Conversion into vanillin

The successful conversion of lignms into vanillin (2) and its analogs by nitrobenzene oxidation stimulated a search for economically viable processes for the manufacture of vanillin (2) from technical lignms In 1942, Pearl found that vanillin (2) could be obtained in a yield of about 20% of the lignin when... 

Smith, C.Z., Utley, J.H.F. and Hammond, J.K. (2011) The electro-oxidative conversion at a laboratory scale of a lignosulfonate into vanillin in an FMOl filter press reactor preparative and mechanistic aspects. Journal of Applied Electrochemistry, 41, 363-375. 

Another example of the direct use of a renewable chemical is the biocatalytic conversion of D-glucose into vanillin used as a flavoring agent in food and beverages [12]. The use of a recombinant Escherichia coli biocatalyst in fermentation offers many advantages over the synthetic vanillin manufactme based on the use of... 

For the biosynthesis of vanillin, several other enzymes are of interest. First of all, phenylalanine ammonia lyase (PAL) this enzyme converts phenylalanine into the cinnamic acid type of compounds, the first intermediates in the vanillin biosynthesis after the primary metabolism. PAL activity could be detected in green beans, but after scalding this activity is lost. The chain shortening enzyme (CSE), responsible for the conversion of a C Cs compound into a C6Ci compound, was found to be localised in the cytosol of cells of the placental tri-chomes in the green beans [23]. 

The conversion of eugenole into iso-eugenole and of safrole into iso-safrole is accomplished by boiling with alcoholic potassium hydroxide. The oxidation products of these ethers are other important essential oil constituents. Eugenole yields the corresponing aldehyde which is known as vanillin, the chief constituent of vanilla beans from which vanilla extract is made. Safrole by oxidation yields a compound known as piperonal also as heliotropine. It has the odor of heliotrope flowers and is used as artificial heliotrope essence. These latter compounds and also constituents of other essential oils will be considered in detail later in their proper chemical relationship (p. 66i, etc.). 

The production of multiple products is generally seen as necessary to increase the economic viability of biomass conversion. This is encapsulated in the concept of a biorefmery , which according to the National Renewable Energy Laboratory (NREL) is a facility that integrates conversion processes and equipment to produce fuels, power and chemicals from biomass [23], Examples of chemicals that can be produced from biomass include ethanol, methanol, furfural, paper, lignin, vanillin, lactic acid, dimethylsulfoxide and xylitol. In many cases, using biomass as a feedstock for chemical production requires an initial step to separate or fractionate the three main components into usable fractions [20, 22], This also maximises the usage of the different biomass components. 

Vanillin crystallizes in needles which melt at 81°. It gives a blue coloration with an aqueous solution of ferric chloride, reduces an ammoniacal solution of silver nitrate, and decomposes carbonates. It is oxidized by moist air to vanillic acid, which is formed as the result of the conversion of the aldehyde group into a carboxyl group. 

Vanillin is the second largest food additive used each year. As a consequence of limited natural sources of this ubiquitous flavor and fragrance, the bulk of commercial vanillin is synthesized from phenol (19). A two-step synthesis of vanillin from glucose via DHS intermediacy was recently reported (Figure 3) (17). DHS dehydratase-catalyzed conversion of DHS into PCA is followed by formation of the methyl ether, catalyzed by catechol-O-methyltransferase, an enzyme not native to E. coli but for which the gene (comt) has been cloned from rat liver (20). In a separate step, incubation of the resulting vanillic acid with aryl-aldehyde dehydrogenase (21) affords vanillin. 

The only viable conversion of lignin into a single aromatic compound is the synthesis of vanillin, which is a very useful commodity, but not in the realm of polymer chemistry. A recent investigation reports its oxidation to vanillic acid [74] and another, the use of this compound for leather tanning [75]. 

Another example on the use of metabolicaUy related enzymes was outbned by Zachariou [33] for the production of 3-carboxymuconate (3CM) from vanillin (Scheme 3.12). This study exploited the metabolic degradation of the cheap starting material vanillin by three different enzymes (4-hydroxy benzaldehyde dehydrogenase, vanillate monooxygenase, and protocatechuate 3,4-dioxygenase) to the monomer feedstock 3CM. After cloning the full pathway into E. coU, the authors were able to produce 3CM in very good overall HPLC yield (100% conversion) at concentrations up to 1 mM. 

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