Musk

Musk was a name originally given to a substance with a strong odor obtained from a gland of the musk deer. The term has come to encompass a wide variety of aromatic substances with similar odors, despite their often differing chemical structures and molecular shapes. The organic compound primarily responsible for the characteristic odor of musk is muscone.

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Natural musk

The musk gland is found only in adult males. The Siberian musk deer (Moschus moschiferus) is a musk deer found in the mountain forests of Northeast Asia. The Siberian musk deer is classified as Vulnerable by the IUCN. The Siberian musk deer is hunted for its musk gland, which fetches prices as high as $45,000 per kilogram. Natural musk is one of the most expensive animal products in the world. The trade quantity of natural musk is controlled by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), but illegal poaching and trading continues. Natural musk is usually packed in hermetically-sealed vessels and wooden boxes lined with tin foil because of its powerful diffusion of odor. Glandular substances with musk-like odors are also obtained from the musk duck, the muskox, the musk shrew and several other animals. The plant sources include the musk flower (Mimulus moschatus) of western North America, the muskwood (Olearia argophylla) of Australia and the musk seeds (Abelmoschus moschatus) from India.

Synthetic musks

Natural musk was used extensively in perfumery until the late 19th century when economic and ethical motives led to the adoption of synthetic musk, which is now used almost exclusively. Obtaining deer musk requires killing the endangered animal, so nearly all musk fragrance used in perfumery today is synthetic. Synthetic musks can be divided into three major classes: aromatic nitro musks, polycyclic musk compounds and macrocyclic musk compounds. Synthetic musks in a narrower sense are chemicals modeled after the main odorants in animal musk: muscone in deer musk and civetone in civet. Synthetic musks are lipophilic compounds and tend to deposit and persist in fat tissues.

Nitro musks

Albert Baur obtained an artificial musk in 1888 by condensing toluene with isobutyl bromide in the presence of aluminium chloride and nitrating the product. The discovery was accidental, a result of Baur's attempts at producing a more effective form of trinitrotoluene (TNT). Musk xylene is a very persistent and very bioaccumulative pollutant in the aquatic environment. It is banned in the EU, since no company has applied for authorisation.

Polycyclic musks

The creation of the cyclic-ether class of musks, which includes Galaxolide, was largely prompted by the need to eliminate the nitro group from the nitro musks, because of their photochemical reactivity and their instability in alkaline media. While poorer in smell strength, the compounds of this class performed well and were stable in harsh detergents, which led to their common use. Galaxolide has become the key synthetic musk ingredient belonging to the polycyclic musk group as defined by the International Fragrance Association. Galaxolide has a measured bioconcentration factor in fish in a range of 600-1600. In March 2003 the European Chemicals Bureau concluded that Galaxolide is not a persistent, bioaccumulative and toxic substance.

Macrocyclic musks

Macrocyclic ketone musks consist of a single ring of more than 6 carbons, often 10 to 15. Of all artificial musks, these most resemble the primary odoriferous compound of Tonkin musk in their large-ringed structure. The macrocyclic musks extracted from plants are large-ringed lactones, while all animal-derived macrocyclic musks are ketones. Muscone is a macrocyclic ketone, a 15-membered ring ketone with one methyl substituent in the 3-position. Muscone had been isolated in 1904 but was not identified as 3-methylcyclopentadecanone until Ružička suspected a macrocycle. In 1926 Leopold Ružička was able to synthesize muscone in very small quantities. Compounds of this class were not commercially produced and commonly used until the late 1990s, because of the difficulties in their synthesis and their consequently higher price. Natural muscone is found as the (-)-enantiomer, (R)-3-methylcyclopentadecanone, and muscone has been synthesized as the pure (-)-enantiomer as well as the racemate. Muscone is very slightly soluble in water and miscible with alcohol. The human musk-recognizing receptor OR5AN1 is reported to bind to muscone and related musks such as civetone through hydrogen-bond formation from tyrosine-260, along with hydrophobic interactions with surrounding aromatic residues in the receptor.

Alicyclic musks

Alicyclic musks, otherwise known as cycloalkyl ester or linear musks, are a relatively novel class of musk compounds. They are dramatically different in structure from the aromatic, polycyclic and macrocyclic musks, in that they are modified alkyl esters. The first compound of this class was introduced in 1975 with Cyclomusk. Alicyclic musks were discovered before 1980, but a compound of the class was produced at a commercial scale only in 1990, with the introduction of Helvetolide. Romandolide was introduced ten years later.

Regulation of individual musks

IFRA Standards prohibit musk xylene, musk ambrette, musk alpha, musk KS, musk moskene and musk tibetene, set a specification for musk ketone and restrict cyclopentadecanolide.