Cleavage of syringylglycerol β-syringyl ethers during soda cooking

The degradation of model compounds of the syringylglycerol beta-syringyl ether type [2-(2,6-dimethoxyphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol and 2-(2,6-dimethoxy-4-methylphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol] on soda cooking under comparatively mild conditions was studied. The formation of the degradation products detected, can be envisioned to proceed via a homolytic cleavage of the beta-ether bond in an intermediate quinone methide. 2,6-Dimethoxyphenol, sinapyl alcohol, syringaldehyde, 1,2-bis(4-hydroxy-3,5-dimethoxyphenyl)ethanedione, bis(4-hydroxy-3,5-dimethoxyphenyl)methanone and traces of syringyl alcohol were detected in the reaction product obtained from 2-(2,6-dimethoxyphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol. 2,6-Dimethoxy-4-methylphenol, sinapyl alcohol, 3-hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)- 1-propanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)-2-propen-1-one, syringaldehyde and syringyl alcohol were detected in the reaction product obtained from 2-(2,6-dimethoxy-4-methylphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol. Traces of sinapaldehyde, 2-aryloxyacroleins and, possibly, syringaresinol were present in the reaction mixtures from both model compounds. Reaction routes that explain the differences in product patterns of the two model compounds are presented. The reactions of phenolic arylglycerol beta-aryl ethers under the conditions prevailing during alkaline pulping are discussed on the basis of the results.