INTRODUCTION

Terpenoids (or isoprenoids), a subclass of the prenyllipids (terpenes, prenylquinones, and sterols), represent the oldest group of small molecular products synthesized by plants and are probably the most widespread group of natural products.
Terpenoids can be described as modified terpenes, where methyl groups are moved or removed, or oxygen atoms added. Inversely, some authors use the term "terpenes" more broadly, to include the terpenoids.

During the 19th century, chemical works on turpentine led to name "terpene" the hydrocarbons with the general formula C₁₀H₁₆ found in that complex plant product. These terpenes are frequently found in plant essential oils which contain the "Quinta essentia", the plant fragrance.

They are universally present in small amounts in living organisms, where they play numerous vital roles in plant physiology as well as important functions in all cellular membranes. On the other hand, they are also accumulated in many cases, and it is shown that the extraordinary variety they then display can be due to ecological factors playing an evolutionary role (Ourisson G, Pure Appl Chem 1990, 62, 1401).
They may be defined as a group of molecules whose structure is based on a various but definite number of isoprene units (methylbuta-1,3-diene, named hemiterpene, with 5 carbon atoms). 

Terpenoids are extraordinarily diverse but they all originate through the condensation of the universal phosphorylated derivative of hemiterpene, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) giving geranyl pyrophosphate (GPP).

In higher plants, IPP is derived from the classic mevalonic acid pathway in the cytosol but from the methylerythritol phosphate pathway in plastids. It is generally accepted that the cytosolic pool of IPP serves as a precursor of sesquiterpenes and triterpenes, whereas the plastid pool of IPP provides the precursors of mono-, di- and tetraterpenes (Bohlmann J et al., Proc Natl Acad Sci USA 1998, 95, 4126). Some exceptions have been described showing that interactions between the two biosynthetic pathways may exist (Dudareva N et al., Proc Natl Acad Sci USA 2005, 102, 933). 
A metabolic map giving the products of isoprene metabolism  and their pathways may be found at the Prof Nicholson web site

A rational classification of the terpenes has been established based upon the number of isoprene (or isopentane) units incorporated in the basic molecular skeleton:

Mono-, sesqui-, di-, and sesterpenes contain the isoprene units linked in a head to tail fashion. The triterpenes and carotenoids (tetraterpenes) contain two C15 and C20 units respectively linked head to head. More than 1000 monoterpenes, 7000 sesquiterpenes and more than 3000 diterpenes have been described.

Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes or ketones are also found. These derivatives are frequently named terpenoids.

Mono- and sesquiterpenes are the chief constituents of the essential oils while the other terpenes are constituents of balsams, resins, waxes, and rubber.
Oleoresin is a roughly equal mixture of turpentine (85% C10-monoterpenes and 15% C15- sesquiterpenes) and rosin (C20-diterpene) that acts in many conifer species to seal wounds and is toxic to both invading insects and their pathogenic fungi (Steele CL et al., Plant Physiol 1998, 116, 1497). A number of inducible terpenoid defensive compounds (phytoalexins) from angiosperm species are well known (Stoessl et al., Phytochemistry 1976, 15, 855). These include both sesquiterpenoid and diterpenoid types.

Isoprenoid units are also found within the framework of other natural molecules. Thus, indole alkaloids, several quinones (vitamin K), alcohols (vitamin E, vitamin A formed from b-carotene), phenols, isoprenoid alcohols (also known as terpenols or polyprenols) also contain terpenoid fragments. The origin of the ubiquitous isoprene unit and its conversion into various compound has been extensively studied.The biogenesis, molecular regulation and function of plant terpenoids has been extensively reviewed by Bouvier F et al. (Prog Lipid Res 2005, 44, 357).
According to Bohlmann J et al. (PNAS 1998, 95, 4126), there are in excess of 1000 monoterpenes, more than 7000 sesquiterpenes and more than 3000 diterpenes.

MONOTERPENES
SESQUITERPENES
DITERPENES
SESTERPENES
TRITERPENOIDS
CAROTENOIDS
RUBBER

HISTORY

Terpenes history spans various civilizations. As they are largely found in essential oils, they were used in the Ancient Egypt for various religions aims. Camphor was introduced in Europe from the East by the Arabs around the 11th century.
The process of obtaining plant essential oils by fatty extraction was known by the early Middle Ages. In the 12th century, Arnaud de Villanosa described distillation of oils from rosemary and sage. He made an "oleum mirabile" from oils of turpentine and rosemary. It is noticeable that some 60 oils were described in the Nuremberg edition of "Dispensatorium valerii cordi" written in 1592.
Analyses of oils of turpentine were made in 1818 by JJ Houston de la Billardière. Dumas proposed in 1866 the name "terpene", derived from turpentine, instead of camphor for crystalline oxygenated substances extracted from essential oils. In 1887, Wallach O proposed that one isoprenic unit of 5 carbon atoms (C₅H₈) is always present in the molecule of terpenes (Justus Lieb Ann Chem 1887, 238, 78). 
The structure of camphor was established by Bredt in 1893, that of pinene by Wagner in 1894 and that of citral by Tiemann in 1895. b-Carotene was isolated in 1837 by Wackenrodder from carrots, and its correct molecular form was determined in 1907 by Willstätter.
The period since 1945 has seen an extensive explosion in natural product chemistry due to the advent of chromatographic and spectroscopic techniques. The discovery of the isoprene unit is the basis of the concept of the "isoprenic rule" edicted in 1953 by Ruzicka L (Experientia, 1953, 9, 357) and completed by Lynen F et al. (Angew Chem 1958, 70, 738) and Bloch K et al. (J Biol Chem 1959, 234, 2595).
Mevalonic acid was shown in 1956 to be a biosynthetic precursor of cholesterol (Tavormina PA et al., J Am Chem Soc 1956, 78, 4498) and later, its incorporation into a number of terpenoids has been demonstrated. Actually, an increasing number of terpenoids are described in the plant kingdom and many of them were shown to have important biological activities. Thus, several sesquiterpenes and diterpenes have antibiotic properties, some sesquiterpenes and diterpenes are insect and plant hormones, respectively.