Aroma enantiomeric excess

IR-l-0cten-3-ol (Matsutake alcohol). CgH, 0, Mr 128.21, bp. 175 °C, [alg -20.2° LDjo (rat p.o.) 340 mg/kg. A typical mushrooms aroma constituent (olfactory threshold 1 ppb ) with an earthy-fungal odor impact compound in mushroom and Camem-bert flavor (see cheese flavor), but also occurring in many essential oils and flavors, e.g., in lavender, peppermint, rosemary, and thyme oil, as well as seafood flavors. Only the (/ )-enantiomer occurring in 90-97% enantiomeric excess possesses the pure fungus odor, the (S)-compound has a more vegetable-like odor. 

The production of new materials for the aroma and fragrance industry was the powerful driving force in the research on the hydroxylation of terpenes [924, 1105-1107]. For instance, 1,4-cineole, a major constituent of eucalyptus oil, was regioselectively hydroxylated by Streptomyces griseus to give 8-hydroxycineole as the major product along with minor amounts of exo- and cnJo-2-hydroxy derivatives, with low optical purity (Scheme 2.151) [1108]. On the other hand, when Bacillus cereus was used, (2R)-exo- and (2/ )-en o-hydroxycineoles were exclusively formed in a ratio of 7 1, both in excellent enantiomeric excess [1109]. 

As demonstrated by Mosandl and his group [90], Es-GC-0 is a valid tool for the simultaneous stereodifferentiation and olfactive evaluation of the volatile optically active components present in essential oils. It is worthwhile to point out that the preponderance of one of the enantiomers, defined by the enantiomeric excess, results in a characteristic aroma [91], and is of great importance for the olfactive characterization of the sample. 

In the case of chiral aroma substances, elucidation of the absolute configuration and determination of the enantiomeric ratio, which is usually given as the enantiomeric excess (ee), are of especial interest because the enantiomers of a compound can differ considerably in their odor quality and threshold. The compound 3a,4,5,7a-... 

Table 5.13. Enantiomeric excess (ee) of chiral aroma substances in some foods...

Chiral compounds are frequently foimd among the flavor volatiles of fruits and, like many naturally occurring chiral compounds, one enantiomer usually exists with a greater preponderance when compared with its antipode. Chiral odor compounds may show qualitative and quantitative differences in their odor properties (7). For example, (/ )-(+)-limonene has an orange-like aroma while (5)-(-)-limonene is turpentine-like (5)-(+)-carvone is characteristic of caraway while its enantiomer has a spearmint odor (2). However, other chiral compounds, such as y 6-lactones, show very little enantioselectivity of odor perception (7). The occurrence of chiral flavor compounds in enantiomeric excess provides the analyst with a means of authenticating natural flavorings, essential oils, and other plant extracts. The advent of cyclodextrin-based gas chromatography stationary phases has resulted in considerable activity in the analysis of chiral compounds in flavor extracts of fruits, spices and other plants (i-7). 

Enantiopure cis- and trans-linalool oxides are found in several plants and fruits and constitute the main aroma components of oolong and black tea. These compounds were prepared from 2,3-epoxylinalyl acetate (Fig. 11.2-17) 119. The key step consists of a separation of the diastereomeric mixture of the starting epoxide by employing an epoxide hydrolase preparation derived from Rhodococcus sp. NCIMB 11216, which furnished the product diol and remaining epoxide in excellent diastereomeric excess (de >98%). Further follow-up chemistry gave both linalool oxide isomers on a preparative scale in excellent diastereomeric and enantiomeric purities. 

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