Everything about Carvone totally explained
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Carvone is a member of a family of chemicals called
terpenoids. Carvone is found naturally in many
essential oils, but is most abundant in the oils from seeds of
caraway (
Carum carvi) and
dill.
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.
Occurrence
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.
Preparation
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 major use of d-limonene is as a precursor to
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.
The
biosynthesis of carvone is by oxidation of
limonene.
Chemical properties
Reduction
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
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 di
lactone 9, while
KMnO4 gives
10.
Conjugate additions
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.
Uses
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.
Food applications
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.
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.
Organic synthesis
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:
Metabolism
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. (4
S)-(+)-carvone is likewise converted to (4
S,6
S)-(+)-carveol. This mainly occurs in the liver and involves
cytochrome P450 oxidase and
(+)-trans-carveol dehydrogenase.
Further Information
Get more info on 'Carvone'.
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