Carvone, isomers

Explain why the retention times for both carvone isomers are the same. 

Thus the unsaturated ketones carvone, eucarvone and 1-acetyl-cyclohexane formed the cyclopropyl ketones (6), (7) and (8) while benzalaceto-phenone (9) affords l-benzyl-2-pheny 1-cyclopropane as a mixture of as and irons isomers (10) and (H) 233,234... 

Bioassay of alternate molecular forms supports the view that the ORs are capable of resolving isomeric distinctions in neutral (non-biological) odourants. Stereochemical pairs of odours were tested for differential sensitivities in the blind subterranean mole rat (Spalax ehrenbergi). The subjects responded to one enantiomer, but not to its stereoisomer. Both sexes were attracted to the odour of R-(-)-carvone but unresponsive to S-(+)-carvone in contrast, males and females were repelled by the odour of (+)-citronellol, but not by (-)-citronellol (Heth et al., 1992). The lack of responsiveness by mole rats could be central due to lack of salience, or peripheral due to hyposmia/anosmia for one isomer. Both carvones have distinct odours for the human nose. 

At the cellular level, the various types of receptor, transporter, enzyme and ion charmel are all chiral in form. Thus although the enantiomers of a drug may have identical physicochemical properties, the way in which they may interact with chiral targets at the level of the cell will give rise to different pharmacod)mamic and pharmacokinetic properties. A few simple examples will illustrate how taste and olfactory receptors can differentiate between enantiomers. Thus R-carvone tastes like spearmint whereas the S-isomer tastes like caraway. Similarly, R-limolene smells like lemon whereas the S-enantiomer tastes of orange. 

Aldehydes in this group are as follows carvone 36, dihydrocarvone 37, isom-enthone 38, piperitone 39, pulegone (piperitenone) 40, isopulegone 41 (Structure 4.9). 

It was also reported that there were slight differences in the activity of enantiomers. (R)-(-i-)-Limonene and (iI)-(-t)-carvone were more biologically active than their isomers (S)-(-)-limonene and (S)-(-)-carvone [115],... 

Limonene (92) is the most widely distributed terpene in nature after a-pinene [68]. The (+)-isomer is present in Citrus peel oils at a concentration of over 90% a low concentration of the (-)-isomer is found in oils from the Mentha species and conifers [26]. The first data on the microbial transformation of limonene date back to the sixties. A soil Pseudomonad was isolated by enrichment culture technique on limonene as the sole source of carbon [69]. This Pseudomonad was also capable of growing on a-pinene, / -pinene, 1-p-menthene and p-cymene. The optimal level of limonene for growth was 0.3-0.6% (v/v) although no toxicity was observed at 2% levels. Fermentation of limonene by this bacterium in a mineral-salts medium resulted in the formation of a large number of neutral and acidic products. Dihydrocarvone, carvone, carveol, 8-p-menthene-1,2-cw-diol, 8-p-menthen-1 -ol-2-one, 8-p-menthene-1,2-trans-diol and 1 -p-menthene-6,9-diol were among the neutral products isolated and identified. The acidic compounds isolated and identified were perillic acid, /Msopropenyl pimelic acid, 2-hydroxy-8-p-menthen-7-oic acid and... 

The studies that eventually led to the synthesis of 6 started by installation of one of the all-carbon stereocenters by addition of vinylmagne-sium bromide to the lithium enolate of (i )-carvone oxide 120. This reaction afforded compound 121 as a single isomer, albeit in only 30% isolated... 

Often the difference in odor between two such forms is surprisingly great. For example, dextro carvone is the main odor constituent of caraway oil, while its optical isomer levo carvone is typical of spearmint. Among other materials that occur in important dextro and levo forms are citronellol and rose oxide. 

The d- and /-carvones are optical isomers that exist in different oils d-carvones are found in caraway oil, whereas /-carvones are found in spearmint oil. 

The importance of the scent molecule linking to a specific receptor in the nose is thought to be related to the shape of the molecule. The optical isomers d- and /-carvone are mirror images of each other, with different 3D molecular arrangements (Fig 5.10). This means that the d- and /-forms do not fit the same receptor sites and are responsible for different aromas. The c/-carvone smells of caraway, while the /-form has a minty odour. 

The two ketones are all flavour compounds too. The first, (-)-carvone, is the chief component (70%) of spearmintoil. Carvone is an interesting compound in Chapter 16 you will meet mirror-image isomers known as enantiomers, and (-)-carvone s mirror image (+)-carvone, is the chief component (35%) of dill oil. Our taste can tell the difference, though an NMR machine can t and both carvones have identical NMR spectra. See Chapter 16 for more detailf The second ketone is raspberry ketone and is largely responsible for the flavour of... 

Optional) The major essential oil of dill is (+) carvone and the main constituent of spearmint oil is (—) carvone. These are optical isomers. Would the column used in this experiment separate these optical isomers Discuss how you could affect a separation of these two optical isomers using HPLC. 

Since initially neither the stereochemistry of the trisub-stituted double bond nor the absolute configuration of component XVI was known, we prepared all four of the possible geometric and optical isomers starting from either (J3)-(+)-carvone (Figure 6) or (R)-(-)-carvone, and only the 3Z,6R isomer was found to be attractive to the males (48). This is not a practical route to XVI a shorter synthesis of racemic XVI was subsequently published by Still and Mitra (51). 

Optical isomers of carvone have totally different flavors. The D+ form is characteristic of caraway the L- form is characteristic of spearmint. 

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