Aldehydes citronellal

Citronellol can be characterised by conversion into citronellyl-3>hthalate of silver, which is prepared in an exactly similar manner to the corresponding geranyl compound, and melts at 125° to 126°. It can also be oxidised in the same manner as geraniol, yielding the aldehyde citronellal, which can be identified as described later (vide citronellal). 

The work of Harries and Himmelmann provides considerable confirmation of the individuality of the aldehydes citronellal and rhodinal. By the action of ozone on the aldehyde, results differ materially according to the source of the citronellal, and those chemists are of opinion that the two complexes—... 

Laboratory colonies of worker ants were fed sugar-water containing l-C14-acetate, 2-C14-acetate, or in a carefully controlled simultaneous feeding, 1-C14- or 2-C14-mevalonate. After an appropriate period, the ants were frozen and extracted with methylene chloride and the terpene aldehydes (citronellal and citral) isolated by thin-layer chromatography. These were then converted into their dini-... 

The malonic ester anion performs a nucleophilic attack on the aldehyde citronellal (9). 

This process represents a Knoevencigel condensation,3 a reaction in which a compound with an acidic methylene group, such as dimethyl malonate (8). condenses with a carbonyl compound like citronellal (9) to give an alkene. Reaction occurs in a weakly basic or neutral medium. Catalytic amounts of piperidinium acetate suffice to deprotonate malonic ester 8. The resulting anion 15 adds to the aldehyde citronellal (9) to give alkoxide 16. 

As described above, hydrolysis of the optically active enamine 3 proceeds without racemization and produces an optically active aldehyde, citronellal, with a very high optical purity (>98% ee). The optical purity of citronellal) available from natural sources is known to be no more than 80% ee [5], The present asymmetric isomerization of the allylamine 1 is utilized as the key step for the industrial production of (-)-menthol (Scheme 3.3). 

Figure 3.8 Aldehydes. Citronellal (C H180), monoterpene aldehyde. Courtesy Spiring Enterprises Ltd.

Before leaving asymmetric hydrogenation reactions, we should mention one other related process that has acquired immense importance, again because of its industrial application. You have come across cit-ronellol a couple of times in this chapter already the corresponding aldehyde citronellal is even more important because it is an intermediate in the a synthesis of L-menthol by the Japanese chemical company Takasago. Takasago manufacture about 30% of the 3500 ton annual worldwide demand for L-menthol from citronellal by using an intramolecular ene reaction (a cycloaddition you met in Chapter 35). 

The chiraUy modified cluster HRu3(CO)10(i -OCNCH2CH2CH2CHCH2 OCH3) catalyzes the conversion of the prochiral allylic alcohol nerol (78) into the chiral aldehyde citronellal (79)... 

Molecular structures of monoterpenoids are present as three major types acyclic, monocyclic and bicyclic. Oxygenated derivatives of acyclic monoterpenes are more widespread in nature than acyclic monoterpenes themselves. Such derivatives are the monoterpene alcohols citronellol and geraniol, or the monoterpene aldehydes citronellal and geranial. Mononocyclic monoterpene hydrocarbons are exemplified by limonene, which is a major component of orange and lemon peel oils, and p-phellandrene, which is emitted by conifer trees under biotic stress. These compounds are relatively common in nature. Bicyclic monoterpenes are represented by the aforementioned a-pinene, one of the most abundant and important monoterpenes. 

The principal constituents of the oil are limonene and citral, together with small quantities of pinene, phellandrene, octyl and nonyl aldehydes, citronellal, geraniol, geranyl acetate, and the stearopten, citraptene. 

Apart from certain carbohydrates, the most inexpensive source of chiral compounds is the terpenes. These are readily obtained from plant sources and encompass examples of many important functional groups These include alcohols such as (+)-menthol (22) and (-)-bomeol (23), ketones such as (+)-camphor (24), (+)-pulegone (25), (-)-menthone (26) and (-)-carvone (27), the aldehyde (+)-citronellal (28), (+)-camphor-10-sulphonic acid (29), and alkenes such as (+)-limonene (30) and (+)-a-pinene (31). (a)-Pinene provides a good illustration of the fact that naturally derived chiral compounds are not necessarily enantiomerically pure. Both enantiomers are readily available but the normal samples are only of around 90% e.e. Fortunately this is not a serious problem since procedures have been... 

Analysis agreed with the formula CgH g or CgH, . It is probably octy-lene. Pinene appears to be present in traces, as well as camphene, phellandrene, and y-terpinene. The bulk of the oil, however, consists of d-limonene which is present to the extent of about 90 per cent. Methyl-heptenone, octyl aldehyde, nonyl aldehyde, citronellal, terpineol, linalyl acetate, geranyl acetate, bisabolene, cadinene and methyl anthranilate have all been found present in traces. 

The mechanisms of the reactions of the cluster Ru3(CO)i2 with halide ions, alkoxide ions and amines, all of which involve initial rapid nucleophilic addition at a carbonyl hgand, have been reviewed.In a related study, addition of 5-proline methylester or 5-methoxymethyl pyrrolidine to a carbonyl ligand of Ru3(CO)j2 has yielded chiral carbamoyl clusters of the type (84) R = C02Me or CH20Me, Eq. (16). Such chiral clusters may have potential as new enantioselective catalysts, as shown by the observation that cluster (84), R = CH20Me) catalyzes the isomerization of the prochiral allylic alcohol nerol to give the chiral aldehyde citronellal with an enantiomeric excess of 12%. 

Further, d-citronellal, the corresponding aldehyde, may be converted into citronellic acid through its oxime and nitrile. Citronellic acid, when treated with thionyl chloride in benzene solution, yields a chloride of a chlorinated acid which is converted by the action of alcohol into the hydrochloride of ethyl citronellate, or hydrochloride of ethyl rhodinate,... 

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. 

The aldehydes of the geraniol series are of very great commercial importance. The only two which are of common occurrence are citral and citronellal. 

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 —... 

Schauwecker, that when citronellal acetal is oxidised by permanganate, there is formed the acetal of the semi-aldehyde of j8-methyladipic acid and a glycol. The indistinct melting-points observed in citronellal derivatives (for example in the semi-carbazone) can also thereby be explained. It would appear that commercial citronellal contains the two isomeric forms in varying proportions. 

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. 

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. 

The synthetic problem is now reduced to the development of a feasible, large-scale preparation of enantiomerically pure (/ )-citro-nellal (36), which has a single stereogenic center. One way in which the aldehyde function in 36 could be introduced is through the hydrolysis of a terminal enamine. (/ )-CitronelIal (36) can thus be traced to citronellal ( )-diethylenamine (44), the projected product of an enantioselective isomerization of prochiral diethylgera-... 

Isolation of Citronellal and Citral. At the close of each experiment (7 to 10 days), the nests were frozen intact. Groups of 200 workers were placed in a micro-Soxhlet apparatus and extracted for 8 hours with methylene chloride. A few milligrams of carrier citronellal and citral were added and the mixture was applied to a thin-layer chromatoplate (silica gel G) which was developed with hexane-ethyl acetate (92 to 8) to separate citronellal and citral (3). The aldehydes were detected by spraying with a solution of 2, 4-dini-trophenylhydrazine in tetrahydrofuran (20) and the citronellal and citral peaks were scraped off and allowed to react with excess dinitro-phenylhydrazine reagent for a further 12 hours. 

Biochemisch werden Aldehyde i. a. zu den entsprechenden Alkoholen reduziert, wobei sie gelegentlich der mikrobiologischen Cannizzaro-Disproportionierung zu Carbonsaure und Alkohol unterliegen konnen [z. B. 3,7-Dimethyl-octen-(6)-al (Citronellal) zu 8-Hydroxy -2,6-dimethyl-octen-(2) und 3,7-Dimethyl-octen-(6)-sdure]1. 

CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Chrysene, 58,15, 16 fzans-Cinnamaldehyde, 57, 85 Cinnamaldehyde dimethylacetal, 57, 84 Cinnamyl alcohol, 56,105 58, 9 2-Cinnamylthio-2-thiazoline, 56, 82 Citric acid, 58,43 Citronellal, 58, 107, 112 Cleavage of methyl ethers with iodotri-methylsilane, 59, 35 Cobalt(II) acetylacetonate, 57, 13 Conjugate addition of aryl aldehydes, 59, 53 Copper (I) bromide, 58, 52, 54, 56 59,123 COPPER CATALYZED ARYLATION OF /3-DlCARBONYL COMPOUNDS, 58, 52 Copper (I) chloride, 57, 34 Copper (II) chloride, 56, 10 Copper(I) iodide, 55, 105, 123, 124 Copper(I) oxide, 59, 206 Copper(ll) oxide, 56, 10 Copper salts of carboxylic acids, 59, 127 Copper(l) thiophenoxide, 55, 123 59, 210 Copper(l) trifluoromethanesulfonate, 59, 202... 

As aldehydes, commercially available rac-citronellal and a synthesized aromatic aldehyde and also two commercially available 1,3-diketones, 1,3-dimethylbarbituric acid and Meldrum s acid, were selected [18]. By 2 x 2 combinations of these reactants, four different cycloadducts were generated ([OS 57]). 

P 39] Protocol for single-run processing reservoirs of the 110 Caliper chip were filled with the following solutions [18] 0.10 M solutions of rac-citronellal and a synthesized aromatic aldehyde in methanol/water (80 20) and 0.12 M solutions of 1,3-dimethylbarbituric acid and Mel drum s acid in methanol/water (80 20) with 10% molar catalyst efhylenediamine diacetate (EDDA). 

Robles and Bochet showed that nitrotoluene derivatives 106 (Scheme 50) can be used as a photoremovable protecting group for aldehydes. Irradiation of 106 released aldehydes in good yields, and the authors demonstrated that aldehydes such as phenylacetalaldehyde and citronellal, which are important for flavor and fragrances, are released efficiently from 106. 

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. ... 

The hydrogenation of Claisen rearrangement products (R)-(E)- and (S)-(Z)-3,7-dimethyl-4-octenal [obtained highly selectively from (7 )-( )-6-methyl-4-vinyloxy-2-heptene, see p421] to give (S)- and (7 )-3,7-dimethyloctanal, respectively. Authentic samples of these aldehydes were obtained from (5)- and (7 )-citronellal 14°. 

---