Synthesis of lignin models of β-5 type

Abstract

A β-5 lignin model of the phenylcoumaran type, trans-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-dihydrobenzo[b] furan, was prepared by acid-catalysed cyclization of 1-(3,4-dimethoxyphenyl)-2-(2-hydroxy-3-methoxyphenyl)-1,3-propanediol. The cyclization was accomplished by treatment with 0.2 M HCl in dioxane-water (1:1) at 50°C. The reaction was first order with respect to the substrate (τ1/2 36min). 1-(3,4-Dimethoxyphenyl)-2-(2-hydroxy-3-methoxyphenyl)-1,3-propanediol, which is a model compound representative of a second type of lignin structure of β-5 type, was in turn obtained by synthesis starting from o-vanillin and 3′,4′-dimethoxyacetophenone. In the first step equimolar amounts of these compounds were treated with alkali to give 1-(3,4-dimethoxyphenyl)-3-(2-hydroxy-3-methoxyphenyl)-2-propen-1-one. Conversion of this compound into its tetrahydropyran-2-yl ether and subsequent epoxidation gave 1-(3,4-dimethoxyphenyl)-3-[3-methoxy-2-(tetrahydropyran-2-yloxy)phenyl]-2,3- epoxypropanone. Acid-catalysed (boron trifluoride) rearrangement of this compound (the tetrahydropyran-2-yl group was removed simultaneously), reduction of the resulting product with sodium borohydride and subsequent Chromatographic purification gave a mixture of the erythro and threo forms of 1-(3,4-dimethoxyphenyl)-2-(2-hydroxy-3-methoxyphenyl)-1,3-propanediol (yield, 57%). The erythro form predominated in the mixture and could be isolated by fractional crystallization. Separation of the diastereomers could be accomplished by ion exchange chromatography. © Acta Chemica Scandinavica 1997.