The efficient valorization of lignin could dictate the success of the 2nd generation biorefinery. Lignin, accounting for on average a third of the lignocellulosic biomass, is the most promising candidate for sustainable production of value-added phenolics. However, the structural alteration induced during lignin isolation is often depleting its potential for value-added chemicals. Recently, catalytic reductive depolymerization of lignin has appeared to be a promising and effective method for its valorization to obtain phenolic monomers. The present study systematically summarizes the far-reaching and state-of-the-art lignin valorization strategies during different stages, including conventional catalytic depolymerization of technical lignin, emerging reductive catalytic fractionation of protolignin, stabilization strategies to inhibit the undesired condensation reactions, and further catalytic upgrading of lignin-derived monomers. Finally, the potential challenges for the future researches on the efficient valorization of lignin and possible solutions are proposed.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.202001213

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review.

Lignin valorization to produce high-value chemicals selectively is an enormous challenge in biorefinery. In this study, 4-alkylphenol, formed by breaking the robust Caryl -OCH3 bonds solely with the retention of other structures in lignin-derived methoxylalkylphenols, was produced selectively over a Co1 -Fe0.1 @NC catalyst from real lignin oil as feedstock, which was obtained by a "lignin-first" strategy from either birch or cornstalk. A yield of 64.7 or 88.3 mol % of 4-propylphenol was obtained if birch lignin oil or eugenol was used as the substrate, respectively. The catalysts were characterized by using methods that include Brunauer-Emmett-Teller measurements, XRD, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and temperature-programmed desorption with synchrotron vacuum ultraviolet photoionization mass spectrometry. The results of catalyst characterization and comparison experiments indicated that CoNx was the main active phase for demethoxylation and hydrogenation, and the incorporation of Fe weakens the adsorption of 4-propylphenol to the catalyst, which inhibits the excessive hydrogenation of 4-propylphenol. This work shows the potential to produce high-value-added 4-alkylphenol from renewable raw biomass.

Selective Preparation of 4-Alkylphenol from Lignin-Derived Phenols and Raw Biomass over Magnetic Co-Fe@N-Doped Carbon Catalysts.

Bridged polycycles are privileged molecular skeletons with wide occurrence in bioactive natural products and pharmaceuticals. Therefore, they have been the pursing target molecules of numerous chemists. The rapid and convenient generation of sp3-rich complex three-dimensional molecular skeletons from simple and easily available aromatics has made dearomatization a highly valuable synthetic tool for the construction of rigid and challenging bridged rings. This review summarizes the-state-of-the-art advances of dearomatization strategies in the application of bridged ring formation, discusses their advantages and limitations and the in-depth mechanism, and highlights their synthetic value in the total synthesis of natural products. We wish this review will provide an important reference for medicinal and synthetic chemists and will inspire further development in this intriguing research area.

Construction of bridged polycycles through dearomatization strategies

Total synthesis has been an effective and broadly practiced approach for structure validation (or revision) of complex natural products. It appears that computational methods for structure elucidat...

Structure Validation of Complex Natural Products: Time to Change the Paradigm. What did Synthesis of Alstofolinine A Prove?

A highly regioselective and stereoselective cascade annulation of indoles with C2-symmetric enone tethered-cyclohexadienones provides rapid access to complex indole alkaloid-like scaffolds in high ...

Stereoselective Desymmetrization of Cyclohexadienone-Tethered Enones: Efficient Access to Highly Strained Polycyclic Indoles

A reaction cascade of aza-Achmatowicz rearrangement followed by indole nucleophilic cyclization was developed to generate the common indole-fused azabicyclo[3.3.1]nonane core of the macroline family alkaloids. The key to the success of the strategy relies on the careful manipulation of protecting groups and judicious selection of chemoselective furan oxidation conditions. The synthetic utility was further demonstrated on the asymmetric total synthesis of (-)-alstofolinine A.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201906637

Total Synthesis of (−)‐Alstofolinine A through a Furan Oxidation/Rearrangement and Indole Nucleophilic Cyclization Cascade

Indole-fused tetracyclic ring systems containing nitrogen atoms are common core skeletons of many indole alkaloids such as sarpagine, macroline, and ajmaline. Efficient and stereoselective construction of these ring systems can promote the development of the corresponding alkaloid syntheses. In this article, we briefly summarize our current progress toward the application of the aza-Achmatowicz reaction and indole nucleophilic addition reaction cascade for the first asymmetric total synthesis of the macroline-type indole alkaloid (–)-Alstofolinine A. Our synthetic strategy is based on furan oxidation/rearrangement and proceeds from easily accessible materials such as indole and furan derivatives.

Ye Zhang

Papers

Total Synthesis of (−)‐Alstofolinine A through a Furan Oxidation/Rearrangement and Indole Nucleophilic Cyclization Cascade

Total Synthesis of (–)-Alstofolinine A: Selected Furan Oxidation/ Cyclization Cascade