Citronellol enantiomers

The odor quality and strength of the two citronellol enantiomers were found to be different.The (35)(—)-enantiomer (Rhodinol) possesses a much finer rose odor than the (3i )(+)-enantiomer. (35)(-)-Citronellol has a sweat, peach-like flavor, while (3i )(- -)-citronellol has a bitter taste. (35 )(—)-Citronellol has been found in a number of geranium and rose oils. (3i )(+)-Citronellol occurs in the oils of Ceylon and Java citronella, Cymhopogon winttrianusff Boronia citriodora. Eucalyptus citriodoraff geranium, Spanish verbena and others. Because of the... 

FIGURE 7.4 Representation of the mass spectra similarity of p-citronellol enantiomers. 

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. 

Stereospecific syntheses of the eight stereoisomers [94] used a variation of the methodology developed in Mori s previous synthesis of the stereoisomers of 6,10,13-trimethyltetradecanol (see Scheme 5) [91], using chiral synthons derived from commercially available enantiomers of citronellol and methyl 3-hydroxy-2-methylpropanoate, and the iterative series of steps outlined in Scheme 13A for one of the stereoisomers [94]. A key step involved moving the... 

In many natural products citronellol occurs as a mixture of its two enantiomers the pure (+) or (-) form is seldom found. (+)-Citronellol dominates in oils from Boronia citriodora (total citronellol content ca. 80%) and Eucalyptus citriodora (citronellol content 15-20%). (-)-Citronellol is the predominant enantiomer in geranium and rose oils, both of which may contain up to 50% citronellols. 

Fontes et al. (1998b) studied the enantioselectivity of cutinase and found that it was very selective toward one enantiomer with an enantiomeric excess of almost 100%. They found that the enantioselectivity was very sensitive to changes in water content. Bornscheuer et al. (1992) studied hydrolysis, esterification, and transesterification in carbon dioxide to try to find the best method for producing enantiomerically pure substances in carbon dioxide. They found that the thermodynamically favored hydrolysis led to higher enantiomeric excess with less enzyme in the shortest time. Michor et al. (1996b) also examined more than one system to determine a better route to product and found that while the transesterification of -menthol was fast and resulted in high enantiomeric excess, resolution of -citronellol was not feasible. The reaction rate for the reaction of -citronellol was 10-20 times of that of -menthol, but was not selective. 

Lesser tea tortrix. A minor component of the sex pheromone blend of this moth is 10-methyl-l-dodecanol acetate, V, (Figure 10) (48). Mori has synthesized the enantiomers both compounds were constructed from (R)-(+)-citronellol (49). The racemic target compound was synthesized by alkylating 10-undecenoic acid (as its dianion) with ethyl iodide (50). Reduction of the carboxyl to a methyl group was accomplished by standard procedures. Hydroboration of the olefinic link with disiamylborane and oxidative workup yielded the primary alcohol which was then acetylated to give the racemic pheromone structure. The overall yield from undecenoic acid was 40-45% without any extensive effort to optimize. 

The molecules we have seen so far have usually been incorporated into the target molecule complete. There are two further and most important groups of larger molecules, the sugars and the terpenes, from which pieces are usually snipped out for incorporation. The simple monoterpenes (C10 compounds) citronellol 75, citronellal 76, and citronellic acid 77 are good examples. They are not cheap but both enantiomers are available, not always in excellent ee. 

Reduction of geraniol 28 shows remarkable chemoselectivity only the allylic alcohol is reduced to give citronellol 29 of higher enantiomeric excess than that found naturally. The catalyst is used at 7 mol% that means 3 mg/g of geraniol. The catalyst must have acetate as the counterion or it is not so chemoselective.10 You will see an alternative synthesis of the other enantiomer of citronellal shortly. 

The present asymmetric technology has enabled the manufacturing of enantiomeric pairs of aroma chemicals that is a strategically powerful means in the fragrance business. Both enantiomers of citronellol 16 (Fig. 2) are precious fragrances inaccessible before this technology. We are supplying a pair of isomers... 

Other olefins lacking a carboxylic or amido fimctionality give less satisfactory optical results. The Ru complexes of atropisomeric diphosphines circumvent this problem, at least with prochiral ally lie alcohol. Either enantiomer of citronellol is obtained in 96-99% e.e. starting from geraniol or nerol ° . 

The yeast hydrogenation of a-methyl-a,/(-unsaturated aldehydes which already contain one stereogenic center leads to synthons with two stereogenic centers36. These aldehydes were prepared by selenium dioxide oxidation of (+ )-(/ )-citronellol obtained from yeast reduction (as shown above)34 or of its enantiomer (commercially available). 

Allylic alcohols make good substrates for ruthenium/BINAP-catalysed hydrogenation. Geraniol (2.81) and nerol (2.82) are (E) and (Z) isomers, and these substrates afforded opposite enantiomers of the product citronellol (2.83). 

ISO standard 3849 shows character and data for the Sri Lanka-type oil and ISO standard 3848 for the Java-type oil. Synthetic citronellal, citronellol, and geraniol were used. In addition citronella oil terpenes were used to cover such adulterations. Lawrence (1996a) mentions the ratio of citronellal in Java type with R- +) enantiomer is 90%. Casabianca (1996) found chiral ratio of (/ )-(+)-citronellol and (5)-(-)-citronellol to be 75,0%-79.0% and 21.0%-25.0%, respectively. 

Examples of the use of these chiral building blocks in the synthesis of drugs and natural products abound in the literature. They include the synthesis of grahamimycin the macrocyclic component of the antibiotic elaiophylin, fungicides norpyrenophorin, pyrenophorin and vermiculin, phorcantholide I and J, which are defensive secretions of the eucarypt longicorn beetle,citronellol and a pheromone of the western corn rootworm. Clearly the monomeric chiral derivatives of PHB have a role to play in enantiomer-specific synthesis. 

The role of fragrances since ancient times has been to cover unpleasant smell and to provide a pleasing impression (e.g., fmity, floral, marine etc.). Fragrance and flavor raw materials are obtained either from natural sources (e.g., terpenes, plant essential oils, animal secretions or from chemical synthesis. As the enantiomers of many odorant molecules differ in strength and in odor/taste description, the selective (and often catalytic) synthesis of the more appreciated isomer is of great interest. This avoids the dilution effect by the non-desired isomer and reduces the amount of active ingredient in the final product. Table 5.3.19 shows the example of (Sj-citronellol, an important perfumery raw material with a rose note. 

Optically pure (5)-(+)-citronellol from citroneUa oil has a specific rotation of+5.3°. An enantiomer of optically pure (5)-(+)-citroneUol is obtained from geranium oil. What is its specific rotation ... 

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