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Humulene, also known as α-humulene or α-caryophyllene, is a naturally occurring monocyclic
sesquiterpene Sesquiterpenes are a class of terpenes that consist of three isoprene units and often have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be cyclic or contain rings, including many unique combinations. Biochemical modificat ...
(C15H24), containing an 11-membered ring and consisting of 3
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
units containing three nonconjugated C=C double bonds, two of them being triply substituted and one being doubly substituted. It was first found in the
essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
s of ''
Humulus lupulus ''Humulus lupulus'', the common hop or hops, is a species of flowering plant in the hemp family Cannabinaceae, native to Europe, western Asia and North America. It is a perennial, herbaceous climbing plant which sends up new shoots in early spr ...
'' (
hops Hops are the flowers (also called seed cones or strobiles) of the hop plant ''Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to whi ...
), from which it derives its name. Humulene is an
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...
of
β-caryophyllene Caryophyllene (), more formally (−)-β-caryophyllene, (BCP), is a natural bicyclic sesquiterpene that is a constituent of many essential oils, especially clove oil, the oil from the stems and flowers of ''Syzygium aromaticum'' (cloves), the esse ...
, and the two are often found together as a mixture in many aromatic plants.


Occurrence

Humulene is one of the components of the
essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
from the flowering cone of the hops plant, ''
Humulus lupulus ''Humulus lupulus'', the common hop or hops, is a species of flowering plant in the hemp family Cannabinaceae, native to Europe, western Asia and North America. It is a perennial, herbaceous climbing plant which sends up new shoots in early spr ...
'', from which it derives its name. The concentration of humulene varies among different varieties of the plant but can be up to 40% of the essential oil. Humulene and its reaction products in the brewing process of beer gives many beers their “hoppy” aroma.
Noble hop Hops are the flowers (also called seed cones or strobiles) of the hop plant ''Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to whi ...
varieties have been found to have higher levels of humulene, while other bitter hop varieties contain low levels. Multiple
epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for ...
s of humulene are produced in the brewing process. In a scientific study involving
gas chromatography–mass spectrometry Gas chromatography–mass spectrometry (GC-MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fir ...
analysis of samples and a trained sensory panel, it was found that the hydrolysis products of humulene epoxide II specifically produces a “hoppy” aroma in beer. α-Humulene has been found in many aromatic plants on all continents, often together with its isomer β-caryophyllene. Proven α-humulene emitters into the atmosphere are pine trees, orange orchards, marsh elders, tobacco, and sunflower fields. α-Humulene is contained in the essential oils of aromatic plants such as ''
Salvia officinalis ''Salvia officinalis'', the common sage or just sage, is a perennial, evergreen subshrub, with woody stems, grayish leaves, and blue to purplish flowers. It is a member of the mint family Lamiaceae and native to the Mediterranean region, thoug ...
'' (common sage, culinary sage), '' Lindera strychnifolia'' Uyaku or Japanese spicebush,
ginseng Ginseng () is the root of plants in the genus ''Panax'', such as Korean ginseng ('' P. ginseng''), South China ginseng ('' P. notoginseng''), and American ginseng ('' P. quinquefolius''), typically characterized by the presence of ginsenosides an ...
species, up to 29.9% of the essential oils of ''
Mentha spicata Spearmint, also known as garden mint, common mint, lamb mint and mackerel mint, is a species of Mentha, mint, ''Mentha spicata'' (, native to Europe and southern temperate Asia, extending from Ireland in the west to southern China in the east. I ...
'', the
ginger Ginger (''Zingiber officinale'') is a flowering plant whose rhizome, ginger root or ginger, is widely used as a spice A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices ...
family (Zingiberaceae), 10% of the leaf oil of ''Litsea mushaensis'', a Chinese laurel tree, 4% of the leaf extract of ''Cordia verbenacea'', a bush in coastal tropical South America (erva baleeira), but with 25% ''trans''-Caryophyllene and is one of the chemical compounds that contribute to the taste of the spice ''Persicaria odorata'' or
Vietnamese coriander ''Persicaria odorata'', with common names Vietnamese coriander, rau răm, laksa leaf, Vietnamese cilantro, phak phai, praew leaf, hot mint and Cambodian mint, is an herb with aromatic leaves, used in Southeast Asian and Northeast Indian cooking. ...
and the characteristic aroma of ''
Cannabis ''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: ''Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternatively ...
''.


Preparation and synthesis

Humulene is one of many sesquiterpenoids that are derived from farnesyl diphosphate (FPP). The formation of humulene from FPP is catalyzed by sesquiterpene synthesis enzymes. This biosynthesis can be mimicked in the laboratory by preparing allylic stannane from farnesol, termed Corey synthesis. There are diverse ways to synthesize humulene in the laboratory, involving differing closures of the C-C bond in the macrocycle. The McMurry synthesis uses a titanium-catalyzed carbonyl coupling reaction; the Takahashi synthesis uses intramolecular alkylation of an allyl halide by a protected cyanohydrin anion; the Suginome synthesis utilizes a geranyl fragment; and the de Groot synthesis synthesizes humulene from a crude distillate of eucalyptus oil. Humulene can also be synthesized using a combination of four-component assembly and palladium-mediated cyclization, outlined below. This synthesis is noteworthy for the simplicity of the C−C bond constructions and cyclization steps, which it is believed will prove advantageous in the synthesis of related polyterpenoids. : To understand humulene's regioselectivity, the fact that one of the two triply substituted C═C double bonds is significantly more reactive, its conformational space was explored computationally and four different conformations were identified.


Research

In laboratory studies, humulene is being studied for potential
anti-inflammatory Anti-inflammatory is the property of a substance or treatment that reduces inflammation or swelling. Anti-inflammatory drugs, also called anti-inflammatories, make up about half of analgesics. These drugs remedy pain by reducing inflammation as o ...
effects. In 2015 researchers in Brazil identified α-Humulene as an active contributor to the
insect repellent An insect repellent (also commonly called "bug spray") is a substance applied to skin, clothing, or other surfaces to discourage insects (and arthropods in general) from landing or climbing on that surface. Insect repellents help prevent and cont ...
properties of
commiphora leptophloeos The genus of the myrrhs, ''Commiphora'', is the most species-rich genus of flowering plants in the frankincense and myrrh family, Burseraceae. The genus contains approximately 190 species of shrubs and trees, which are distributed throughout the ...
leaf oil, specifically against “the yellow fever mosquito,”
Aedes aegypti ''Aedes aegypti'', the yellow fever mosquito, is a mosquito that can spread dengue fever, chikungunya, Zika fever, Mayaro and yellow fever viruses, and other disease agents. The mosquito can be recognized by black and white markings on its legs ...
.


Atmospheric chemistry

α-Humulene is a biogenic
volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
, emitted by numerous plants (see occurrence) with a relatively high potential for secondary organic aerosol formation in the atmosphere. It quickly reacts with ozone in sunlight (photooxidation) to form oxygenated products. α-Humulene has a very high reaction rate coefficient (1.17 × 10−14 cm3 molecule−1 s−1) compared to most monoterpenes. Since it contains three double bonds, first-, second- and third-generation products are possible that can each condense to form secondary organic aerosol. At typical tropospheric ozone mixing ratios of 30 ppb the lifetime of α-humulene is about 2 min, while the first- and second-generation products have average lifetimes of 1 h and 12.5 h, respectively.


References

{{reflist, 2 Cycloalkenes Sesquiterpenes Humulus