Citronellol and Citronellal

Reaction of citronellal (3.30) with diethanolamine produces the oxazolidine derivative (3.35). If this product is dissolved in 98% sulfuric acid, two things happen. Firstly, the nitrogen atom is protonated to form the hydrogen sulfate salt. This process no doubt helps the organic material to dissolve in the acid. Sulfuric acid then adds across the double bond at the far end of the molecule to give the sulfate ester (3.36) as 

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Until rather recently, our choice among chemicals repellant to insects was very limited (9), and some of the available remedies seem to have been almost equally repellent to their human users. The most familiar of them undoubtedly is the classical oil of citronella, a mixture of plant terpenes which consists principally of geraniol, citronellol, and citronellal. It is a remarkable coincidence that at least one insect species, an ant discussed by Dr. Happ, also makes use of some of these same terpenes as repellents against other insects. It biosynthesizes them de novo rather than simply taking them from plant sources. Many other examples of insect repellency have been observed (9), and Roth and Eisner (28) list over 30 compounds which have been identified as defense substances of anthropods. 

The difference between citronellol and citronellal is typical of that between an alcohol and an aldehyde. Although they are clearly related in odor, the aldehyde is, as we would expect from quite a small molecular structure, very much more powerful and harsh. In hydroxycitronellal the addition of an—OH and—H across the double bond (in effect by adding a molecule of water, a process known as hydration) produces an alcohol group near to the other end of the molecule, away from the aldehydic group. The material still maintains some of its aldehydic character but now, in addition, has some of the softness and floral character associated with an alcohol. The presence of more than... 

Plants especially from the Lamiaceae family can inhibit the growth of several weeds by releasing phytotoxic monoterpenes (a-pinene, P pinene, camphene, limonene, a-phellandrene, p-cymene, 1,8-cineole, borneol, pulegone, and camphor) (Angelini et al., 2003). The herbicide effect of 1,4-cineole and 1,8-cineole is also described by Dayan et al. (2012). Plants that are exposed to essential oils often metabolize them, and when citral was added geraniol, nerol and their acids appeared. When citronellal metabolization was tested, citronellol and citronellic acid were formed, and with pulegone (iso)-menthone, isopulegol and menthofuran were found (Dudai et al., 2000). 

The most important commercial source of natural geraniol is citronella (especially Cymbopogon winterianus) in which oil it is present at a level of 30%. The oil also contains citronellol and citronellal and the geraniol must be separated from these by... 

Bernier et al. extensively reviewed attraction-inhibitors. Included in the review were compounds produced by plants such as catnip oil (specifically the isomers of nepetalactone), geraniol, linalool, citronellol and citronellal, and human-produced compounds. In human-produced compounds, attraction-inhibition was observed for some carboxylic acids (saturated acid combinations of Cg-C,o... 

Further evidence of the difference between rhodinol and citronellol is forthcoming, in that the former yields on oxidation an aldehyde, rhodinal, whose oxime does not yield citronellic acid nitrile when treated with acetic anhydride, nor citronellic acid when the nitrile is treated with alkalis, wheras citronellal, the aldehyde of citronellol, does yield the nitrile and citronellic acid. 

Harries and Comberg have also supplied much evidence, which, taken with the above-mentioned researches, places the chemical isomerism of citronellol and rhodinol practically beyond dispute. By ozonisation experiments decomposition products were obtained, which proved that natural citronellal, obtained from citronella oil, is a mixture of about... 

According to Skita, the reaction proceeds in a different manner if the reduction be effected with palladium chloride and hydrogen. In this case the citral in alcoholic solution is mixed with an aqueous solution of palladium chloride and the whole thickened with gum-arabic. Hydrogen gas is then forced into this solution under pressure. The products of the reduction include citronellal and citronellol and a di-molecular aldehyde, C Hj O, which probably has the following constitution —... 

Citronellic acid, C Hj-. COOH, is the acid corresponding to the alcohol citronellol, and is present in the essential oil of Barosma jmlcliel-lum. It has the following characters —... 

Figure 15.7 Response of the molecular imprinted polymer quartz crystal microbalance (MIP-QCM) sensor to monoterpene analogues ( ) L-menthol, (A) D-menthol, ( ) citronellol, (A) citronellal, and (O) menthone. Reprinted from Percival et al. (2001). Copyright 2001 American Chemical Society.

Selective catalytic hydrogenation with chromium-promoted Raney nickel is reported (e.g. citral and citronellal to citronellol) NaHCr2(CO)io and KHFe(CO)4 reduction of a/3-unsaturated ketones (e.g. citral to citronellal) has been described (cf. Vol. 7, p. 7). The full paper on selective carbonyl reductions on alumina (Vol. 7, p. 7) has been published." Dehydrogenation of monoterpenoid alcohols over liquid-metal catalysts gives aldehydes and ketones in useful yields. ... 

Uses. Citronellal is used to a limited extent for perfuming soaps and detergents. Its main use is as a starting material for the production of isopulegol, citronellol, and hydroxydihydrocitronellal. 

In addition to the main component, citronellal, the oil contains citronellol and isopulegol (5-10% each) [502-504a]. 

A study with houseflies (7) shows clearly that very small differences in the molecular structure can result in drastically different biological effects as exemplified in Table III by the optical Isomers (-)-limonene, a fly attractant, and (+)-limonene, a fly deterrent. A difference in oxidation state In the functional group as in citronellol, a fly attractant, and citronellal, a fly deterrent, also causes different responses. A difference in the length of the carbon chain as in farnesol (C15), a fly attractant, and geranlol (CIO), a fly deterrent, also confers different... 

Uses and Reactions. The largest use of myrcene is for the production of the terpene alcohols nerol, geraniol, and linalool. The nerol and geraniol are further used as intermediates for the production of other latge-volume flavor and fragrance chemicals such as citronellol, dimethyloctanol, citronellal, hydroxycitronellal, racemic menthol, citral, and the ionones and methylionones. 

Olefine Terpenes.— The olefine terpenes and the alcohols and aldehydes derived from them are found in several essential oils, e.g.y oil of geranium contains geraniol and citronellol (p. 167), and oil of lemony citral and citronellal (p. 170). 

HPLC on an octadecyl column can be applied to the analysis of caffeine from tea or cola syrup (Chapter 8—use 20% methanol, 0.8% acetic acid, and 79.2% water as eluant), the acetylation of cholesterol (Chapter 10) pulegone and citronellal from citronellol (Chapter 25), cholesterol from gallstones (Chapter 22), the isolation of eugenol from cloves (Chapter 28— use 10% methanol, 5.4% acetic acid, and 84.6% water as eluant), isolation of lycopene and /3-carotene (Chapter 9), and the product obtained from enzymatic reduction of ethyl acetoacetate (Chapter 59). 

In the present experiment pyridinium chlorochromate (PCC) is used to oxidize citronellol, a constituent of rose and geranium oil, to the corresponding aldehyde. If PDC in CH2CI2 were used, the reaction would stop at this point and citronellal could be isolated in 92% yield. Or if sodium acetate (a buffer) were added to the PCC reaction, the oxidation would also stop at the aldehyde stage, in 82% yield. It has been found that PCC deposited on alumina will effect the same oxidation in less time and even higher yield. ... 

The flavour properties of the citrus oils are based on compounds like aldehydes, esters and alcohols. Aldehydes play the most important role in citrus flavours. Mainly the saturated C-8, C-10 and C-12 long-chain constituents as well as the terpenoid aldehydes citral (a mixture of neral and geranial) and citronellal form the basic notes of the fresh and pleasant citms flavour. Also sesquiterpene aldehydes and ketones, such as sinensal or nootkatone, are significant for citms flavours. The aldehyde content, therefore, is frequently used as a quality criterion for citms oils. Important citms alcohols are linalool, octanol, a-terpineol (especially in distilled lime oil) and ter-pinen-4-ol. Furthermore, esters contribute to the flavour properties of citms oils. Especially the acetates of geraniol and nerol and, to a lesser extent, citronellol are... 

B. Oxygenated 2,6-Dimethyloctanes, One Double Bond Citronellol, Citronellal, and Citronellic Acid... 

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. 

Based on the experimental infoimation we can conclude that the main routes for the hydrogenation of citral over alumina supported nickel goes through citronellal to citronellol and finally to 3,7-dimethyloctanol, whereas the contribution of nerol and geraniol to the hydrogenation path is of very minor importance. The hydrogenation kinetics was described with first order rate laws with respect to the substrate molecules [2],... 

I will now outline the details of our second synthesis of 104 published in 1990.103 The purpose of our 1990 synthesis was to provide extremely pure products. In our 1978 synthesis of 104 all the stereoisomers were pheromonally active. In that synthesis, however, we employed (R)-citronellic acid of 92% ee as our starting material (cf. Figure 4.54). In our 1990 synthesis, we chose enantiomerically pure (R)-citronellol and ethyl (R)-3-hydroxybutanoate as our starting materials to ensure the preparation of the stereoisomers of 104 in 100% enantiomerically pure forms. Bioassay of the products then unambiguously would verily the previous conclusion that all the stereoisomers are bioactive. 

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