The odorous compounds of plants are volatile and are usually separated from the plant material by steam distillation. They are known as the volatile or essential oils, and consist of hydrocarbons, alcohols, ethers aldehydes and ketones. In the evaluations of conifers and in the oils from citrus fruits and from eucalyptus trees, alicyclic hydrocarbons of the composition C.sub.10 H.sub.16 were found to be especially abundant, and it is to these compounds that the term "terpene" was applied in the restricted sense. It soon became evident, however, that compounds containing 15, 20, 30 and 40 carbon atoms also are closely related to terpenes, and the term "terpene" in its broadest sense now includes all such compounds, which comprise repeating iso-C.sub.5 units.
Many of the essential oils are employed in various flavors and fragrances, and their medicinal or biocidal potential has been the subject of continued investigation. For example, the cyclic terpenones, .alpha.-ionone and .beta.-ionone, were reported to exhibit moderate antibacterial activity against S. mutans by I. Kubo et al., J. Agric. Food Chem., 41, 2447 (1993). This bacterium is responsible for causing dental caries. Carvone, the chief component of spearmint oil, was reported to exhibit antifungal activity by V. Moleyar et al., Food Microbiol., 3, 331 (1986). Kubo et al., J. Natural Products, 57, 9 (1994) subsequently reported that a number of cyclic and acyclic terpene alcohols, including geranylacetol, farnesol and farnesyl acetol, exhibited activity against Pr. acnes, the bacterium responsible for acne. However, the linear ketone derived from farnesylacetol, farnesylacetone, was found to be inactive.
While some of these natural products may be potent enough for practical use, the synthesis or extraction of highly branched cyclic or alicyclic terpenes can be complex. Furthermore, terpenes such as ionone, a component of cedar oil, can cause allergic skin reactions. Nonetheless, essential oils and other phytochemicals are by definition biodegradable and renewable. Therefore, a continuing need exists for compounds of natural origin which exhibit useful levels of biocidal activity.