Synthesis of Beta-Aryl Lignin Model Compounds

Abstract

A Claisen-Schmidt type aldol addition reaction was used to synthesize erythro- and threo-ethyl-3-hydroxy-3-(4-benzyloxy-3-methoxyphenyl)-2-phenylpropanoate (la, lb) and erythro-2,3-bis(4-hydroxy-3-methoxyphenyl)-1,3-propanediol (9). The key aldol addition step was facilitated by adding a molar equivalent of Zn (II) cations to stabilize the addition product, thus inhibiting the reverse aldol reaction. This allowed the reaction to be carried out at moderate temperatures (-20 to 0°C) and rather short reaction times (10 to 20 minutes). The H-alpha–H-beta coupling constants in the PMR spectra were used for assignment of the stereoisomers.