The Biogenesis of Terpenes

The name "terpenes" has been applied to a group of compounds distinguished by a singular chemical composition. Their empirical formula contains a multiple of five carbon atoms, and their chemical structure can be described as consisting of isopentane units. In the volatile plant materials, the essential oils, occur substances containing five, 10, 15 and small amounts of compounds with 20 carbon atoms. These have been called hemiterpenes, mono-, sesqui- and diterpenes, respectively. Still larger terpene homologues are not volatile enough to be found in the steam distillate but are obtained from the plants by solvent extraction. These include the resin adds, saponins, earotenoids, and rubber, which belong, respectively, to the di-, tri-, tetra- and polyterpenes. In addition, there are found structures which do not contain multiples of C_5 units, but are related to the terpenoid group since part of the molecule contains isopentane units (1). These compounds are sometimes indicated as isoprenoids. To this class belong the irones (I), ketones with 14 carbon atoms. Recent studies by Ruzicka et al. (2) and Naves et al. (3) have shown that this fragrant principle of orris root cannot be constructed entirely of isopentane units.