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Nonphenolic β-0-4  erythro  and  threo  lignin model diastereomers with various γ-groups (CH 3 , CH 2 -O-CH 3 , and CH 2 OH) and C α -substituents (OH, OCH 3 ) were synthesized, and the alkaline hydrolysis rates and activation parameters determined. In addition, two of the diastereomer pairs were computationally modeled and the thermodynamic values for the ionization of the α- or γ-hydroxyl, and subsequent displacement of the phenolate group to form an epoxide intermediate, were determined. The results suggest that the  erythro  γ-hydroxyl may participate in the hydrolysis to a significant extent, which results in a relatively high  erythro / threo  rate ratio for the α,γ-di-OH isomers. The influence of the  erythro  γ-hydroxyl on the hydrolysis rate may be due to the relatively favorable stability of the  erythro  γ-oxyanion. The electronic effect of the g-substituent appears to influence how fast the α-hydroxyl displaces the phenoxyl. We had previously suggested that the γ-substituent sterically inhibits hydrolysis of the  threo  isomer, and computational modeling confirmed this.

Effect of the α- and γ-Hydroxyls on the Alkaline Hydrolysis Rate of Nonphenolic β-0-4 Lignin Diastereomers