Regiopreference of aromatic C6- over C5-carbon in acid condensation of guaiacyl type lignin

Veratryl alcohol ((3,4-dimethoxyphenyl)methanol) was treated with an excess amount of homoveratrol (Ho, 1,2-dimethoxy-4-methylbenzene) or creosol (Cr, 2-methoxy-4-methylphenol) under acidic conditions in aqueous 1,4-dioxane (Diox) solutions containing triflic acid with various Diox contents (defined as Ho or Cr system, respectively) to quantitatively simulate acid condensation of lignin. Five or three products were identified by LC/MS and NMR analyses in the Ho or Cr system, respectively, and three condensation products, α-6-type dimer, α-5-type dimer, and α-6/α-6-type trimer, were quantified by HPLC analysis in either system. The α-6-type dimer was the only major product, whereas the α-5-type dimer was a minor product in either system, suggesting that guaiacyl lignin has a strong regiopreference for acid condensation at the aromatic C6- over C5-carbon. The concentration ratio of the α-6- to α-5-type dimer was about 100 in the Ho systems and 11–12 in the Cr systems, the regiopreference being 8–9 times weaker in the latter. This implies that a phenolic hydroxy group at the aromatic C4-carbon markedly promotes acid condensation at the aromatic C5-carbon, the ortho-position of the aromatic C4.