Fig. 1. Summary of aromatic N-heterocyclic compounds from lignin.

Fig. 2. (a) Palladium-catalyzed formal cross-coupling between monophenols and pyrrolidines/indolines to give several N-cyclohexyl-substituted pyrrole or indole products. Reprinted with permission from Ref. [55]. Copyright 2017, Royal Society of Chemistry. (b) The reaction mechanism. Reprinted with permission from Ref. [55]. Copyright 2018, Wiley-VCH.

Fig. 3. Sequence reaction of carbazole synthesis from phenol in heterogeneous catalyst. Reprinted with permission from Ref. [65]. Copyright 2018, Springer Nature

Fig. 4. By fusing the benzene ring at specific positions on the imidazole ring, constructing Benzimidazole and imidazopyridine frameworks.

Fig. 5. Synthesis of imidazopyridine from the β-O-4 model compound via copper-catalyzed oxidative cyclization. Reprinted with permission from Ref. [72]. Copyright 2017, American Chemical Society.

Fig. 6. Synthesis of imidazopyridine derivatives from lignin β-O-4 segments via a one-pot multicomponent reaction. Reprinted with permission from Ref. [73]. Copyright 2023, Cell Press.

Fig. 7. DDQ-catalyzed construction of imidazole derivatives from the lignin β-O-4 model compound and o-phenylenediamine, and the possible reaction mechanism. Reprinted with permission from Ref. [75]. Copyright 2022, Wiley-VCH.

Fig. 8. Formation of isoxazole and nitrile derivatives through hydroxylamine participation in lignin depolymerization. Reprinted with permission from Ref. [79]. Copyright 2018, American Chemical Society.

Fig. 9. Vanadium-complex catalyzed tandem reaction to construct diverse triazole derived from lignin model compound. Reprinted with permission from Ref. [83]. Copyright 2024, Wiley-VCH.

Fig. 10. DEA-based lignocellulose fractionation for the valorization of carbohydrate and lignin. Reprinted with permission from Ref. [96]. Copyright 2024, Springer Nature.

Fig. 11. Construction of quinoline and acridine from the β-O-4 major decomposition derivative.

Fig. 12. Sequence reaction of quinoline and acridine synthesis from phenol in heterogeneous catalyst. Reprinted with permission from Ref. [65]. Copyright 2018, Springer Nature

Fig. 13. Integration of multiple reactions for acridine synthesis from phenols. Reprinted with permission from Ref. [106]. Copyright 2024, Royal Society of Chemistry.

Fig. 14. [Ir]- and [Cr]-complex-catalyzed synthesis of 3-oxo quinoline derivatives from the β-O-4 model compound. Reprinted with permission from Ref. [107]. Copyright 2023, American Chemical Society. Reprinted with permission from Ref. [108]. Copyright 2024, Royal Society of Chemistry.

Fig. 15. Direct synthesis of quinoline derivatives from β-O-4 model compounds without transition metals. Reprinted with permission from Ref. [109]. Copyright 2022, Wiley-VCH.

Fig. 16. Coordination-mediated Co-Cu dual single-atom catalysts for the cascade transformation of the β-O-4 model compound to quinoline. Reprinted with permission from Ref. [110]. Copyright 2024, Wiley-Blackwell.

Fig. 17. Clean synthesis strategy for generating biologically active isoquinoline molecules from lignin. Reprinted with permission from Ref. [115]. Copyright 2024, Wiley-VCH.

Fig. 18. Sustainable route for the synthesis of bio-based pyridazine-based compounds from guaiacol. Reprinted with permission from Ref. [119]. Copyright 2019, Springer Nature.

Fig. 19. One-pot synthesis of phenazine from lignin depolymerization-derived catechol and ammonia solution, and the main reaction processes. Reprinted with permission from Ref. [127]. Copyright 2022, Royal Society of Chemistry.

Fig. 20. KOH-mediated one-pot highly coupled reaction pathway for quinoxaline synthesis. Reprinted with permission from Ref. [128]. Copyright 2022, Wiley-VCH.

Fig. 21. Synthesis of pyrimidine-like compounds from lignin β-O-4 model compounds without transition-metal catalysts. Reprinted with permission from Ref. [131]. Copyright 2022, Springer Nature.

Fig. 22. Direct construction of N-aryl substituted pyrrolidine via Pd/C-catalyzed coupling of aryl ethers and pyrrolidines without additives. Reprinted with permission from Ref. [134]. Copyright 2021, Royal Society of Chemistry.

Fig. 23. Transformation of lignin via a three-step approach for the synthesis of heptacyclic N-heterocyclic compounds. Reprinted with permission from Ref. [138]. Copyright 2019, American Chemical Society.