From Eugenol

Eugenol is the main component of the essential oil of the clove tree Syzygium aro-maticum) and is one of the cheapest and most common flavor ingredients easily obtained from natural sources. Its low market price makes it an attractive feedstock for vanillin production, but up to now the yields are still unsatisfactory. 

To date, the most efficient process for the bioconversion of eugenol to vanillin employed a recombinant strain of Pseudomonas sp. HR199 with the vanillin dehydrogenase gene vdh) knocked out, yielding 2.9gl ofvanillin [34, 35]. Several other wild-type or recombinant strains have been selected for the same biotransformation, but the reported yields did not exceed 0.5 gl [36-38]. 

Even though more expensive than eugenol, isoeugenol is still considered a suitable precursor for biotransformation processes. It is extracted from essential oils such as ylang-ylang Cananga odorata), and is used as a natural flavor ingredient as well. 

Isoeugenol is converted to vanillin through an epoxide-diol pathway leading to vanillyl alcohol as a precursor. The intermediates isoeugenol oxide and isoeugenol diol were identified in the biotransformation of isoeugenol with Bacillus pumilus S-1, which afforded 3.75 gl yield of vanillin in preparative biotransformation [39]. 

Several other Bacillus strains isolated from soil were found to be efficient for this biotransformation remarkably, with Bacillus fusiformis SW-B9, an overall production of 32.5 gl was reached [40]. 

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Siegel S.M. (1957) Non-enzymic macromolecules as matrices in biological synthesis. The role of polysaccharides in peroxidase catalyzed lignin polymer formation from eugenol // J. Amer. Chem. Soc. V. 79. P. 1628-1632... 

Bolton, J. L. Comeau, E. Vukomanovic, V. The influence of 4-alkyl substituents on the formation and reactivity of 2-methoxy-quinone methides evidence that extended ji-conjugation dramatically stabilizes the quinone methide formed from eugenol. Chem.-Biol. Interact. 1995, 95, 279-290. 

Synthesis from Eugenol. The sodium or potassium salt of eugenol is isomerized to isoeugenol by heating. Isomerization can also be carried out catalytically in the presence of ruthenium [164] or rhodium [165] compounds. 

The flavoring vanillin occurs naturally as glucovanillin (a glucoside) in the vanilla bean (Section 20-5). It is made commercially in several ways. One is from eugenol, itself a constituent of several essential oils ... 

Since isoeugenol is the important intermediate in the vanillin production from eugenol, the isomerization of eugenol attracted a considerable amount of attention. The first report on the isomerization reaction appeared in 1891, in which Tiemann reported 50% conversion in 24 hours using an ethanolic KOH solution at 80°C [13]. 

The classical synthesis of vanillin from eugenol or isoeugenol was developed in 1896 and it remained the preferred method for about 50 years. Vanillin is now prepared industrially in large amounts by the Reimer-Tiemann reaction, starting with... 

Fig. 15.4. The routes (a) from eugenol to ferulic acid and (b) from ferulic acid to vanillin in Pseudomonas strains (Gasson etal., 1998 Mitra etal., 1999).

Krawczik AR, Lipkowska E, Wrobel JT (1991) Horseradish peroxidase-mediated preparation of dimers from eugenol and isoeugenol. Collect Czech Chem Commun 56 1147-1150... 

Eugenol.—The firm v. Hey den Echfg.4 obtains vanillin elec-trolytically from eugenol. The latter is rearranged by alkalies into isoeugenol and then oxidized electrolytically in alkaline solution ... 

Isoeugenol methyl ether was prepared from eugenol methyl ether by mixing eugenol methyl ether (150 parts) with potassium hydroxide (1.5 parts) and absolute ethyl alcohol (3.2 parts) at 160 to 165°C with stirring. The temperature of the mixture rose rapidly to 190 to 195° C and after 30 minutes the heat was turned off, by which time the temperature had fallen to 170°C. When the mixture reached room temperature, water was added. The oily layer was separated off, dried and vacuum distilled. The distillate (130 parts) was substantially all cis and tran.s isoeugenol methyl other. Source Bush 1968... 

Vanillin has a vanilla-like odour with a threshold of 0.02 ppm in water at 20°C [21 [, Ethyl vanillin (Fig. 3.60), which is 2 to 4 times stronger than vanillin, has not yet been found in nature. It is synthesized from eugenol, isoeugenol, or safrole. Both vanillin... 

Derivation From eugenol by isomerization with caustic potash. 

Figure 4.71 Perfumery materials from eugenol and safrole isoeugenol...

Isoeugenol to Vanillin. The formation of vanillin from eugenol is an example of an oxidation of a side chain to an aldehydic group in which it is convenient to protect one substituent against oxidation. Eugenol obtained from oil of cloves is heated with an alkali such as sodium hydroxide to convert it to isoeugenol, the hydroxyl group is protected by acetylation, and the substance is oxidized and then saponified to vanillin. 

Yokota, H., Hoshino, J., and Yuasa, A., Suppressed mutagenicity of benzo[a]pyrene by the liver S9 fraction and microsomes from eugenol-treated rats, Mutot. Res., 172, 231, 1986. 

Sanchez-Perez, and A. Garcia-Diez. 1999. Occupational allergic contact dermatitis from eugenol, oil of cinnamon and oil of cloves in a physiotherapist. Contact Dermat. 41(6) 346-347. 

Contact allergy to propanidid, (epontol), developed from occupational exposure in an anesthetist. This compound is derived from eugenol and cross-reacts with eu-genol and balsam of Peru (Sneddon and Glew 1973) Another case was reported by Bandmann and Doenicke (1971). 

However, ferulic acid is also obtainable from eugenol in an aerobic process, in the presence of Pseudomonas strains. In Asia, ferulic acid is obtained form rice bran. 

Kanerva L, Jolanki R, Estlander T (1996b) Offset printer s occupational allergic contact dermatitis caused by cobalt-2-ethylhexoate. Contact Dermatitis 34 67-68 Kanerva L, Henriks-Eckerman M-L, Jolanld R, Estlander T (1997) Plastics/acrylics material safety data sheets need to be improved. Clin Dermatol 15 533-546 Kanerva L, Estlander T, Jolanki R (1998) Dental nurse s occupational allergic contact dermatitis from eugenol used as a restorative dental material with polymethylmethacrylate. Contact Dermatitis 38 339-340... 

Vilaplana J, Giimalt F, Romaguera C, Craiellana F (1991) Contact dcmnatitis from eugenol in mouthwash. Contact Dermat 24 223—224... 

It appears as well in many other natural sources found in the tropical world. It is now made synthetically from eugenol, which, itself has a pleasant dove-like smell. 

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