Isoprenoid Compounds

The odor of a freshly crushed mint leaf, like many plant odors, is due to the presence in the plant of volatile C10C10 and C15C15 compounds, which are called terpenes. Isolation of these substances from the various parts of plants, even from the wood in some cases, by steam distillation or ether extraction gives what are known as essential oils. These are widely used in perfumery, as food flavorings and medicines, and as solvents. Among the typical essential oils are those obtained from cloves, roses, lavender, citronella, eucalyptus, peppermint, camphor, sandalwood, cedar, and turpentine. Such substances are of interest to use here because, as was pointed out by Wallach in 1887 and reemphasized by Ruzicka in 1935, the components of the essential oils can be regarded as derived from isoprene:

Not only are the carbon skeletons of these substances divisible into isoprene units, but the terpene hydrocarbons are usually exact multiples of C5H8C5H8. An example is myrcene (C10H16)(C10H16), which occurs in the oils of bay and verbena and has a carbon skeleton divisible into two isoprene units.

The connection between the isoprene units in myrcene is between the 1- and 4-positions; this turns out to be more common than 1,1 and 4,4 linkages.