Fig. 2

Synthesis of ¹³C-labeled β-O-4 lignin oligomer model compounds. ¹³C-labels in a aromatic rings and the β positions; b α positions; and c methoxy groups. A: Vanillin (1) was refluxed with BrCH₂COOtBu, KI, and K₂CO₃ in acetone for 1.5 h. B: 2 is dissolved in anhydrous THF, followed by LDA addition with stirring at −30 °C (1 h). The mixture was then stirred at 0 °C–5 °C for 1.5 h. C: 3 is dissolved in tBuOH with NaBH₄, followed by the addition of MeOH and stirring at 65 °C–70 °C for 7 h