INTRODUCTION

105Supplementary references

3.1 INTRODUCTION

3.2 Essential oils

3.3 Diterpenoids and gibberellins 3.4 Triterpenoids and steroids 3.5 Carotenoids

3.1 INTRODUCTION

107 110 124 129

138

An enormous range of plant substances are covered by the word 'terpenoid', a term which is used to indicate that all such substances have a common biosYnthetic origin. Thus, terpenoids are all based on the isoprene molecule CH2=C(CH3)-eH=CH2and their carbon skeletons are built up from the union of two or more of these C_sunits. They are then classified according to whether they contain two (CIO), three (CI5), four (C20), six (C3Q) or eight (C4Q) such units. They range from the essential oil components, the volatile mono-and sesquiterpenes (CIO and CIS)' through the less volatile diterpenes (C20) to the involatile triterpenoids and sterols (C3Q) and carotenoid pigments (C4Q)' Each of these various classes of terpenoid (Table3.1)are of significance in either plant growth, metabo- lism or ecology.

Although terpenoids are derived biogenetically from the molecule of isoprene, which does occur as a natural product, this substance is not the in vivoprecursor. Instead, the compound actually involved is isopentenyl pyrophosphate, CH2=C(CH3)CH2CH20PP, which is formed itself from acetate via mevalonic acid, CH20H-eH2C(OH,CH3)CH2C02H.

Isopentenyl pyrophosphate exists in living cells in equilibrium with the isomeric dimethylallyl pyrophosphate, (CH3)2C=CHCH20PP. In biosYn- thesis, a molecule of isopentenyl pyrophosphate and one of dimethylallyl pyrophosphate are linked together to give geranyl pyrophosphate (CIO).

the key intermediate in monoterpene formation; geranyl pyrophosphate

Condensation., Ring closure

•

108

Isopentenyl and

dimethylallyl pyrophosphates

The terpenoids

C CI

c...

^C

I I

c...c ^C

CI c... ...c

Geranyl pyrophosphate

C CI C... ...C

I I

C...C...C

I C...CC C,o limonene skeleton

C,s abscisic acid skeleton

Ring

~closure

Farnesylpyrophosphate

+Cs Geranyl-geranyJ -... pyrophosphate

I

^x2 C20 diterpenoids^{+c1osureIRing} C30 squalene and

triterpenoids C40 carotenoids

Fig. 3.1 The path of terpenoid biosynthesis in plants (for simplicity, only the carbon skeletons are shown).

and isopentenyl pyrophosphate are, in turn, linked to give farnesyl pyrophosphate (C_1S),the key intermediate of sesquiterpene synthesis. Dif- ferent combinations of these C_s, C_lO and C_1S units are then involved in the synthesis of the higher terpenoids, triterpenoids being formed from two farnesyl units and carotenoids from the condensation of two geranylgeranyl units (see Fig. 3.1). Most natural 'terpenoids' have cyclic structures with one or more functional groups (hydroxyl, carbonyl, etc.>

so that the final steps in synthesis involve cyclization and oxidation or other structural modification.

Chemically, terpenoids are generally lipid-soluble and are located in the cytoplasm of the plant cell. Essential oils sometimes occur in special glandular cells on the leaf surface, whilst carotenoids are especially asso- ciated with chloroplasts in the leaf and with chromoplasts in the petal.

Terpenoids are normally extracted from plant tissues with light petro- leum, ether or chloroform and can be separated by chromatography on silica gel or alumina using the same solvents. There is, however, often difficulty in detection on a microscale, since all (except carotenoids) are

Introduction

Table 3.1 The main classes of plant terpenoids

109

Number

of

isoprene Carbon

units number Name or class Main types and occurrence 1 Cs isoprene detected inHamamelis japonicaleaf 2 ClO monoterpenoids monoterpenes in plant essential oils

(e.g. menthol from mint)

monoterpene lactones (e.g. nepetalactone) tropolones (in gymnosperm woods) 3 CIS sesquiterpenoids sesquiterpenes in essential oils

sesquiterpene lactones (especially common in Compositae) abscisins (e.g. abscisic acid) 4 C20 diterpenoids diterpene acids in plant resins

gibberellins (e.g. gibberellic acid) 6 C30 triterpenoids sterols (e.g. sitosterol)

triterpenes (e.g. f.\-amyrin) saponins (e.g. yamogenin) cardiac glycosides

8 C40 tetraterpenoids carotenoids" (e.g. f.\-carotene) n Cn polyisoprene rubber, e.g. inHevea brasiliensis

"C50-based carotenoids are known in some bacteria.

Cis - trans isomerism

Forms of cycJohexane ring

Geraniol

'Chair'

Nerol

'Boat'

Fig. 3.2 Conformation of terpenoids.

110 The terpenoids

colourless and there is no sensitive universal chromogenic reagent for them. Reliance often has to be placed on relatively non-specific detection on TLC plates with conc.H2S0₄and heating.

Isomerism is common among terpenoids, and pairs of isomeric forms may be isolated from plants; one such pair are the monoterpenes geraniol and nerol (see Fig. 3.2). In addition, terpenoids are mostly alicyclic com- pounds and because the cydohexane ring is usually twisted in the so- called 'chair' form (see Fig. 3.2), different geometric conformations are possible, depending on the substitution around the ring. The stereochem- istry of the cyclic terpenoids is therefore highly involved and often dif- ficult to determine. From the practical point of view, it must be remembered that both isomerization and structural re-arrangement within the molecule may occur quite readily under relatively mild condi- tions and artifact formation is always possible during isolation procedures.

A considerable number of quite different functions have been ascribed to plant terpenoids. Their growth-regulating properties are very well documented; two of the major classes of growth regulators are the sesquiterpenoid abscisins and the diterpenoid-based gibberellins. The important contribution of carotenoids to plant colour is well known and it is now certain that these C₄₀ terpenoids are also involved as accessory pigments in photosynthesis. The importance of mono- and sesquiterpenes in providing plants with many of their distinctive smells and odours is also familiar to most scientists. Less is generally known of the role of terpenoids in the more subtle interactions between plants and animals, e.g. as agents of communication and defence among insects, but this is now an area of active research. Finally, it should be mentioned that certain non-volatile terpenoids have been implicated as sex hormones among the fungi.

Reviews of many aspects of plant terpenoids are included in Volume 4 of a comprehensive treatise on plant biochemistry (Stumpf, 1980). The ecological chemistry of plant terpenoids is reviewed in Harbome and Tomas-Barberan (1991).