Guaiacyl groups, oxidation

Oniki, T. and Takahama, U. (1997). Free radicals produced by the oxidation of compounds containing syringyl and guaiacyl groups. Mokuzai Gakkaishi, 43(6), 493 98. 

Sodium periodate specifically oxidizes guaiacyl groups to quinones. Hydrogen and sodium peroxides in alkali are also somewhat selective in oxidizing lignin and destroy chromophoric groups such as quinones and carbonyl functions while also degrading only aromatic units with free phenolic hydroxyls to dibasic acids. 

FIGURE 15.9 Oxidation of guaiacyl groups to c>-quinonoid structures by manganese... 

Takahama, U. Oxidation of hydroxycinnamic acid and hydroxycinnamyl alcohol derivatives by laccase and peroxidase—interactions among p-hydroxyphenyl, guaiacyl and syringyl groups during the oxidation reactions. Physiol. Plantarum 1995, 93, 61-68. 

Our findings lead us to assume that the oxidative degradation of guaiacyl compounds with saturated side chains in the 4-position (para to the hydroxyl group) is proceeding via 0,0 -coupling. Dimerization is then followed by ring fission reactions. 

A method developed by Adler et al. (1958) is based on the oxidation of simple phenolic guaiacyl (Adler and Hernestam 1955) or syringyl (Chang et al. 1975) compounds with aqueous sodium periodate solution to ortho-quinone structures in which process nearly lmol of methanol per mol of phenolic hydroxyl group is released [Eq. (1)]. 

The phenolic hydroxyl and etherified benzyl alcohol group can be selectively oxidized by periodate [385] and 2,3-dichloro-5,6-dicyano-l, 4-benzoquinone (DDQ) [386], respectively. The periodate oxidation of guaiacyl and syringyl nuclei yields 0-quinones plus methanol and has been used to estimate the phenolic hydroxyl group content of lignin. DDQ oxidation has been applied to estimate the content of lignin-carbohydrate linkages [28]. 

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