Citronellal structures

The adjacent iodine and lactone groupings in 16 constitute the structural prerequisite, or retron, for the iodolactonization transform.15 It was anticipated that the action of iodine on unsaturated carboxylic acid 17 would induce iodolactonization16 to give iodo-lactone 16. The cis C20-C21 double bond in 17 provides a convenient opportunity for molecular simplification. In the synthetic direction, a Wittig reaction17 between the nonstabilized phosphorous ylide derived from 19 and aldehyde 18 could result in the formation of cis alkene 17. Enantiomerically pure (/ )-citronellic acid (20) and (+)-/ -hydroxyisobutyric acid (11) are readily available sources of chirality that could be converted in a straightforward manner into optically active building blocks 18 and 19, respectively. 

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). 

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... 

These regular monoterpenes constitute a small class which includes the trienes myrcene 4 and ocimenes (5-7) and the alcohols geraniol 15, nerol 16, citronel-lol 17, linalool 18, etc (Structure 4.5). 

Citral is the naturally occurring mixture of the aldehydes geranial 19 and neral 20 (Structure 4.6). Citronellal 21 is another acyclic aldehyde within this grouping. Variation of the 2,6-dimethyloctane skeleton is easily noticeable. 

As we would expect the perfume is clearly composed around a classical structure of top, middle, and base notes. Apart from all the essential elements of a chypre there is the bold use of strong materials such as citronellal (1%), the aliphatic aldehydes (1%), styrallyl acetate (4%), and styrax in an accord that gives the perfume its enormous individuality. It was also part of Carles s technique to take structural materials such as vetiveryl acetate, methyl ionone, and hydroxycitronellal and dress them up with a number of natural and synthetic ma-... 

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... 

Semiochemicals are usually compounds with structures much simpler than that of brevenal. But even with simple compounds, there are possibilities of misassigning their absolute configuration. Male-produced pheromone components of the flea beetle Aphthona flava were isolated and identified in 2001 by Bartelt etal6 They proposed himachalene-type sesquiterpene structures 2—5 (Figure 2) to the components. In 2004, Mori and coworkers synthesized 2-5 and their enantiomers ent-2—mt-S from enantiomers of citronellal, and the... 

A simple example of the examination of a proposed structure through synthesis is provided in this section. In 2005, Takita12 proposed the structure of the male-produced aggregation pheromone of the stink bug Eysarcoris lewisi as the sesquisabinene alcohol, ( )-2-methyl-6-(4 -methylenebicyclo[3.1.0]hexyl)hept-2-en-l-ol (8) (Scheme 1). Mori13 synthesized (61 )-8 and (6S)-8 from the enantiomers of citronellal (10). The key steps were the intramolecular addition of an ct-keto carbene to the alkene bond (11—>12) and ( )-selective olefination of 13 to give 14. The 1H- and 13C-NMR spectra of 8 around the trisubstituted double bond at C-2 were different from those of the natural pheromone. 

Geraniol-nerol, linalool, citronellol, citronellal and citral are five of the most important terpenes as far as the perfume industry is concerned. Apart from citral, all are used as such in perfumes, and the alcohols and their esters are particularly important. All of them are key starting materials for other terpenes, as discussed later. Scheme 4.1 shows the structures of these materials and how they can be interconverted simply by isomerization, hydrogenation and oxidation. The ability to manufacture any one of these, therefore, opens up the potential to produce all of them and, hence, a wide range of other terpenes. Obviously, if one company produces geraniol-nerol initially and another linalool and both do so at the same cost per kilogramme, then they will not be able... 

Figure 15.2 Comparison between the structures of the tetrahydropyranyl ethers of 4-t-butylphenol and c s-4-methylcyclohexanol, and those of hydroxy citronellal and Lilial . The structures have been drawn with similar orientation of the polar group to better visualize similarities in size and shape of the hydrocarbon region...

Figure 4.1 shows the structures of pheromones, which have been synthesized by us starting from (+)-tartaric acid and (+)-citronellic acid or (+)-citronellal. Malic acid, glutamic acid, serine, glyceraldehydes, limonene, carvone and others were employed as our starting materials in pheromone syntheses.7... 

S)-Diester E prepared from (.S )-citronellal was cyclized under the Dieckmann conditions to give (S)-F. Its hydrolysis and decarboxylation were followed by methylation to furnish (S)-G, whose Robinson annu-lation yielded (-)-A (= 81 ). The structure of the crystalline ketone (-)-A was solved by X-ray analysis, and then (-)-A was further converted to (-)-B, (-)-C and (—)-D. Although C (= 83 ) and D (= 84 ) possess the absolute configuration given to the dextrorotatory natural pheromone components, our synthetic C and D were levorotatory in hexane. In addition, none of them showed pheromone activity.48... 

Citronellal. 3,7-Dimethyl-b-octenal. C, 11l80. mol wt 154.24. C 77.86%, H 11.76%, O 10.37%. Chief constituent of citrondla oil also found in many other volatile oils, such as lemon, lemon grass, melissa-. Tiemann, Ber. 32, 834 (1899) Spoon. Chem. IFeekbi. S4, 236 (1958). Structure Naves, Bull Soc. Chim. France 1951, 505 Eschinazi, J. Org. Chem. 26, 3072 (1961). 

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. 

An intramolecular hetero-DiELS-ALDER reaction of a,P-unsaturated carbonyl compounds, e.g. 12 [19] derived from citronellal, results in structurally complex 3,4-dihydro-2//-pyrans, e.g. the system 13 derived from tetrahydrocannabinol (see p 269) ... 

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). 

The o-quinone methide derived from 4-hydroxypyrid-2-one and citronellal undergoes an intramolecular Diels-Alder reaction to give the chroman (5) structurally related to the free radical scavenger pyridoxatin and the antiinsectan Leporin A (94TL531). 

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