Vanillin, commercial source

Alkaline oxidation of lignin is the commercial source of vanillin. This procedure converts a significant portion of the lignin to aromatic fragments that are, for the most part, aromatic aldehydes. The mechanism probably involves two steps, hydrolysis of the aryl ether linkages followed by side chain oxidation. 

The chemical and enzymatic oxidative degradation of lignin (and coal) is used to obtain not only vanillin and benzoic acid, but also other aromatics (Baciocchi et al. 1999, references therein). In principle, lignin could be a major nonfossil and renewable source of aromatic compounds, a feedstock for synthesis of useful products. The problem deserves finding new ion-radical routes to cleave lignin. At present, there is some shortage in oil, gas, and even coal, which had usually been well-available natural sources of aromatics. In the near future, biomass may (and must) replace fossil-originated materials in the manufacture of commercial carbon-based products. 

Vanillin has been known as a flavouring substance since about 1816, and by 1858 the pure chemical had been obtained from ethanolic extracts of vanilla beans. It was not until 1872 that Carles established its correct formulation and in 1874, Tiemann and Haarmann reported it as 3-methoxy-4-hydroxy-benzaldehyde (Fig. 3.60). Finally, Reimer synthesized vanillin from guaiacol and thus proved its chemical structure. For many years, the most important source of vanillin was eugenol, from which it was obtained by oxidation. Today, the major portion of commercial vanillin is obtained by processing waste sulfite liquors, the rest through fully synthetic processes starting from guaiacol [21 ]. 

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. 

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