Effect of the alpha- and gamma-hydroxyls on the alkaline hydrolysis rate of nonphenolic beta-0-4 lignin diastereomers

Nonphenolic beta-O-4 erythro and threo lignin model diastereomers with various gamma-groups (CH3, CH2-O-CH3, and CH,OH) and C-alpha-substituents (OH, OCH3) were synthesized, and the alkaline hydrolysis rates and activation parameters determined. In addition, two of the diastercomer pairs were computationally modeled and the thermodynamic values for the ionization of the alpha- or gamma-hydroxyl, and subsequent displacement of the phenolate group to form an epoxide intermediate, were determined. The results suggest that the erythro gamma-hydroxyl may participate in the hydrolysis to a significant extent, which results in a relatively high erythro/threo rate ratio for the alpha,gamma-di-OH isomers. The influence of the erythro gamma-hydroxyl on the hydrolysis rate may be due to the relatively favorable stability of the erythro gamma-oxyanion. The electronic effect of the gamma-substituent appears to influence how fast the alpha-hydroxyl displace,s the phenoxyl, We had previously suggested that the gamma-substituent sterically inhibits hydrolysis of the threo isomer, and computational modeling confirmed this.