Cleavage of syringylglycerol beta-syringyl ethers during soda cooking

The degradation of model compounds of the syringylglycerol beta-syringyl et her type [2-(2,6-dimethoxyphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-pr opanediol and 2-(2,6-dimethoxy-4-methylphenoxy)-1-(4-hydroxy-3,5-dimethoxyp henyl)-1,3-propanediol] on soda cooking under comparatively mild conditions was studied. The formation of the degradation products detected, can be en visioned to proceed via a homolytic cleavage of the beta-ether bond in an i ntermediate quinone methide. 2,6-Dimethoxyphenol, sinapyl alcohol, syringal dehyde, 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-hyd roxy-3,5-dimethoxyphenyl)-2-propen-1-one, syringaldehyde and syringyl alcoh ol were detected in the reaction product obtained from 2-(2,6-dimethoxy-4-m ethylphenoxy)-1-(4-hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol. Traces of sinapaldehyde, 2-aryloxyacroleins and, possibly, syringaresinol were presen t 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 th e conditions prevailing during alkaline pulping are discussed on the basis of the results.