes

GPP geraniol citral A (geranial) citronellal

OPP OH

linalyl pyrophosphate linalool

CHO OPP OH

neryl pyrophosphate nerol citral B (neral)

Other common open chain terpenes :

myrcene cis-ocimene 6. Terpenes: C5 to C20 (Dayrit) 29 Cyclic monoterpenes • In order to form the cyclic monoterpenes, it is postulated that GPP isomerizes to neryl or linalyl diphosphate. Cyclization produces a 1-methyl 4-isopropyl cyclohexane ring system. This is the basis for limonene-type structures. Ring formation can occur via three cyclization routes which produces bicyclic structures. • Ring formation produces optically active cyclic monoterpenes. Both (+) and (-) enantiomers have been found for many of the cyclic monoterpenes; these enantiomers can be found in different plants but sometimes both enantiomers can be found in the same plant.

6. Terpenes: C5 to C20 (Dayrit) 30 OPP

OPP Route to the OPP various mono- and bicyclic GPP linalyl pyrophosphate neryl pyrophosphate monoterpenes. -OPP -OPP

a b e + a c 1,2-H shift

+ bornane + d skeleton b

c + + -H

+ OH pinene skeleton

limonene borneol

[O] 4-carene

-pinene -pinene OH camphor -terpineol terpinolene

thujane Cyclization of monoterpenes can occur with opposite conformations giving rise to enantiomers.

D-Limonene: L-Limonene:

PPO PPO H displacementH displacement from above from below

H H PPO equatorial equatorial H H PPO

6. Terpenes: C5 to C20 (Dayrit) 32 Labeling studies for the conversion of MVA into cyclic monoterpenes.

o _ O2C OPP OPP OPP o o

o: full label o: half-label o OPP o

OPP o o OPP

o o

sabinene + 99% o o o

50% + o o each o o o o

thujone O camphor o 90-99% 80% O o 50% o o each 50% 33 each o o + +

Wagner- H H Meerwein rearrangement + of cationic intermediates + +

in cyclic + monoterpenes. +

+ + + +

-H+

HO HO

fenchol camphene Iridoids • Iridoids are rearranged monoterpenes which have a characteristic fused 5/6-membered ring structure. The 6-membered ring contains an acetal carbon where one oxygen forms part of the ring as a C-O- C bond, and the other oxygen is usually glycosylated. • About 600 iridoids are known. However, these are mostly glycosides; only about 100 non-glycosidic iridoids are known • Iridoids are generally plant terpenes. The name, however, comes from iridomyrmecin, a compound isolated from ants of the genus Iridomyrmex. Iridoid monoterpenes also are known from other - O-Glu O Glu insects, such as aphids. CH 3 H O • Loganin is a typical iridoid. O O HO Secologanin is a rearranged H H iridoid which retains only the CO CH CO CH 2 3 6-membered ring. Loganin 2 3 Secologanin

6. Terpenes: C5 to C20 (Dayrit) 35 _ HO H H CH3

o o CHO o [O] o _ NADPH O2C OH CHO OPP OH o OHC H o O o: full label OHC o nerol o: half-label H

OGlu OGlu OH CH CH3 CH3 3 CH3 H O [O], Glu, O [O] H O [CH 3 ] O o o o HO o O o o o o H

CO2H CO2CH3 o CHO o o CHO o loganin Iridoids are rearranged [O] monoterpenes. Loganin is a typical example of an HO OGlu CH2 OGlu iridoid. Opening of the

O cyclopentane ring yields H-O O o OHC secologanin secologanin. Secologanin is o o o incorporated into numerous

CO2CH3 indole alkaloids. o CO2CH3 o Iridoids

• Because the biosynthesis is relatively long and involves steps not commonly seen in other pathways, it would not be expected to have arisen often in the course of evolution. • The iridoids are produced by plants primarily as a defense against herbivores or against infection by microorganisms. Iridoids are often characterized by a deterrent bitter taste. • Iridoids are found in many medicinal plants and may be responsible for the some of their pharmaceutical activities. Isolated and purified, iridoids exhibit a wide range of bioactivity including cardiovascular, antiheptatoxic, chlorectic, hypoglycemic, anti-inflammatory, antispasmodic, antitumor, antiviral, immunomodulator and purgative activities. • Iridoids are incorporated into the large family of indole alkaloids. 6. Terpenes: C5 to C20 (Dayrit) 37 Irregular monoterpenes The irregular monoterpenes are a miscellaneous group that include the following types of compounds: 1. compounds that are formed from ring expansion; 2. compounds that are degraded so that the resulting compound has less than 10 carbons; and 3. compounds that are formed via a head-to-head condensation of DMAPP. A well-known group is the pyrethrins which have the characteristic cyclopropyl ring system.

6. Terpenes: C5 to C20 (Dayrit) 38 1. Irregular monoterpenes that are formed from ring expansion.

CO2H CO2H CO2H

[O]

thujic acid sonanic acid [O] H O OH H-O O HO O O 4-carene O O

OH HO O O

O HO

-thujaplicin -thujaplicin -thujaplicin 6. Terpenes: C5 to C20 (Dayrit) 39 2. Irregular monoterpenes that are degraded so that the resulting compound has less than 10 carbons.

OH O [O] [O]

geraniol -phellandrene cryptone

6. Terpenes: C5 to C20 (Dayrit) 40 3. Irregular monoterpenes that are formed via head-to-head condensation of DMAPP: chrysanthemic acid. The cyclopropyl group is characteristic of the pyrethroids, which is a well-known insecticide which has low mammalian toxicity.

OPP o o o o HO X-Enz o PPO o o HO o X-Enz o : full C-label [O] o : half C-label O o o

O N chrysanthemic acid

CO2H O o O

Tetramethrin (an active ingredient in Raid insect spray) 6. Terpenes: C5 to C20 (Dayrit) 41 Sesquiterpenes, C15 Sesquiterpenes (sesqui = “one and a half”) are derived from C15 farnesyl diphosphate (FPP). The sesquiterpenes comprise a very large group of over 1,000 individual compounds with over 100 skeletal types. The great variety of structures arises from the following types of transformation: 1. There are 3 double bonds in FPP; two of these double bonds can isomerize into cis  trans configurations. This gives 4 double-bond geometric isomers: all-trans; 2-cis,6-trans; 2- trans,6-cis; and 2-cis,6-cis isomers. The most important geometric isomers are the all-trans and 2-cis,6-trans isomers. Each double bond isomer gives rise to a different branch of the sesquiterpene family. 10 6 2 OPP

6. Terpenes: C5 to C20 (Dayrit) 42 Many sesquiterpenes, however, are derived from either the all-trans isomer, 1, or the 2-cis, 6-trans isomer, 2.

E E OPP 10 6 2 all-trans, 1

E Z

10 6 2 Z 2-cis, 6-trans, 2 OPP 10 6

Z Z 10 6 2 OPP

2-trans, 6-cis, 3 E 2-cis, 6-cis, 4

OPP

6. Terpenes: C5 to C20 (Dayrit) 43 2. When FPP cycles, the position of initial cyclization is the second major source of variation. There are several ways in which FPP can fold leading to a number of cyclization modes. Cyclization can involve either nucleophilic displacement of OPP by any of the -orbitals of the double bonds, or nucleophilic attack of the double bond (usually the terminal double bond) on a proton or other electrophile. Two main groups arise from cyclization of: • all-trans FPP • 2-cis, 6-trans FPP

OPP 2 10 6 2 10 6 OPP

6. Terpenes: C5 to C20 (Dayrit) 44 E E E Z OPP 10 6 2 10 6 2 all-trans, 1 2-cis, 6-trans, 2 OPP

Figure 7.18 Skeletal types a obtained from b OPP b all-trans FPP. a

humulane skeleton +

illudanes and protoilludanes

+ +

germacrane skeleton

eudesmanes, valeranes 45 guaianes, pseudoguaiananes 3. Farnesyl diphosphate is achiral. However, as in the case of geranyl diphosphate, it can fold up into two conformational forms which give rise to enantiomers; the conformational isomers are pro-chiral. Further modification of thse enantiomers leads to diastereomers which are now chemically distinct compounds.

OPP

-OPP- +

- -OPP +

OPP

6. Terpenes: C5 to C20 (Dayrit) 46 4. The remaining double bonds can react forming more C-C bonds leading to bicyclic systems. The stereochemistry of the second cyclization and the conformation of folding gives rise to further types of isomers. From the all-trans FPP arises the germacrane subgroup. A. Simple germacrane metabolites:

germacrane germacrone

_ _ B. Overview of three secondary cyclization modes, X nucleophile is usually OH . _ X Mode A: Mode B: Mode C:

+ + + _ _ X X i. chair conformation, base attack from -face:

H+

+ Metabolites + HO H _ OH + from all- OH trans FPP:

H germacrane H OH OH OH -eudesmol -eudesmol -eudesmol skeleton and (ovicidal compound from cyclization Vitex negundo) ii. chair conformation, base attack from -face:

mode A: + H H _ overview. OH + + HO H + HO

[O[

valeranone

OH H-O O

iii. boat conformation, base attack from -face:

H+ _ OH HO + + + HO H

occidentalol

H OH i. chair conformation, base attack from -face:

H+

+ + HO H _ OH + OH

H H OH OH OH -eudesmol -eudesmol -eudesmol (ovicidal compound from Vitex negundo)

Metabolitesii. chair fromconform aalltion-,trans base attFPP:ack from germacrane-face: skeleton and cyclization mode A: + i. chairH conformation, base attack fromH -face. _ ii.O chairH conformation, base attack from -face. + iii. boat conformation, base+ attack from -face. HO H + HO

[O[

valeranone

OH H-O O

iii. boat conformation, base attack from -face:

H+ _ OH HO + + + HO H

occidentalol

H OH i. chair conformation, base attack from -face:

H+

+ + HO H _ OH + OH

H H OH OH OH -eudesmol -eudesmol -eudesmol (ovicidal compound from Vitex negundo)

ii. chair conformation, base attack from -face:

+ H H _ OH + + HO H + HO

[O[

valeranone

OH H-O O

Metabolitesiii. bo afromt confo rallmat-iotransn, base aFPP:ttack fr o mgermacrane -face: skeleton and cyclization mode A: i. chair conformation,H+ base attack from -face. ii. chair_ conformation, base attack from -face. OH HO iii. boat conformation, +base attack from -face. + + HO H

occidentalol

H OH i. chair conformation, base attack from -face:

H+

+ + HO H _ OH + OH

H H OH OH OH -eudesmol -eudesmol -eudesmol (ovicidal compound from Vitex negundo)

ii. chair conformation, base attack from -face:

+ H H _ OH + + HO H + HO

[O[

valeranone

OH H-O O

iii. boat conformation, base attack from -face:

H+ _ OH HO + + + HO H

occidentalol

H OH

Metabolites from all-trans FPP: germacrane skeleton and cyclization mode A: i. chair conformation, base attack from -face. ii. chair conformation, base attack from -face. iii. boat conformation, base attack from -face. Metabolites from all-trans FPP: germacrane skeleton and cyclization mode B.

_ OH OH H OH + + H H + H H +

H H O OH OH bulnesol patchoulenone

6. Terpenes: C5 to C20 (Dayrit) 52 E E E Z OPP 10 6 2 10 6 2 all-trans, 1 2-cis, 6-trans, 2 OPP

Figure 7.17

b a Skeletal types c d OPP obtained from d a

2-cis, 6-trans c b FPP (1) + +

bisabolene skeleton caryophyllenes, + longifolanes, longibornanes carotane skeleton begamotanes, santalanes, cedranes, cadinanes, copaenes, chamigranes, cupranes ylanganes, sativanes Skeletal types obtained from 2-cis, 6-trans FPP (2)

Part of a biogenetic map for sesquiterpenes showing all structural types found in the oils from Toona ciliata, Cedrela odorata, and C. fissilis. (modified from: http://www.scielo.br/img/fbpe/jbchs/v11n6/3595f3.gif) Sesquiterpenes, C15

5. Various skeletal rearrangements may occur through a number of mechanisms, such as 1,2- and 1,3-H shifts, 1,2-methyl shifts, double-bond migration (this generally occurs via H migration), and Wagner-Meerwein type rearrangement. 6. Other modifications such as oxidation, reduction, etc. The sesquiterpenes serve as plant defense compounds (e.g., polygodial, -eudesmol), sensory attractants or fragrance odors (e.g., ylanganes, patchoulenone). The structures displayed by these compounds attest to the rich chemistry that is found in plants.

6. Terpenes: C5 to C20 (Dayrit) 55 The major skeletal types germacrane skeleton O which contain the O elemanolide -methylene-

O -lactone O functionality psuedoguaianolide belong to the germacrane O O O O group. eudesmanolide germacrenolide

guaianolide O

O O eremophilanolide

O 6. Terpenes: C5 to C20 (Dayrit) 56 xanthanolide O Sesquiterpene -methylene--lactones • The -methylene--lactone group is derived by oxidation and cyclization of the isopropyl side chain of the cyclized sesquiterpene.

OH [O] [O] O  O 

Germacrane skeleton -methylene--lactone

• The -methylene--lactone functionality is characteristic of sesquiterpenes, and is found most widely in the Compositae family.

6. Terpenes: C5 to C20 (Dayrit) 57 • This distinctive structural feature is accompanied by the observation that many of these compounds show antimutagenic properties which are attributed to the electrophilic exocyclic methylene group which can react rapidly with nucleophilic moieties, such as the nucleophilic 8-carbon of adenine and guanine in DNA.

Reaction scheme for attack by nucleophile, OX: O NH2

N O O N

O O N O P O 5' N O O- 3' X X- O

6. Terpenes: C5 to C20 (Dayrit) 58 Examples of sesquiterpene -methylene--lactones.

OH O O H O OH O O O HO O

O HO O OH O Tagitenin A Hyporadiolide O Miloanokrypten (germacrenolide) (guaianolide) (guaianolide)

O R1 R2 H OR2 CH3 O 1 H O C CH3 O OR1 O O CH3 O OH O 2 H O Helenalin C CH Blumealactones 3 from Blumea balsamifera (sambong) O (Fujimoto, et al., Phytochem., 27, 1109 (1988).) 3 Ac H

6. Terpenes: C5 to C20 (Dayrit) 59 Terminal cyclohexyl ring formation: Abscisic acid • A terminal cyclohexyl group is readily formed from 2-trans, 6- trans farnesyl diphosphate by electrophilic attack. This relatively simple cyclization mode is surprisingly not common among the sesquiterpenes. The best representative of this group is abscisic acid. • Abscisic acid (“abscission”  shedding of leaves, fruits or flowers) is a very important plant growth regulator. In particular, abscisic acid (commonly called ABA), as its name suggests, is the plant hormone responsible for dormancy of leaves and the abscission of leaves, flowers and fruit (the natural process of removal, cutting, or falling off). Thus, ABA plays an important role in normal plant development. In tissue cultures, ABA has also been shown to inhibit plant cell elongation.

6. Terpenes: C5 to C20 (Dayrit) 60 Terminal cyclohexyl ring formation leads to the biosynthesis of the important plant growth regulator, abscisic acid.

E OPP OPP H+ E +

OH CO H O 2 Abscisic acid

6. Terpenes: C5 to C20 (Dayrit) 61 Sesquiterpenes from cascading “linear” cyclization: Polygodial

• 2-Trans, 6-trans farnesyl diphosphate can cyclize in a cascading “linear” conformation to form a trans-decalin structure. This process is initiated by attack of an electrophile at the terminal double bond, formation of a cyclohexyl ring followed by a second cyclization to form the fused trans- decalin structure. • This mode of cyclization is commonly observed in the longer diterpenes and triterpenes, but is unusual in sesquiterpenes.

6. Terpenes: C5 to C20 (Dayrit) 62 Appropriate folding of all-trans farnesyl diphosphate and electrophilic attack at the terminal double bond with sequential cyclization leads to a trans-decalin structure. The -OPP- group is not displaced during the initial cyclization. This mode of cyclization is very common for diterpenes and triterpenes, and unusual in sesquiterpenes. Polygodial is an antipest compound produced by plants. OPP OPP CHO 2 + CHO E E 6 10 H+ H

Polygodial OPP OPP

E 7 + 2 E H+ 11 H 6. Terpenes: C5 to C20 (Dayrit) 63 Sesquiterpenes from 2-cis, 6-cis-farnesyl diphosphate: Gossypol

A few biologically important sesquiterpenes are formed from 2- cis, 6-cis-farnesyl diphosphate. The best known metabolite from this group is gossypol. Gossypol is an unusual sesquiterpene since it is has a naphthalene structure and is dimerized. Although gossypol can be easily mistaken for a polyketide, the isopropyl group hints at its terpenoid origins. The final proof of biogenetic origin comes from labeling studies which are consistent with its being a sesquiterpene metabolite.

CHO OH OH CHO

HO OH

Gossypol

6. Terpenes: C5 to C20 (Dayrit) 64 Gossypol is formed from 2-cis, 6-cis-farnesyl diphosphate. It is a dimeric naphthalene. Gossypol has attracted interest because it is an insecticidal defense compound found in the seeds of the cotton plant.

OPP CHO OH Z HO . . 6 2 Z . HO . . o o

. = 14C label [O] o = partially labeled

CHO OH OH CHO HO . . OH

HO . . OH

o o o o Gossypol 6. Terpenes: C5 to C20 (Dayrit) 65 Propionic acid in starter unit: Insect juvenile hormones

Insect development is characterized by discrete stages in its life cycle in going from larva to adult. Two types of hormones – juvenile hormones (JH) and moulting hormones (MH) -- initiate these changes. Juvenile hormones are required at the initial metamorphosis from the 1st to 2nd stage larva, while molting hormones are required at all stages of development: MH/JH MH MH 1st stage Larva  2nd stage Larva  Pupa  Adult Many juvenile hormones use propionyl CoA as the starting unit to form homo-mevalonic acid. Juvenile hormones are linear sesquiterpenes with various oxidized groups (epoxides, alcohols). Moulting hormones, on the other hand, are steroidal compounds.

6. Terpenes: C5 to C20 (Dayrit) 66 Juvenile hormones are linear sesquiterpenes which use a propionyl CoA as the starting unit to make homo mevalonic acid. Starter unit: propionyl CoA: O HO . . O O SCoA . O -CO2 SCoA - O2C - O O OH OPP SCoA - O2C - SCoA homo-mevalonic acid

. . . . 3 X OPP OPP

farnesyl pyrophosphate analogue

CO2CH3 O Neotenin 6. Terpenes:Neotenin is C5a juvenile to C20 hormone (Dayrit) isolated from thousands 67 of male butterflies of the species Hyalophora cecropia L. Diterpenes, C20 Addition of another isopentenyl diphosphate (C5) group to farnesyl diphosphate (C15) forms geranylgeranyl diphosphate (GGPP, C20). GGPP is the starting point for the biogenesis of the diterpenes. As in the case of the sesquiterpenes, there is a great variety of structures that are formed due to five important structural features of diterpenes : 1. GGPP has four double bonds. Three of the double bonds can take the cis- or trans- configuration. This gives seven possible double-bond geometric isomers. • The largest group of diterpenes is formed from the all-trans GGPP. • A number of important diterpenes arise from the 2-trans,6- cis,10-trans isomer.

6. Terpenes: C5 to C20 (Dayrit) 68 The majority of the diterpenes are cyclic compounds. However, some unusual open-chain di-, tri- and tetraene diterpenes (along with sesquiterpenes) have been isolated from the skin glands of alligators and crocodiles. These terpenes are believed to act as pheromones. (The most commonly-occurring open-chain terpene on the skin of many animals, including humans, is squalene.) (Schultz, Krückert and Weldon, J. Nat Prod., 2003, 66, 34-38)

6. Terpenes: C5 to C20 (Dayrit) 69 Diterpenes, C20 2. Initial cyclization of GGPP can occur at different sites. As in the case of FPP, the position of initial cyclization contributes to structural variety. This includes the important consideration of whether the cyclization involves the displacement of the -OPP group or not. • Cyclization in the all-trans mode does not displace -OPP, while the other modes cause displacement of the -OPP group. 3. Further cyclization steps may occur using the remaining double bonds to form additional smaller rings. The stereochemistry of these cyclization steps depends on the double-bond configurations and the conformation of the ring.

6. Terpenes: C5 to C20 (Dayrit) 70 GGPP cyclizes in + a variety of ways. Cembrane Theoretically, Taxane - 2-trans, - Fusicoccin -OPP - -OPP there are seven 6-cis, -OPP all- trans 10-trans double-bond

14 10 6 2 geometric OPP isomers possible Geranylgeranyl pyrophosphate, GGPP all- trans - 2-cis,6-trans, -OPP from -OPP- geranylgeranyl 10-cis diphosphate. The OPP most important + diterpenes are Artemisene formed from the Labdane all-trans GGPP and 2-trans,6- cis,10-trans isomer.

6. Terpenes: C5 to C20 (Dayrit) 71 Pimarane Kaurane Gibberelane Diterpenes 4. Skeletal modifications can occur via migration of hydrogen and/or methyl, Wagner-Meerwein-type rearrangements, and others. 5. Other modifications may occur, such as oxidation, reduction of double bond (+2[H]), etc.

There are a number of well-known compounds which belong to the diterpene group, in particular, the cembranes, pimaranes and the gibberelanes. The cembrane structure is a recurring theme among marine natural products. The pimaranes are major constituents of pine oleoresin, which is heavily used in paper sizing and other coating applications. The gibberellins are important plant growth hormones. Taxol, the anticancer drug, is also a diterpene. 6. Terpenes: C5 to C20 (Dayrit) 72 Diterpenes from all-trans GGPP

• The largest group of diterpenes is produced from all-trans GGPP which lead to the labdanes. The biosynthesis starts from two conformations of chair-like folding of all-trans GGPP: one conformation leads to the 10- series while the alternative conformation leads to the 10- series. This is an example of a very important phenomenon in generation of diversity in natural products where the same starting compound produces stereoisomeric products—in this case, diastereomers. • Electrophilic attack at the terminal double bond accompanied by two rapid ring-forming steps produces the two trans-decalin stereoisomers with a stable tertiary cation intermediate. It should be noted that initial cyclization retains the -OPP group which is used in subsequent transformations.

6. Terpenes: C5 to C20 (Dayrit) 73 The largest group of diterpenes is produced from all-trans GGPP which leads to the labdanes. The biosynthesis starts from two conformations of chair-like folding of all-trans GGPP leading to the 10- /10- series. -Series

14 3 H1 H 6 X+ OPP X 10 9 OPP 10 1 5 4 8 +

OPP 11 1 9 10 8 + 10-Me  3 5 4 X H

-Series

OPP 1 OPP 10 8 1 9 6 X + X+ 3 4 5 14 10 H OPP 11 1 9 10-Me  10 8 + 5 6. Terpenes: C5 to C20 (Dayrit)3 4 74 X H Diterpenes from all-trans GGPP

The initially formed decalin cation subsequently undergoes various transformations leading to sub-groups, as illustrated by the following reactions. Examples from both the 10- and 10- series are known in nature, but both are rarely found in the same plant.

1. Loss of H+: Loss of H+ limits modifications of the ring to the decalin system. This produces an exocyclic methylene group at the 8-position of the decalin. Secondary modifications may take place at the side chain. This group is exemplified by the labdanes.

6. Terpenes: C5 to C20 (Dayrit) 75 OPP

+ Loss of H from + the intermediate decalin produces an exocyclic 10-series 10-series methylene group a the 8-position. Secondary 10 8

modifications H H C may take place at 3 CH2OH Ozol the side chain. OH This leads to the 13 OH 13 labdanes. 10 8 10 8

H H H3C CH2OH 18

Ent-18-hydroxy-13-epi-manool (+)-Manool

13 OH OH 20 10 8 15 10 8 20 15

H H H3C CH2OH 18 6. Terpenes: C5 to C20 (Dayrit) 76 Enantio-labda-8(20)-en-15,18-diol Labda-8(20)-13-dien-15-ol Diterpenes from all-trans GGPP

2. Attack of nucleophile (e.g., OH-) at intermediate cation on 8- position: Attack of hydroxide (OH-) at the cationic carbon at the 8-position is controlled by steric considerations. Because the 10- methyl blocks approach from the bottom side, the hydroxide has the 8- configuration. Similarly, the 10- methyl series gives rise to the 8- hydroxide configuration.

6. Terpenes: C5 to C20 (Dayrit) 77 Attack of hydroxide (OH-) at the cationic carbon at the 8-position is controlled by steric considerations. The 10- methyl blocks nucleophilic attack from the bottom, while the 10- methyl blocks attack from the top. OH 10-Series H H OH- 10 OH 8 R 10 + 8 CH3 steric H hindrance OH- 10-Methyl- -labdane

10-Series OH OH- steric hindrance CH 3 R 10 10 8 OH 8 + H H OH- 10-Methyl- -labdane 6. Terpenes: C5 to C20 (Dayrit) 78 Diterpenes from all-trans GGPP

3. Tricyclic diterpenes: The 8-methylene exocyclic group participates in a nucleophilic attack on the -OPP group on the side chain and forms the pimaryl tricyclic system. Two stereoisomeric groups of pimaranes are formed from the 10- and 10- methyl series.

6. Terpenes: C5 to C20 (Dayrit) 79 The 8-methylene 10-Series

exocyclic group H H H H participates in a OPP OPP nucleophilic attack + 1. -H+ on the -OPP group 2. OPP: 1,3-migration

on the side chain -OPP- and form the pimaryl tricyclic H H system. This figure shows the + pimarane 10- and 10- series.

H H OH HO H H CO H CO2H 2 7-Hydroxypimara-8(14),15-diene Oblongifoliol 19-oic acid

6. Terpenes: C5 to C20 (Dayrit) 80 10-Series

OPP The 8-methylene OPP

exocyclic group + 1. -H+ 2. OPP: 1,3-migration participates in a H H nucleophilic attack on the -OPP group on the side chain and form the +

pimaryl tricyclic H system. Isopimara-7,15-diene H+

H+

H CO H 2 CO2H Pimaric acid

1,2-methyl shift

6. Terpenes: C5 to C20 (Dayrit) 81 CO2H Abietic acid Diterpenes from all-trans GGPP

4. Tetracyclic diterpenes: Further cyclization of the tricyclic cationic intermediate leads to a tetracyclic cationic intermediate known as the kaurenes. Two stereoisomeric groups are formed. Many of the kaurenes display remarkable complexity of structure.

6. Terpenes: C5 to C20 (Dayrit) 82 10-Series Further from cyclization of pimarane H H H H 10 12 the tricyclic H + 17 + 14 13 cationic H +15 intermediate path A leads to a path B tetracyclic

cationic H H H H H 12 H 12 intermediate 17 H 17 14 13 14 13 known as the 15 + 15 kaurenes. This + 1. 1,2-bond shift: (C13>C15) 1. 1,3-H shift: (C12>C15) figure features 2. -H+ (C17) 2. 1,2-bond shift: (C13>C12) 3. -H+ (C17) the kaurane 10- H H  series. H H

H H

H H (-) Kaurene (Ent-kaurene) Atisirene 10-Series Further H cyclization of from pimarane the tricyclic 10 + H + + cationic H H H intermediate -H+ leads to a 1,2-bond shift tetracyclic cationic 1,2-H shift intermediate H

known as the H H (+) Kaurene kaurenes. This ((+) Phyllocladene) figure features the kaurane 10- H series. H + + H H +

H H H -H+ 1,2-bond shift

H 6. Terpenes: C5 to C20 (Dayrit) 84 H H Diterpenes from all-trans GGPP

5. Kaurene B-ring contraction: the gibberellins: From the 10– kaurene series comes the important group of plant hormones, the gibberellins. The gibberellins are synthesized in the protoplasm of plants and increase the rate and amount of growth. There are over 66 compounds which have been isolated which belong to this structural group. Interestingly, gibberellic acid was first isolated in a fungus, Gibberella.

CO2HCHO

6. Terpenes: C5 to C20 (Dayrit) 85 H H H O Contraction of the H O kaurene B-ring H 7 HO2C H 6 X H CO H X leads to the (-) Kaurene 2 1. -X- gibberellins. The 2. 1,2-C migration (7>6) gibberellins are 3. -H+ plant growth H H H

OHC hormones which H HO2C increase the rate CO2H CHO C19 gibberellic acids C20 gibberellic acids [O] [O] and amount of CHO H H growth.

HO H H

CO2H CO2H CO2H CO2H 1. -CH2O GAR 2. [O] 1. [O] H 2. -CH2O 3. [O] O O H OH HO H O CO2H O GA1 H CO H -2H 2 GA15 H

O O OH HO H C6.O2 Terpenes:H C5 to C20 (Dayrit) 86 GA3 (Gibberellic Acid) Diterpenes from 2-trans,6-cis,10-trans GGPP: Cembranes

Isomerization of the 6-double bond of all-trans GGPP to the cis configuration allows nucleophilic attack of the terminal double bond on the 1-position accompanied by displacement of -OPP and formation of a 14-membered cembrane ring.

6. Terpenes: C5 to C20 (Dayrit) 87 Diterpenes from 2-trans,6-cis,10-trans GGPP: Cembranes

The name cembrane is taken from the simplest member of this group, cembrene, which was isolated form the oleoresin of Pinus sibirica. The most interesting members of the cembrane group are found in marine soft corals. The 14-carbon ring marine cembranes are in many ways structurally analogous to the 10-carbon ring sesquiterpenes. While the sesquiterpene germacranes form - methylene--lactones, the diterpene cembranes form - methylene--lactones. Like the sesquiterpene lactones, the cembrane lactones also exhibit powerful cytotoxic and antitumor properties.

6. Terpenes: C5 to C20 (Dayrit) 88 Isomerization of the 6-double bond of all-trans GGPP to the cis configuration allows nucleophilic attack of the terminal double bond on the 1-position accompanied by displacement of –OPP- and formation of the cembrane ring.

E Z 2 10 Z E 6 PPO Cembrene Cembrene itself was isolated from the oleoresin of Pinus sibirica .

-Methylene- -lactones in marine natural products: O OH O O H

H HO O OAc O Crassin acetate Sinulariolide from Psuedoplaxaura porosa from Sinularia flexibilis 6. Terpenes: C5 to C20 (Dayrit) 89 Diterpenes from all-trans GGPP with loss of OPP: Taxanes

The biosynthetic pathway to the tricyclic taxoid skeleton involves an alternative cyclization attack of C10 on C15, accompanied by attack of C14 on C1 with displacement of -OPP. This forms a 6- membered A ring, with an 8-membered B-ring.

1 12 11

4 5 H H

6. Terpenes: C5 to C20 (Dayrit) 90 The taxoid skeleton is formed from all-trans GGPP. The initial step involves attack of C14 on C1 with displacement of -OPP and attack of C10 on the cationic center which forms on C15. H 6 H 10 E + - E -OPP + 15 1 14 OPP H H

-H+ + H H H H

6. Terpenes: C5 to C20 (Dayrit) 91 Taxol is a well-known diterpenoid from the Pacific yew tree, Taxus brevefolia. Over 350 compounds belonging to the taxoid group have been isolated. In a short span of seven years from 1992-1999, over 250 taxoids were isolated and characterized. (Baloglu and Kingston, J. Nat. Prod. 1999, 62, 1448-1472.)

O AcO O 19 OH CH Ph NH O H3C 3 7 17 Ph O H3C CH3 4 H OH 14 2 H O HO AcO Taxol PhCO2 (Paclitaxel)

6. Terpenes: C5 to C20 (Dayrit) 92 Potential labeling patterns of monoterpenes and the sesquiterpene nerolidol by 2 2 [ H2]-DOX (A) and [ H2]- MVL (B).

93 HO

trans-Nerolidol

Emission and in vivo labeling kinetics of nerolidol during feeding of 2 snapdragon flowers with [ H2]-MVL. Filled circles represent emission from 2 [ H2]-MVL-fed flowers, and open circles represent control flowers fed with sucrose solution. Open triangles represent percent of unlabeled compounds, and filled squares and triangles represent percent of nerolidol labeled by +4 amu; open squares represent percent of nerolidol labeled by +6 amu. In the case of snapdragon petals, only one of the two pathways is operating in the formation of volatiles isoprenoids. The MEP pathway, localized in the plastids, provides IPP and DMAPP precursors for both monoterpene biosynthesis (in plastids) and sesquiterpene biosynthesis (in the cytosol) and determines their rhythmic emission. 6. Terpenes: C5 to C20 (Dayrit) 94 Summary The following biosynthetic patterns among terpenes can be observed: 1. The terpenes are readily recognizable from the characteristic presence of 1,5-methyl substituents and the carbon number which is a multiple of 5. 2. The key intermediates, geranyl diphosphate, farnesyl diphosphate and geranylgeranyl diphosphate, have a -OPP group at the C1 position and 1,5-diene groups. 3. Monoterpenes: There are three main types: linear, cyclic and the iridoids. 4. Sesquiterpenes: This a very diverse group of over 100 skeletal types. The initial source of diversity is the variation of the cyclization step.

6. Terpenes: C5 to C20 (Dayrit) 95 Summary 5. Diterpenes: Diversity starts from the initial cyclization step from the four double bonds in geranylgeranyl diphosphate. The most prominent subgroups are the labdanes which arise from cyclization of the all-trans GGPP, the cembranes which come from cyclization of 2-trans,6-cis,10-trans GGPP, and the taxanes which arise form cyclization of all-trans GGPP with loss of -OPP. Terpene chemistry is an exquisite example of diversity, stereochemistry and biochemical control in natural products. Despite the diversity of structure, however, the majority of chemical reactions are actually limited to a few types: Sn2 displacement of -OPP by an olefinic group; E2 elimination to yield an olefin; 1,3-allyl shift of -OPP; epoxidation; and protonation of olefins followed by cationic rearrangement. 6. Terpenes: C5 to C20 (Dayrit) 96 Summary 6. MVA and MEP Pathways: The co-occurrence of two completely distinct pathways for isoprenoid formation in plant cells is remarkable because a similar situation does not hold for any other major metabolic route. The plastidial pathway probably arose from genes contained in a cyanobacterium-like symbiont that served as the progenitor of modern chloroplasts. However, this scenario still does not explain the persistence of both pathways in contemporary plants. The answer may lie in the enormous variety of isoprenoids formed by plants, which could require two separate pathways composed of completely different enzymes and different intermediates to facilitate separate regulation. Further study of when and where the two pathways are active in plants should shed further light on questions regarding their evolutionary origin and maintenance. (Dudareva et al. PNAS 2005)

6. Terpenes: C5 to C20 (Dayrit) 97