ON THE MECHANISM OF FORMATION OF NONCYCLIC BENZYL ETHERS DURING LIGNIN BIOSYNTHESIS .4. THE REACTIONS OF A BETA-O-4-TYPE QUINONE METHIDE WITH CARBOXYLIC-ACIDS IN THE PRESENCE OF PHENOLS - THE FORMATION AND STABILITY OF BENZYL-ESTERS BETWEEN LIGNIN AND CARBOHYDRATES

The reactions of a beta-O-4-type quinone methide with carboxylic acids in the presence of phenols have been investigated in buffered water-dioxane solutions, in order to determine the conditions in which the reactions lead to the formation of non-cyclic benzyl aryl ethers or, alternatively, benzyl esters. It was found that under neutral conditions, at pH 6, the formation of benzyl esters was kinetically favoured whereas on longer reaction times also the formation of thermodynamically more stable benzyl aryl ethers was observed. Under more acidic conditions, at pH 4, the formation of benzyl esters competed with the formation of benzyl alcohols. No formation of benzyl aryl ethers was detected at this pH. In the experiment with a mixture of vanillyl alcohol and D-glucuronic acid, the formation of benzyl glucuronate, the LCC benzyl ester model, was observed only at lower pH values. In neutral conditions no formation of the glucuronate ester was detected. The conclusions that can be drawn concerning the biosynthesis of lignin in the cell walls are discussed.