Synthesis of Lignin Model Compounds

Model compounds were prepared and their NMR spectra were acquired in DMSO to enable the assignments made in a previous paper.² Coniferyl alcohol and sinapyl alcohol were prepared from commercially available coniferaldehyde and sinapaldehyde using borohydride exchange resin.^{105 (link)}p-Coumaryl alcohol was synthesized from p-coumaric acid.¹⁰⁶ Coniferyl alcohol dimers were synthesized from in vitro radical coupling reactions using MnO₂ in dioxane :H₂O (1 : 1, v/v).¹⁰⁷ Sinapyl alcohol dimers were prepared using FeCl₃·6H₂O in dioxane :H₂O (5 : 2, v/v).¹⁰⁸ p-Coumaryl alcohol dimers were synthesized with horseradish peroxidase with hydrogen peroxide in acetone :water (1 : 10, v/v) or with FeCl₃·6H₂O in acetone: H₂O (5 : 1, v/v). Each metallic oxidative radical reaction was stirred for 1 to 4 h, and the metal salts were filtered off through a silica gel bed in fine sintered glass filters. The peroxidase reactions were conducted for about 15 h. Reaction solutions were poured into EtOAc, and washed with satd. aqueous NH₄Cl. EtOAc extracts were dried over anhydrous MgSO₄, and concentrated under reduced pressure. Model dimers were separated on preparative TLC plates with CHCl₃–MeOH (10 : 1, v/v). The fully authenticated NMR data for model compounds will be deposited in the “NMR Database of lignin and cell wall model compounds” available via the internet.⁴⁸