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Stereoisomerism and odor
Carvone forms two mirror image forms or enantiomers: S-(+)-carvone smells like caraway. Its mirror image, R-(–)-carvone, smells like spearmint. The fact that the two enantiomers are perceived as smelling differently is proof that olfactory receptors must contain chiral groups, allowing them to 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.
The two forms are also referred to by older names, with dextro-, d- referring to S-carvone, and laevo-, l- referring to R-carvone.
S-(+)-Carvone is the principal constituent (50-70%) of the oil from caraway seeds (Carum carvi),, which is produced on a scale of about 10 tonnes per year. 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 present at levels greater than 51% in spearmint oil (Mentha spicata), which is produced on a scale of around 1500 tonnes annually. This 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 lower concentrations of carvones.
Caraway was used for medicinal purposes by the ancient Romans, but carvone was probably not isolated as a pure compound until Varrentrapp obtained it in 1841. It was originally called carvol by Schweizer. Goldschmidt and Zűrrer identified it as a ketone related to limonene, and the structure was finally elucidated by Wagner in 1894.
The dextro-form is obtained practically pure by the fractional distillation of caraway oil; the laevo-form from the oils containing it, by first forming its addition compound with hydrogen sulfide, decomposing this by potassium hydroxide in ethanol, and distilling the product in a current of steam. It may be synthetically prepared from limonene nitrosochloride, alcoholic converting this compound into 1-carvoxime, which on boiling with dilute sulfuric acid yields l-carvone.
The biosynthesis of carvone is by oxidation of limonene.
There are three double bonds in carvone capable of reduction; the product of reduction depends on the reagents and conditions used. Catalytic hydrogenation of carvone (1) can give either carvomenthol (2) or carvomenthone (3). Zinc and acetic acid reduce carvone to give dihydrocarvone (4). MPV reduction using propan-2-ol and aluminium isopropoxide effects reduction of the carbonyl group only to provide carveol (5); a combination of sodium borohydride and CeCl3 (Luche reduction) is also effective. Hydrazine and potassium hydroxide give limonene (6) via a Wolff-Kishner reduction.
Oxidation of carvone can also lead to a variety of products. 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 epoxide 8 is formed. Carvone may be cleaved using ozone followed by steam, giving dilactone 9, while KMnO4 gives 10.
As an α,beta;-unsaturated ketone, carvone undergoes conjugate additions of nucleophiles. For example, carvone reacts with lithium dimethylcuprate to place a methyl group trans to the isopropenyl group with good stereoselectivity. The resulting enolate can then be allylated using allyl bromide to give ketone 11.
Both carvones are used in the food and flavor industry. R-(-)-Carvone is also used for air freshening products and, like many essential oils, oils containing carvones are used in aromatherapy and alternative medicine.
As the compound most responsible for the flavor of caraway, dill and spearmint, carvone has been used for millennia in food. Wrigley's Spearmint Gum is gum soaked in R-(–)-carvone and powdered with sugar.
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.
Carvone is available inexpensively in both enantiomerically pure forms, making it an attractive starting material for the asymmetric total synthesis of natural products. For example, (S)-(+)-carvone was used to begin a 1998 synthesis of the terpenoid quassin:
In the body, in vivo studies indicate that both enantiomers of carvone are mainly metabolized into dihydrocarvonic acid, carvonic acid and uroterpenolone. (4R,6S)-(–)-carveol is also formed as a minor product via reduction by NADPH. (4S)-(+)-carvone is likewise converted to (4S,6S)-(+)-carveol. This mainly occurs in the liver and involves cytochrome P450 oxidase and (+)-trans-carveol dehydrogenase.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Carvone". A list of authors is available in Wikipedia.|