Oxidation of thymol

Excellent reviews on dioxirane-mediated oxidations have appeared. One of the most eharacteristic points is that dioxiranes can be applied to the epoxidation of labile olefins such as enol ethers, enol acrylates, allenes and others. Dioxiranes have also been utilized for phenolic oxidation, but in relatively rare cases. Oxidation of simple phenols and anisoles with dimethyldioxirane (544) provided only a complex mixture, so that hindered phenols are more favorable. On treatment with dimethyldioxirane (4 equiv.) in acetone, 2,4-di(terf-butyl)phenol (216) was oxidized to afford in 79% yield the corresponding o-benzoquinone 220, which reacted with 544 and aq. NaHS03 to give catechol 545. Dimethyldioxirane-promoted oxidation of 545 provided again a quantitative yield of 220. Further oxidation of 220 produced a 52% yield of two epoxides 546 and 547 in a ratio of 1 20. Oxidation of thymol (548) was effected with dimethyldioxirane in acetone to afford fhe four oxidation producfs 549-552 in 10, 20, 10 and 10% yields, respectively (Scheme 102). ... 

Under mild conditions the oxidation of thymol has been studied extensively in... 

Methylphenol under similar conditions afforded a cresylcresoquinone analogue of the major product from the oxidation of thymol. From 2,6-dimethylphenol and... 

Thymoquinone has been prepared directly from thymol by sulfonating and oxidizing the sulfonation mixture with manganese dioxide or potassium dichromate the same process has been successfully applied to carvacrol. The oxidation of sa ts of aminothymol with dichromate, ferric chloride, or nascent bromine also leads to satisfactory yields of thymoquinone. The above procedure is based on the observation that the diazonium salt obtained from aminothymol is almost quantitatively con-... 

A variation of this procedure is the nitrosation of a phenol such as thymol in an alcohol solution with hydrochloric acid and sodium nitrite. This procedure is said to avoid the evolution of oxides of nitrogen, since it may involve the intermediate formation of ethyl nitrite as the nitrosating agent [31]. However, from the safety standpoint, the oxides of nitrogen and nitrite esters must be considered hazardous. 

The use of microwaves in the acceleration of digestion and colour development in the determination of total Kjeldahl nitrogen in soil has been examined [1]. Visual method based on the colour development of indothymol blue formed between ammonia and thymol has recently been developed for the determination of ammonia-nitrogen in environmental waters [2]. UV photo-oxidation of reduced forms of nitrogen to nitrate by peroxodisulfate can make a basis for the determination of total nitrogen in urban and industrial waste waters [3]. 

The oxidation of p-cymene (103) usually gives rise to carvacrol (139) contaminated with varying amounts of thymol (140), but thallium trifluoroacetate in trifluoroacetic acid gives a thallium organic compound (137) that can be converted... 

Z-Menthone can be made by the oxidation of rhodinol with a chromic-sulphuric acid mixture.1 A Sabatier-Senderens reduction of thymol gives a mixture containing 30 per cent menthone.2 Z-Menthone is also obtained by treating menthol with copper at 3000.3 The method used in these directions is that of Beckmann.4... 

Extracts derived from most members of the Labiatae family (Table 3) have the potential to reduce the oxidation of food products and extend shelf life. This antioxidant action is dependent on the ability of the constituent phenolics to scavenge free radicals and chelate metals [27]. This dual effect has implications for CHD since it has been proposed in some studies, mainly from Europe, that a high status of iron increases the risk of CHD [28]. A number of the active compounds found in rosemary have also been found in sage and other herbs. In oregano, four flavonoids were identified among the active fraction [29], while in thyme, dimers of thymol and flavonoids have been isolated and characterized. 

On the other hand, ethanol was the main volatile foimd in the head-space of bread in contact with the PPO film and stored at room temperature for 4 days. This compound was resulting from fermentation and/or lipid oxidation as it has been reported by other authors [30]. In this sense, commercial bread samples in contact witii PPT8 or PPC8 films after 4 days of storage were characterized by significantly lower amoimts of ethanol, suggesting a reduction on the lipid oxidation reactions by the presence of thymol and carvacrol. The improvement on the oxidative stability of bread could be attributed to the release of carvacrol and thymol increasing the shelf-life of bread. 

By far the most significant sources of menthone are the dementhohzed oils from corn-mint and peppermint. Synthetic material is available from oxidation of menthol, from carene (Fig. 8.35) and from hydrogenation of thymol (317), but this accounts for only 30 of the 1330 tonnes of menthone produced annually (316). Menthone is used in mint reconstitutions and to some extent in other essential oil reconstitutions and perfumes. 

Add to a 125-ml Erlenmeyer flask about 50 ml of distilled water, pipette into this 10.0 ml of oxidizing agent, and add 2 ml of sodium arsenite solution. Add one or two drops of thymol blue indicator solution (0.1% solution in distilled water) and titrate the mixture carefully with the diluted hydrochloric solution until the color changes from blue to pink (ca. pH 1.7). Carry out titrations in duplicate. Titrations should agree to better than 0.1 ml of acid and the mean volume should be recorded to the nearest 0.05 ml. If x ml of acid is used (about 5-6 ml) dilute 200x ml of acid to exactly 2000 ml with deionized water using a measuring flask. This solution must be prepared fresh whenever a new alkaline sodium citrate solution is used. 

Figure 5.15 The mechanism of thymol as an inhibitor of oxidation and typicai phenoiic antioxidants.

Piperitone is of considerable technical im portance. It is a colourless oil of a pleasant peppermint-like smell. (-)-Piperilone has b.p. 109-5-110-5 C/I5mm. Piperitone yields thymol on oxidation with FeCl. On reduction with hydrogen in presence of a nickel catalyst it yields menthone. On reduction with sodium in alcoholic solution all forms of piperitone yield racemic menthols and womenthols together with some racemic a-phel)andrene. 

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