Convenient preparation of a β-O-4-type lignin model trimer via KOH-catalyzed hydroxymethylation and a new protection method

摘要

Lignin, as the second most abundant biopolymer on earth, is one of the targets for plant biorefinery studies. Its complex chemical behavior is frequently studied by dimeric, trimeric, etc. model compounds, preferably with a β-O-4-type structure. In the present study, a convenient synthesis of a β-O-4-type trimeric model compound possessing a free syringylic hydroxyl has been investigated. Two key modifications were in focus: (1) Protection of the aliphatic hydroxyl groups of the starting phenolic dimer prior to the S_N2 displacement reaction before introducing the syringylic moiety with 2,2-dimethoxypropane. (2) The hydroxymethylation step to introduce the full side chain moiety. When this reaction was performed in dioxane/ water in the presence of a catalytic amount of KOH instead of K_2CO_3, the formation of a side product via dehydration was markedly reduced. In addition, a convenient method for introducing an α,β-epoxide structure in acetophenone is recommended.