Carvone is a member of a family of chemicals called terpenoids.[1] Carvone is found naturally in many essential oils, but is most abundant in the oils from seeds of caraway (Carum carvi), spearmint (Mentha spicata), and dill.[2]
Both carvones are used in the food and flavor industry. As the compound most responsible for the flavor of caraway, dill, and spearmint, carvone has been used for millennia in food.[2] Food applications are mainly met by carvone made from limonene. R-(−)-Carvone is also used for air freshening products and, like many essential oils, oils containing carvones are used in aromatherapy and alternative medicine.
Agriculture
S-(+)-Carvone is also used to prevent premature sprouting of potatoes during storage, being marketed in the Netherlands for this purpose under the name Talent.[2]
File:Carvone.svg(R) gives spearmint its distinctive smell. (S) gives caraway its distinctive smell.
Carvone has two mirror image forms, or enantiomers: R-(−)-carvone, the sweetish minty smell of spearmint leaves. Its mirror image, S-(+)-carvone, has a spicy aroma with notes of rye, and gives caraway seeds their smell.[4][5]
The fact that the two enantiomers are perceived as smelling different is evidence that olfactory receptors must respond more strongly to one enantiomer than to the other. Not all enantiomers have distinguishable odors. Squirrel monkeys have also been found to be able to discriminate between carvone enantiomers.[6]
The two forms are also referred to, in older texts, by their optical rotations of laevo (l) referring to R-(−)-carvone, and dextro (d) referring to S-(+)-carvone. Modern naming refers to levorotatory isomers with the sign (−) and dextrorotatory isomers with the sign (+) in the systematic name.
Occurrence
S-(+)-Carvone is the principal constituent (60–70%) of the oil from caraway seeds (Carum carvi),[7] which is produced on a scale of about 10 tonnes per year.[2] It also occurs to the extent of about 40–60% in dill seed oil (from Anethum graveolens), and also in mandarin orange peel oil. R-(−)-Carvone is also the most abundant compound in the essential oil from several species of mint, particularly spearmint oil (Mentha spicata), which is composed of 50–80% R-(−)-carvone.[8] Spearmint is a major source of naturally produced R-(−)-carvone. However, the majority of R-(−)-carvone used in commercial applications is synthesized from R-(+)-limonene.[9] The R-(−)-carvone isomer also occurs in kuromoji oil. Some oils, like gingergrass oil, contain a mixture of both enantiomers. Many other natural oils, for example peppermint oil, contain trace quantities of carvones.
History
Caraway was used for medicinal purposes by the ancient Romans,[2] but carvone was probably not isolated as a pure compound until Franz Varrentrapp (1815–1877) obtained it in 1849.[1][10] It was originally called carvol by Schweizer. Goldschmidt and Zürrer identified it as a ketone related to limonene,[11] and the structure was finally elucidated by Georg Wagner (1849–1903) in 1894.[12]
Preparation
Carvone can be obtained from natural sources but insufficient is available to meet demand. Instead most carvone is produced from limonene.
The dextro-form, S-(+)-carvone is obtained practically pure by the fractional distillation of caraway oil. The levo-form obtained from the oils containing it usually requires additional treatment to produce high purity R-(−)-carvone. This can be achieved by the formation of an addition compound with hydrogen sulfide, from which carvone may be regenerated by treatment with potassium hydroxide followed by steam distillation.
Carvone may be synthetically prepared from limonene by first treating limonene nitrosyl chloride. Heating this nitroso compound gives carvoxime. Treating carvoxime with oxalic acid yields carvone.[13] This procedure affords R-(−)-carvone from R-(+)-limonene.
The major use of d-limonene is as a precursor to S-(+)-carvone. The large scale availability of orange rinds, a byproduct in the production of orange juice, has made limonene cheaply available, and synthetic carvone correspondingly inexpensively prepared.[14]
Oxidation of carvone can also lead to a variety of products.[1] In the presence of an alkali such as Ba(OH)2, carvone is oxidised by air or oxygen to give the diketone 7. With hydrogen peroxide the epoxide8 is formed. Carvone may be cleaved using ozone followed by steam, giving dilactone9, while KMnO4 gives 10.
Being available inexpensively in enantiomerically pure forms, carvone is an attractive starting material for the asymmetrictotal synthesis of natural products. For example, (S)-(+)-carvone was used to begin a 1998 synthesis of the terpenoid quassin:[16]
↑Handwörterbuch der reinen und angewandten Chemie [Concise dictionary of pure and applied chemistry] (Braunschweig, (Germany): Friedrich Vieweg und Sohn, 1849), vol. 4, pages 686-688. [Notes: (1) Varrentrapp purified carvone by mixing oil of caraway with alcohol that had been saturated with hydrogen sulfide and ammonia; the reaction produced a crystalline precipitate, from which carvone could be recovered by adding potassium hydroxide in alcohol to the precipitate, and then adding water; (2) Varrentrapp's empirical formula for carvone is incorrect because chemists at that time used the wrong atomic masses for the elements; e.g., carbon (6 instead of 12).]
↑Heinrich Goldschmidt and Robert Zürrer (1885) "Ueber das Carvoxim,"Berichte der Deutschen Chemischen Gesellschaft, 18 : 1729–1733.
↑Georg Wagner (1894) "Zur Oxydation cyklischer Verbindungen" (On the oxidation of cyclic compounds), Berichte der Deutschen chemischen Gesellschaft zu Berlin, vol. 27, pages 2270-2276. [Notes: (1) Georg Wagner (1849–1903) is the Germanized form of "Egor Egorovich Vagner", who was born in Russia and worked in Warsaw (See brief biography here.); (2) Wagner did not prove the structure of carvone in this paper; he merely proposed it as plausible; its correctness was proved later.]
↑Karl-Georg Fahlbusch, Franz-Josef Hammerschmidt, Johannes Panten, Wilhelm Pickenhagen, Dietmar Schatkowski, Kurt Bauer, Dorothea Garbe, Horst Surburg "Flavors and Fragrances" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim Script error: No such module "doi".
