Oxygenated Isoprenoid Compounds

A great profusion of oxygen-containing isoprenoid compounds are known. Of particular importance in the acyclic series are the alcohols geraniol, nerol, and linaloöl, and the aldehydes geranial (citral a), neral (citral b), and citronellal:

The alcohols occur in oil of rose and other flower essences. They have geranium or rose odors and are important perfume ingredients. The aldehydes have much stronger citruslike odors and occur as major or minor constituents in many essential oils, such as oil of citronella, oil of lemon, and so on.

Monocyclic and bicyclic oxygenated terpenes include some familiar and interesting substances such as menthone and menthol from peppermint oil, 1,8-cineole from eucalyptus, and ascaridole, which is a naturally occurring peroxide from chenopodium oil:

Camphor is a particularly well-known bicyclic terpene ketone, which has uses in medicine and as a plasticizer for nitrocellulose.:

For many years, the principal source of camphor was the Formosan camphor tree. It now can be synthesized on a large scale from αα-pinene. Some of the other types of naturally occurring bicyclic ketones follow:

Higher oxygenated terpenes include the sesquiterpene alcohol, farnesol, which has a lily-of-the-valley odor and occurs in ambrette-seed oil. On acid dehydration it gives αα-farnesene (Table 30-1) under some conditions, and bisabolene (a component of oil of bergamot) under others:

As we shall see, cyclization reactions of this general type seem to be important in terpene biosynthesis. The 6,7-trans-farnesol has been shown to have hormone action in some insects. It acts to regulate the changes from caterpillar to cocoon to moth.

Two important diterpene alcohols are vitamin A  and phytol, which occurs as an ester of the propanoic acid side-chain of chlorophyll (Figure 20-6):

The phytyl group appears also as a side chain in vitamin K11 and ββ-Carotene  has vitamin A activity and apparently is oxidized in the body at the central double bond to give one mole of vitamin A.

The diterpene acid, abietic acid, is a major constituent of rosin, which is obtained as a nonvolatile residue in the manufacture of turpentine by steam distillation of pine oleoresin or shredded pine stumps. Abietic acid is used extensively in varnishes and as its sodium salt in laundry soaps.