Myrcene, or β-myrcene, is an olefinic natural organic hydrocarbon. It is more precisely classified as a monoterpene. Monoterpenes are dimers of isoprenoid precursors, and myrcene is a significant component of the essential oilof several plants, including bay, cannabis, ylang-ylang, wild thyme, parsley, cardamom, and hops.[3][4] It is produced mainly semi-synthetically from myrcia, from which it gets its name. It is a key intermediate in the production of several fragrances. α-Myrcene is the name for the structural isomer 2-methyl-6-methylene-1,7-octadiene, which is not found in nature and is little used.[3]

Production

Myrcene is produced by the pyrolysis (400 °C) of β-pinene, which is obtained from turpentine.[3] It is rarely obtained directly from plants.[5]

Terpenes arise naturally from dehydration of terpenol geraniol. Plants biosynthesize myrcene via geranyl pyrophosphate (GPP). The mevalonate pathway gives the precursors dimethylallyl pyrophosphate (Fig. 1a) and isopentenyl pyrophosphate (Fig.1b). These two precursors combine to produce GPP (Fig. 1c), which then isomerizes into linally pyrophosphate (Fig. 1d). The rearrangement and release of the pyrophosphate (OPP), and the double bond formation creates the product myrcene (Fig. 1e).[6]

Biosynthesis of myrcene. A is dimethylallyl pyrophosphate, B is isopentyl pyrophosphate, C is geranyl pyrophosphate, D is linally (LPP), E is myrcene.

Occurrence

It could in principle be extracted from any number of plants, for example wild thyme, the leaves of which contain up to 40% by weight of myrcene. Many other plants contain myrcene, sometimes in substantial amounts.[3] Some of these include cannabis, hops Houttuynia, lemon grass, mango, Myrcia, Verbena, West Indian bay tree, and cardamom.[7]

Of the several terpenes extracted from Humulus lupulus (hops), the largest monoterpenes fraction is β-myrcene. One Swiss study of the chemical composition of the fragrance of Cannabis sativa L. found β-myrcene to compose between 29.4% to 65.8% of the steam-distilled essential oil for the set of fiber and drug strains tested.[8]

Use in fragrance and flavor industries

Myrcene is an important intermediate used in the perfumery industry. It has a pleasant odor but is rarely used directly.[3] It is also unstable in air, tending to polymerize. Samples are stabilized by the addition of alkylphenols or tocopherol. It is thus more highly valued as an intermediate for the preparation of flavor and fragrance chemicals such as menthol, citral, citronellol, citronellal, geraniol, nerol, and linalool. Myrcene is converted to myrcenol, another fragrance found in lavender, via hydroamination of the 1,3-diene by diethylamine followed by hydrolysis and Pd-catalyzed removal of the amine.[9]

Both myrcene and mycenol undergo Diels-Alder reactions with several dienophiles such as acrolein to give cyclohexene derivatives that are also useful fragrances.[2]

Myrcene contributes a peppery and balsam aroma in the beer.[10][11]

 

source: wikipedia