Citral oxidation

Influence of Individual Components of Essential Oils and Flavorings on Citral Oxidation... 

Keywords antioxidant activity, citral oxidation, linalool, lemonene, octyl acetate, anise aldehyde, vanillin, eugenol, capillary gas chromatography. 

Figure 2. Influence of lemonene on citral oxidation A - control solution, B - individual lemonene, C -lemon essential oil. (1 - neral, 2 - geranial).

We have tried to study the influence of antioxidant concentration in the samples, containing anise aldehyde and vanillin. Figure 1 (samples 4-7) demonstrates that anise aldehyde possesed AO properties only at relatively high concentration (sample 6 and 7), but at low concentration it was oxidized and didn t inhibit citral oxidation as compared with control sample. We observed another situation in the case of vanillin. This compound is a component of numerous flavorings, which are used in food and perfume industry. That is why studying of it s AO properties is of special interest. In fact, the data obtained by us (Figure 1,... 

Most terpene-based citral (5) produced is based on the catalytic oxidative dehydrogenation of nerol (47) and geraniol (48), or by the Oppenauer oxidation of nerol and geraniol (123—125). 

Citral is prepared starting from isobutene and formaldehyde to yield the important C intermediate 3-methylbut-3-enol (29). Pd-cataly2ed isomeri2ation affords 3-methylbut-2-enol (30). The second C unit of citral is derived from oxidation of (30) to yield 3-methylbut-2-enal (31). Coupling of these two fragments produces the dienol ether (32) and this is followed by an elegant double Cope rearrangement (21) (Fig. 6). 

Oxidations and reductions are amongst the most frequent in situ prechromato-graphic reactions they were exploited as early as 1953 by Miller and Kirchner [9]. They characterized citral as an aldehyde by oxidizing it to geranic acid and reducing it to geraniol. Further examples are listed in Table 10. 

Apopinol, CjoHjgO, is an alcohol, which has been identified in a Japanese essential oil by Keimaza. It yields citral on oxidation, and it is not certain that it is in fact a chemical individual, being, possibly, an impure form of linalol. 

On oxidation by weak oxidising agents, citral yields geranic acid, CjjHjgOj on reduction it yields geraniol. 

Hydrogenation reactions, particularly for the manufacture of fine chemicals, prevail in the research of three-phase processes. Examples are hydrogenation of citral (selectivity > 80% [86-88]) and 2-butyne-l,4-diol (conversion > 80% and selectivity > 97% [89]). Eor Pt/ACE the yield to n-sorbitol in hydrogenation of D-glucose exceeded 99.5% [90]. Water denitrification via hydrogenation of nitrites and nitrates was extensively studied using fiber-based catalysts [91-95]. An attempt to use fiber-structured catalysts for wet air oxidation of organics (4-nitrophenol as a model compound) in water was successful. TOC removal up to 90% was achieved [96]. 

Figure 13.3. Hydrogenation of CAL over Ir catalysts supported over mixed oxides (a.) furfural, (b.) cinnamaldehyde, (c.) citral.

In preparation for scale-up of the strigol synthesis described by Sih (8), efforts were made to improve the yield of some of the seven steps involved in the scheme. Of these steps, nine are satisfactory from the standpoint of yield and experimental conditions. For three of the steps, we have improved the yield and/or experimental conditions such that the yield of (+ )-strigol would be raised to 2.85% overall from citral rather than 1.53% based on Sih s procedure and reported yields. Improvements were developed preparation of a-cyclocitral (III), the oxidation of the hydroxyaldehyde (V) to the ketoacid (VII), and for the preparation of the hydroxybutenolide (XVII). For the remaining five steps, our attempts to change experimental conditions have failed to improve, and in most cases to even obtain, the yields reported in the literature (8). We have considered the preparation of strigol analogs and determined the conditions and limitations for the preparation of a series of alkoxybutenolides (XVI) and a butenolide dimer (XVIII). Modification of the literature procedure (11) to eliminate the use of the mesylate (XX) and the use of polar aprotic solvents gave better yields of the 2-RAS (XXI). 

Table 3 indicates that 5%Pt,l%Bi/C is active for three reaction cycles in the selective oxidation of the chosen alcohols. For primary alcohols the use of water as solvent can promote the aldehyde to carboxylic acid reaction (3). This effect is observed in the selective oxidation of 1-octanol where octanoic acid is formed with 97% selectivity in the first cycle dropping to 81% in the third. In the selective oxidation of geraniol only citral is observed as the oxidation product. The presence of the double bond stabilises the aldehyde even in the presence of... 

Example (t) Citral reacts with 30% H202 in the presence of UV-light for a duration 10 minutes and undergoes catalytic oxidation to yield geranic acid as shown below ... 

Furthermore, the amount of ligand present in the reaction system must be higher than expected from stoichiometry, since the ligand is partly oxidized during the formation of the catalyst [10]. The main substrates investigated in this work were prenal (3-methylcrotonaldehyde) and citral (Fig. 5), although the transferability of the catalyst system to other o, j0-unsaturated aldehydes was also checked. 

Methods for oxidative transformations continue to receive attention. Nickel peroxide on graphite oxidizes geraniol to citral in 89% yield. Three groups report " the oxidative rearrangement of tertiary vinyl carbinols. Linalool is con-... 

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