Carvone, chirality structure

Equation (81)), while the other two C=C double bonds in the structure are intact. Under the same reaction conditions, the racemic carvone is also resolved kinetically with a KR/KS ratio of 33 1. Asymmetric hydrogenation of a,/Tacetylenic ketones to chiral propargylic alcohols is still unavailable. 

A useful synthesis (ref.ll)of patchouli alcohol, an important fragrant constituent of patchouli oil, from (+)-camphor, that onetime important natural product which was employed as a plasticiser for nitrocellulose (itself a semi-synthetic polymer), was complicated by structural revision of the sesquiterpene alcohol. Dihydrocarvone (14) obtained by saturation of the ring double bond in carvone, a major constituent of oil of spearmint has been employed for two very different sesquiterpenes, the ketone campherenone (15) and the alcohol, occidentalol (16). In the first case an enol acetate was converted to a bicyclic intermediate by earlier established methodology and the route emulated a plausible biogenetic sequence giving racemic campherenone (ref.12) as shown. Any chirality in (14) is apparently lost. 

For more conformationally-constrained chiral substrates, however, diastereoselectivity can be expected to be good to excellent. Lithium enolates derived from sterically unencumbered cyclohexanones undergo preferential axial acylation as illustrated by the reductive acylation of (R)-(-)-carvone 4 to afford a 3 1 mixture of esters 5 and 6. whereas equatorial acylation is favored in compounds that possess an alkyl substituent in a 1,3-syn-axial relationship to the reacting center, as in the conversion of tricyclic enone 7 to ester 8 (epimeric with the product from the more traditional sequence of acylation followed by alkylation). (In substrates of this kind it is assumed that the transition state structure is based on a twist-boat conformation which permits the reagent to approach along an axial-like trajectory on the less encumbered, lower face of the substrate.) ... 

Using the Cahn-Ingold-Prelog sequence rules, assign priorities to the groups aroimd the chiral carbon in carvone. Draw the structural formulas for (-1-)- and (-)-carvone with the molecules oriented in the correct position to show the R and S configurations. 

Following are structural formulas for the enantiomers of carvone. Each has a distinctive odor characteristic of the source from which it is isolated. Assign an or S configuration to the single chiral center in each enantiomer. Why do they smell different when they are so similar in structure ... 

Since the reaction is taking place within the (chiral) active site of the enzyme, the reaction mechanism is stereospecific, the addition of H2 occurring with antistereochemistry. There is only one exception reported, the syn-hydrogenation of verbenone, carvone, and cyclohex-2-enone catalyzed by OYE of Nicotiana tabacum (Scheme 2.3) [12]. However, the aforementioned tremendous advances in molecular biology and biotechnology will facilitate recombinant expression and synthetic use of these novel biocatalysts by protein engineering. In this respect, recent examples already report structure-driven mutagenesis, which successfully improved reaction specificity or enantioselectivity [13,14]. 

As we saw with the early measurements of Biot, natural sources of chiral material such as lemon oil are complex mixtures. We will see shortly that not only are there differences in the smell of many enantiomers there are also large differences in threshold sensitivity, that is, the concentration in which the substance can be detected by the nose. So you might have a 99.9% pure sample, but the dominant aroma may be from the 0.1% impurity. It wasn t until the midtwentieth century that chemists were able to prepare "pure" compounds of enantiomers and then verify the different smells. One of the first examples of such compounds are P-(—)-carvone and S-(—)-carvone, which were studied by three different research teams and published independently in 1971 [5-7]. The structures of these enantiomers are shown in Figure 4.3. It is important to remember that these two compounds have identical physical properties unless they are being analyzed or reacted with... 

Our noses can obviously be very sensitive detectors of chirality, but there are few examples as obvious as the enantiomers of carvone. Perusal of the Leffingwell database illustrates this point as, the terms used to describe the different enantiomeric smells are often very similar. It has been shown that animal species other than humans can demonstrate chiral selectivity better than humans can. This is to be expected because of the greater importance of this sense to these animals. Trained human sniffers are better able to discriminate between subtle differences in smell than most of us are, and it is not surprising that much of the current research in this area is associated with the perfume industry. Some of the impetus for obtaining more information concerning the structure-smell relationships is, of course, driven by economic considerations and market competition among perfume producers. However, there is also increasing awareness and concern for the environ-... 

Nature is the world-leading chemist in synthesizing chiral enantiopure substances, and a vast variety of structures isolated from plant or animal sources are available for the synthetic chemist to use as starting materials. Examples of chiral synthons from nature are amino acids, carbohydrates, hydroxy acids, terpenes, alkaloids, and so on (Figure 1.44). The most representative among them are commercially available compounds, such as ascorbic acid, (+)-calcium panthotenate, (—)-carvone, dextrose, ephedrine hydrochloride, (+)-limonene, L-lysine, mannitol, monosodium glutamate, norephedrine hydrochloride, quinidine, quinine, sorbitol, and L-treonine. The chiral pool strategy uses chiral compounds from nature or products derived thereof (e.g., from fermentation processes). Examples of industrial... 

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