Citronellal determination

Figure 26.8 traces the label from its origin in acetic acid to its experimentally determined distribution in citronellal. 

Acetylation.—Gitronellal may be quantitatively estimated by the ordinary acetylation process i when the aldehyde is quantitatively converted into isopulegyl acetate, which is then determined by saponification with potash in the ordinary way. Dupont and Labaume have attempted to base a method for the separation of geraniol from citronellal in citron-ella oils on the fact that the citronellal oxime formed by shaking with hydroxylamine solution at the ordinary temperature is not converted into an ester by subsequent acetylation, but into the nitrile of citronellic acid which is stable towards" alkali during the saponification process. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

More recendy the cis and trans isomers of the mosquito repellent CIC-4, a mixture of citronella isomers, have been separated by preparative hplc and bioassayed for effectiveness (23). Chiral-phase capillary gas chromatography and mosquito repellent activity of some oxazolidine derivatives of (+)-and (— )-citronellal have been studied to find structure—activity relationships (24). Several 2-alkyl- -acetyloxalidines have been synthesized and tested against mosquitoes, with further efforts using nmr to determine the rotational isomers of the more active N-acetyl-2,2-dimethyloxazolidine (25). 

R,S)-Citronellal can be purchased from BASF, and (R)-citronellal from Dragoco, Fluka, or Takasgo Perfumery Co., Ltd., Japan. (R)-Citronellal can also be synthesized from pulegone with ee >99%.5 (S)-Citronellal may be obtained by oxidation of (S)-citronellol,6 which is accessible by different routes with ee 95%.7 The optical purity of citronellal can be determined by GLC after conversion to the acetal of (-)-(2R,4R)-pentanediol.8 For the reactions described, (R,S)-citronellal from BASF, (R)-citronellal from Dragoco, and (S)-citronellol from Fluka were used. (R,S)-Citronellal... 

The determination of aldehydes and ketones is of importance in the analysis of those essential oils characterised especially by aldehydic or ketoruc principles, e g.,the citral contained in lemon and lemongrass oils, citronellal in citronella Oil and some eucalyptus oils, benzaldehyde in bitter almond oil, salicylaldehyde in meadow-sweet oil, anisaldehyde in aniseed and fennel oils, cuminaldehyde m cumin oil, cinnamaldehyde in cinnamon oil, carvone in caraway oil, pulegone in pennyroyal oil and methyl nonyl ketone in rue oil The determination of the aldehydes and ketones presents, however, difficulties and the above methods are moderately exact in only a few cases, especially when the content of aldehydes or ketones is considerable The bisulphite method is applicable particularly to the determination of cinnamaldehyde and benzaldehyde in cinnamon oil and bitter almond oil, and, up to a certain pomt, to that of citral in lemongrass Oil. The sulphite method gives good results in the same cases and for the determination of carvone and pulegone... 

Two possibilities can be distinguished with compound 10 for an exo-E-anti transition state the methyl group might occupy either a pseudoequatorial or a pseudoaxial position. Product 11a arises from the energetically more favorable state 20a, whereas lib is derived from transition state 20b. It is in this way that the methyl group of citronellal (9) determines the ratio of 20a to 20b. 

By comparing the relative volatilities of various components, the ease of separation between these components can be determined. For example, a key terpene-oxy separation involves terpinolene with a relative volatility with respect to limonene of 0.7 and citronellal with relative volatility with respect to limonene of 0.4. The relative volatility of terpinolene to citronellal is then 0.7/0.4 or 2. 

Rate constants and activation energies for liquid- and gas-phase isomerization of a-pinene have been determined.310 The activity of metal sulphate monohydrates in isomerizing a-pinene is correlated with the strength of co-ordination of the water of crystallization to the metal ion.3" Pyrolysis of chrysanthanol acetate (217 R = Ac) gives citronellal and the (E)- and (Z)-3,7-dimethylocta-l, 6-dien-l -ol acetates in 20, 28, and 3% yields respectively formation of the enol acetates is consistent with a biradical or a concerted pathway.312 Further work directed towards C-l—C-7 bond pyrolysis of pinane derivatives shows C-l—C-7 C-l—C-6 bond cleavage ratios of 4 51 for (217 R = Ac), 13 22 for (217 R = H), 6 7 for (218 R = H), and 43 35 for (218 R = Me) the expected acyclic and cyclic alcohol, aldehyde, and ketone pyrolysis products are obtained.313 The ene reaction between /3-pinene and methyl... 

The same dilution technique is usually applied in the analysis of ethereal oils [210]. It can be used in the determination of aldehydes such as citral and citronellal in lemon oil (Oleum citri) and in Oleum verbenae, of anisaldehyde in Oleum anisi stellati. Oleum anisi and Oleum foeniculi, of cinnamaldehyde in Oleum cinnamomi, and of ketones, such as car-vone, in Oleum anethi, of pulegone and diosphenol. In all these cases, the oil is first diluted with ethanol and to this solution an aqueous solution of the supporting electrolyte is added, usually only 0.1 M LiCl. 

The product Is very moisture sensitive and should be handled under dry nitrogen or argon. Despite this precaution, the product 1s always contaminated by small amounts of (+)-c1trone11a1, which has an optical rotation opposite to that of the enamlne. To determine the optical purity of the product enamlne, the specific rotation measured therefore must be corrected for the (+)-c1tronellal Impurity. It is more reliable to base optical purity on the specific rotation of the citronellal obtained by hydrolysis of the enamlne (Part C). The absolute method using HPLC of the diastereomeric amide derivative also may be useful as a check of the optical purity. 

We then synthesized (+)-81, (+)-82, (+)-83 and (+)-84 starting from (R)-citronellal (Figure 4.24).47 They were all pheromonally active 48 We were confident about our stereochemical assignment, because the stereochemistry of (lR,2S)-(-)-A (81 ) was unambiguously determined by X-ray analysis. 

Optical yields were assessed by transforming the hydrolyzed product (citronellal) via isopulegol into menthol which serves as a reliable reference substance for determining optical purity by specific rotation. 

Compounds with similar carbon skeletons but different oxygen-containing functional groups were tested (Table 4). Citronellic acid (69) was the most active, however, no other trends could be determined from the remaining data. 

The other two chiral centres of menthol are formed in the ene reaction when citronellal is cyclised to isopulegol in the presence of zinc chloride. The ene reaction is a 6-centre reaction. As the reaction starts, the six atoms involved come together in a ring and the shape of this ring determines the stereochemistry of the product. This is shown in Figure... 

To illustrate the approach to the identification of unknown materials by using infrared spectroscopy, we will work through an example. Citronellal is the terpenoid responsible for the characteristic aroma of lemon oil, and is used in perfumes and also as a mosquito repellent. The infrared spectrum of citronellal is shown in Figure 4.5a. We can use this spectrum to determine the molecular structure of this compound. 

The structural elucidation of dihydroxyserrulatic acid (120) required evidence for the tetralin ring and the relative arrangement of the substituents around it. To this end, the dimethoxy derivative was deoxygenated at Cl6 and the resulting compound was dehydrogenated with DDQ to provide a naphthalene (121). The orientation of substituents on the aromatic ring and the stereochemistry of the side chain double bond was inferred from spectroscopic analysis. Confirmation of the structure and the relative stereochemistry of the three asymmetric carbons was obtained by an X-ray diffraction study of 120. The absolute stereochemistry was determined by conversion of the methoxy naphthalene (121) to 5-(-)-4-(l, 5 -dimethylhexyl)-l,6-dimethylnaphthalene (122) which proved identical to, except for the sign of optical rotation, a synthetic sample of the / -enantiomer (123) prepared from / -citronellal (Scheme 29) (102). 

---