Vanillin Production Pathways in Alkaline Nitrobenzene Oxidation of Guaiacylglycerol-β-guaiacyl Ether.

Alkaline nitrobenzene oxidation (AN oxidation) is a significant method for chemical analysis of lignin. Despite its importance in lignin chemistry, the detailed chemical reactions involved in AN oxidation are not yet fully understood. Surprisingly, there is almost no experimentally supported information available regarding the reaction pathways in the AN oxidation of guaiacyl glycerol-β-guaiacyl ether (), a common model compound in lignin chemistry. This study reports the results of our investigation into the formation pathway of vanillin (4-hydroxy-3-methoxybenzaldehyde) in the AN oxidation of . Our series of experiments proposed a vanillin formation pathway involving an enol ether with a C side chain, 2-methoxy-4-[2-(2-methoxyphenoxy)-ethenyl]-phenol , as an intermediate, in which is produced by the non-oxidative degradation of by alkali. Another enol ether with a C side-chain, -4-[3-hydroxy-2-(2-methoxyphenoxy)-1-propen-1-yl]-2-methoxyphenol (), and the condensation products formed under alkaline conditions were found to be insignificant as vanillin sources. On the other hand, the comparison of the vanillin yields from and isolated (80.7 and 86.5 mol %, respectively) in their AN oxidation to the yield from in the absence of nitrobenzene (69.9 mol %) also suggested that the vanillin formation from involved unknown pathways in which is not an intermediate.