Essential oils are a group of natural organic compounds that are predominantly composed of terpenes (hydrocarbons) and terpenoids (oxygen containing hydrocarbons). Essential oils also contain simple phenols, sulphur containing mustard oils, methyl anthranilate and coumarins.
ISOPRENE UNITS
Plants build terpenes and terpenoids from the basic building block 3-methyl-3-butenyl pyrophosphate. The 5-carbon unit of this molecule is the source of the "isoprene" unit that makes up all "isoprenoids". Combining two of these units give rise to geranyl pyrophosphate which then forms the skeleton of the MONOTERPENES (10 carbons).
Combining three of these units gives rise to farnesyl pyrophosphate which then forms the skeleton of the SESQUITERPENES (15 carbons).
Combining two geranyl pyrophosphates gives rise to geranylgeranyl pyrophosphates which then forms the skeleton of the DITERPENES (20 carbons).
Farnesyl and geranylgeranylpyrophosphates can dimerize to yield TRITERPENES (30 carbons) and TETRATERPENES (40 carbons).
There are about 400 known monoterpenes which occur widely among gymnosperms and angiosperms. Most are free but some are found as glycosides (combined with sugars).
Monoterpenes make up a major part of the volatile constituents (or essential oils) of plants. In nature, they are frequently involved in plant-animal and plant-plant interactions - such as pollination, seed and fruit dissemination, allomones, kairomones, and allelopathic agents. For example, the monoterpenoid, cantharidin, is produced by the blistering beetle (Cantharis vesicatoria) as an alarm substance.
Some examples of the monoterpenes and (-oids) are: pinene, geraniol, thymol, citral, citronellal, carvone, camphor, apiol, cinnamic acid, and safrol.
SECOIRIDOID MONOTERPENES - TYPES OF BITTER PRINCIPLES
The phrase bitter principles, which refers to the bitter-tasting monoterpenoid lactones known as iridoids are also components of volatile oils and have been used to stimulate gastric secretion in man.
Examples of monoterpene bitter principles include: gentiopicrin, gentianin, procumbid, harpagid, oleuropein, aucubin.
Examples of sesquiterpene bitter principles include: cnicin, artabsin, absinthin.
Examples of diterpene bitter principles include: marrubiin, carnosol (picrosalvin).
Examples of triterpene bitter principles include: quassin and cucurbitacin.
Sesquiterpenes make up the largest group of terpenoid compounds. There are about 1000 known compounds with the distribution being about the same as the monoterpenes. Thus, sesquiterpenes are common essential oil components. Abscisic acid, a plant growth compound, is also a sesquiterpene.
Farnesol, nerolidol are examples of sesquiterpenes found with monoterpenes in essential oils. They are both mite pheromone sex attractants (Regev - Rec. Adv. in Acarology - 1:147, 1979).
Sesquiterpenoids also occur in microorganisms, marine animals, and fighting insects. Some are very toxic, but some are antifungals, carminatives, and insecticides.
SESQUITERPENE LACTONES
Sesquiterpene lactones are restricted in distribution, occurring primarily in Apiaceae, Asteraceae, Lauraceae, and Hepaticae. These compounds possess potent biological activity and are responsibile for the bitter taste and toxic properties of many of the plants in which they occur. Often, these components cause contact dermatitis and some are lethal.
At least 650 diterpenes are found in a large number of plant families. They are chemically complex, difficult to isolate, purify and characterize. They are usually compoents of plant resins but are sometimes encountered as by-products from the isolation of essential oils (e.g., from turpentine production).
Some diterpenes possess a configuration which can give rise to gibberellins.