Some compounds are rare in essential oils. Sometimes they make a dot on the “i” of the fragrant bouquet, just like nitrogen or sulphur compounds. They often have an unpleasant odour. Sometimes they are interesting in a somewhat unusual garlic aromatherapy. Some were anti-parasitic drugs, such as ascaridol, and are forgotten today.
Lactones are a relatively rare group of compounds in essential oils. They are present in other types of plant extracts and their main action is expectorant (facilitates expectoration) and is sometimes choleretic (enhances bile secretion).
There is one commercially available essential oil that contains lactones as dominant compounds. It is an essential oil of elecampane (Inula helenium) which contains over 70% of allantolactone and isoalantolactone. Their structures are shown in the following figure:
Elecampe oil is not used for several reasons. The lactones it contains easily cause allergic reactions on the skin, and the oil itself is a solid at room temperature and needs to be warmed. It is very rarely found on the market and is relatively expensive. As there are many other essential oils that act on the respiratory system, and elecampe itself can be used in the form of other extracts (decoction, tincture), its use is very rare.
The essential oil of the former relative inula (Dittrichia graveolens, formerly Inula graveolens) contains borneol and bornyl acetate as the dominant ingredients, and contains lactones in very small concentrations. Due to its price it is very rarely used in aromatherapy.
Like lactones, peroxides are very rarely dominant compounds. Peroxides are formed in the metabolism of both plants and animals, but they are mostly unstable. Only a small number of molecules, such as artemisinin from the plant Artemisia annua, is stable enough to be isolated. A similar compound is ascaridol, the dominant compound of the essential oil of Mexican-tea plant (Dysphania ambrosioides, formerly Chenopodium ambrosioides). The essential oil of Mexican-tea was widely used at the end of the 19th century and in the early twentieth century as an anthelmintic, an agent against intestinal parasites. Due to its toxicity, this essential oil has been suppressed by safer drugs and thus belongs to the already forgotten past of medicine.
Only one essential oil contains a significant amount of these interesting compounds. It is an essential oil of lovage (Levisticum officinale). The two main phthalides are 3-butylidenephthalide and ligustilide, which, depending on the origin, can be over 70% in this essential oil.
The essential oil of lovage is, unfortunately, in the shadow of the planet-famous plant of Chinese herbalism, danggui (当归 / 當歸), better known in the west as Dong Quai (Angelica sinensis). Both plants belong to the same family Apiaceae. Although in its shadow, Angelica sinensis is credited with a very extensive study of the phthalides it contains. Ligustilide has an anti-inflammatory effect, stimulates osteoblasts to form bone tissue, acts oestrogen-like (Angelica sinensis is known in supplements that help menopausal problems), reduces tissue damage caused by harmful influences (hypoxia due to lack of circulation) and reduces the possibility of platelet aggregation and the formation of dangerous clots. Zhong et al. have shown that it acts by modulating TRPA- 1 protein (transient receptor potential channels), which is not surprising because many small molecules of essential oils act through these proteins. Yu reported that these types of compounds prevent hepatotoxicity or liver damage. In the French school of aromatherapy, lovage root is a favourited essential oil for liver regeneration. But it never came to life in greater application, primarily because of the presence of milk thistle as an herbal remedy for the liver. Therefore, lovage and phthalides as a class of compounds have been suppressed by other plant species and extracts, probably also due to its high cost.
Coumarins are extremely common compounds in nature. But due to their specific structure in essential oils there are very few of them. Coumarins are quite difficult to distill, and we usually find only those coumarins in oils that have a lower molecular weight. Coumarins have a number of beneficial effects on the body, but it is likely that their action in essential oils is in the shadow of other dominant compounds.
The only essential oils that contain more noticeable amounts of coumarin are those obtained by pressing citrus fruits. Bergamot and lime fruit oils contain the most of them. Coumarins in essential oils are better known in a negative context. Furanocoumarins are responsible for phototoxic effects, i.e. toxic effects on the skin by activation of UV rays (sunbathing, solarium). The most famous such compound is bergapten from the essential oil of bergamot fruit. Therefore, bergamot essential oil should not be applied to the skin before exposure to UV rays. Bergamot essential oil without bergapten is commercially available.
Most organic sulphur compounds have a very unpleasant odour. As they are often formed by the decomposition of meat (organic matter), but also come out in dangerous areas (volcanic vapours, anaerobic growth conditions of microorganisms), we have been evolutionarily trained to regard these types of compounds as a warning sign. Apparently, sulphur compounds have nothing to do in essential oils – we expect a pleasant, not unpleasant, odour from them. However, there are very diverse structures of sulphur compounds. In 2002, Andreas Goeke in the article Sulphur containing odorants in fragrance chemistry gave a good overview of sulphur compounds that, he said, “live a hidden life in the chemistry of odours”.
Sulphur compounds can very rarely be dominant compounds in essential oil. One such oil is garlic essential oil. Garlic essential oil has a different composition from classic garlic extracts, but retains the classic effects of garlic: it has an extremely strong antimicrobial effect, lowers blood pressure and affects blood coagulation (prevents blood clots). The main constituents of garlic essential oil are di-2-propenyl trisulfide, di-2-propenyl disulfide, methyl, 2-propenyl trisulfide and di-allyl disulfide. Some of the dietary supplements used to regulate fat metabolism and help with hypertension (high blood pressure) contain garlic essential oil.
Buchu essential oil (Agathosma betulina), also contains a significant amount of sulphur compounds such as 8-mercapto-p-menthan-3-one. It has no significant application in aromatherapy, but it has in perfumery. Paradoxically, buchu essential oil has an unpleasant odour reminiscent of burnt rubber. But in small concentrations it gives a sharp freshness to the perfume. Here we encounter the rule that compounds with a very unpleasant odour in small concentrations emphasize and give a special fragrant beauty to the molecules of a pleasant odour, which is also the rule of traditional perfumery. The unpleasant smell, most likely, awakens in us that animal character which in its essence loves the smell of rot.
Mostly sulphur compounds are trace elements. Here, too, the rule applies that they are in charge of rounding off the fragrant bouquet and the overall impression of the essential oil. Mint-sulphide and iso-mintsulphide stimulate the prickly-deep character of menthol in peppermint essential oil. These same compounds, along with a whole host of other sulphur compounds such as humulene sulphide and caryophyllene sulphide, give even the essential oil of rose, the “queen of all essential oils and perfumes,” the dot on the -i-, though in it otherwise monoterpene alcohols and pleasant-smelling esters predominate. 1-p-menten-8-thiol is found in grapefruit essential oil. Precisely such trace compounds give a characteristic citrus scent in which, otherwise monotonously, limonene or other monoterpenes dominate.
4-mercapto-4-methyl-pentanone is another example of an unpleasant odour compound. In high concentrations it has the smell of cat urine, but in low concentrations it gives a woody-fresh smell. It is contained in the essential oil of the wooden parts of black currant.
As in the case of sulphur compounds, nitrogen compounds are relatively rare in essential oils. The essential oil of petit grain of mandarin is one of the few essential oils that contains methyl-N-methyl-anthranilate, a pleasant-smelling compound, as the dominant compound. It shows spasmolytic activity and is one of the oils used in perfumery. An essential oil such as bitter orange neroli (bitter orange flower) contains up to 1% methyl anthranilate and contributes to the fragrant quality of the essential oil. Except for mandarin essential oil, nitrogen compounds are generally not active compounds. They are more important in their olfactory quality (perfumery, cosmetics, massages, baths).
Similar to sulphur compounds, some nitrogen compounds have a very unpleasant odour. Indole, and especially the compound skatole, has an unpleasant odour of feces. Interestingly, jasmine absolute contains it. If the rose is the “queen of fragrance” then jasmine is their “king”. Indole and skatol, as in the case of sulphur compounds of the rose, are found in very low concentrations and in combination with the esters of a pleasant odour give the final aphrodisiac-sweet scent to the jasmine absolute.