13.1. TERPENOIDS
Many terpenoids are compounds isolated from essential oils of plants by distilling the plant with water or by extraction. These oils are often responsible for a fragrant odour of the particular plant such as rose, geranium, peppermint, bay, citrus plants, conifers, and others. For centuries, the essential oils have been used as perfumes, spices, and medicines.
Chemically, terpenoids are relatively small molecules that have an enormous diversity of structure. A carbon skeleton of terpenes can be acyclic or, more often, cyclic. Some terpenoids are hydrocarbons (called simply - terpenes, but very often both terms are equivalent), but many contain oxygen, mostly as a constituent of a hydroxyl or carbonyl group.
The main structural peculiarity of terpenes is that they constitute of 10, 15, 20, 30, or 40 carbon atoms, i. e. a number divisible by five. Moreover, according to the isoprene rule proposed by the Swiss chemist L.S. Ruzicka (in the 1920's), terpene skeletons are built up from «head-to-tail» joining1 of simple five-carbon blocks (isoprene units). The C-1 atom of isoprene is called the «head» of an isoprene unit, and C-4 is called the «tail».