SYNTHESIS OF LIGNIN MODELS OF BETA-5 TYPE

A beta-5 lignin model of the phenylcoumaran type, phenyl)-3-hydroxymet hyl-7-methoxy-2,3-dihydrobenzo [b]furan, was prepared by acid-catalyse d cyclization of yl)-2-(2-hydroxy-3-methoxyphenyl)-1,3-propanediol. Th e cyclization was accomplished by treatment with 0.2 M HCl in dioxane- water (1:1) at 50 degrees C. The reaction was first order with respect to the substrate (tau(1/2) 36 min). 1-( 3,4-Dimethoxyphenyl)-2-( 2-hy droxy-3-methoxyphenyl )-1,3-propanediol, which is a model compound rep resentative of a second type of lignin structure of beta-5 type, was i n turn obtained by synthesis starting from o-vanillin and 3',4'-dimeth oxyacetophenone. In the first step equimolar amounts of these compound s were treated with alkali to give nyl)-3-(2-hydroxy-3-methoxyphenyl)- 2-propen-1-one. Conversion of this compound into its tetrahydropyran-2 -yl ether and subsequent epoxidation gave etrahydropyran-2-yloxy)pheny l]-2,3-epoxypropanone. Acid-catalysed (boron trifluoride) rearrangemen t of this compound (the tetrahydropyran-2-yl group was removed simulta neously), reduction of the resulting product with sodium borohydride a nd subsequent chromatographic purification gave a mixture of the eryth ro and threo forms of (3,4-dimethoxyphenyl)-2-(2-hydroxy-3-methoxyphen yl )-1,3-propanediol (yield, 57%). The erythro form predominated in th e mixture and could be isolated by fractional crystallization. Separat ion of the diastereomers could be accomplished by ion exchange chromat ography.