Acid reactions of lignin models of beta-5 type

Refluxing of trans-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-di hydrobenzo[b]furan with dioxane-water (9 : 1) in the presence of various ac id catalysts led to the formation of 2-(3,4-dimethoxphenyl)-7-methoxy-3-met hylbenzo[b]furan, the trans and cis forms of 2-hydroxy-3,3',4'-trimethoxyst ilbene and cis-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-dihydr obenzo [b] furan. The proportions of the products were strongly dependent o n the particular acid used as catalyst. HCl and to a greater extent HBr fav ored the formation of the 2-arylbenzofuran (phenylcoumarone) while the tran s-stilbene derivative predominated in reaction products from the experiment s with trifluoromethane sulfonic acid as the catalyst. Isomerization of the starting material occurred, regardless of the nature of the catalyst (smal l amounts of the cis-isomer formed). The number of phenylpropane units in s pruce lignin attached to an adjacent unit by a beta-5 linkage was estimated to be 6-9 % on the basis of H-1 NMR spectrometric measurements of the form ation of phenylcoumarone structures on refluxing of milled wood lignin from spruce with 0.1 M HBr in dioxane-water (9 : 1).