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Isothiourea-catalysed chemo- and enantioselective [2,3]-sigmatropic rearrangements of N,N-diallyl allylic ammonium ylides
T. H. West, S. S. M. Spoehrle, A. D. Smith
The isothiourea-catalysed chemo- and enantioselective [2,3]-sigmatropic rearrangement of N,N-diallyl allylic ammonium ylides is explored as a key part of a route to free functionalised ?-amino esters and piperidines. The [2,3]-sigmatropic rearrangement proceeds with excellent diastereo- and enantiocontrol (>95:5 dr, up to 97% ee), with the resultant N,N-diallyl ?-amino esters undergoing either mono- or bis-N-allyl deprotection. Bis-N-allyl deprotection leads to free ?-amino esters, while the mono-deprotection strategy has been utilized in the synthesis of a target functionalised piperidine.undergoing either - or -allyl deprotection. Bis -allyl deprotection leads to free -amino esters, while the mono-deprotection strategy has been utilized in the synthesis of a target functionalised piperidine.
Davide Di Marino,a David Stöckmann, Stefanie Kriescher, Serafin Stiefela, and Matthias Wessling
Lignocellulosic biomass is an important renewable resource that could substitute fossil feedstocks as a raw material for high value chemicals production. While the cellulosic fraction of biomass gives access to saccharides only, lignin could possibly give access to low molecular weight aromatic compounds. Electrochemical processes represent a green and cheap alternative to homogenous and heterogeneous catalysis as well as to valorization processes that need high temperatures and high pressures. Electrochemical processes involving lignin need strong alkalinity of the reaction medium in order to dissolve lignin and to obtain an electrically conducting solution. However, strong alkalinity is supposed to be one of the reasons for consecutive oxidations to undesired organic acids and carbon dioxide. The high pH also limits the use of only a few metals as electrode materials, due to corrosion. We report the use of pure deep eutectic solvents (DES) in order to dissolve lignin in combination with an electrochemical oxidative depolymerisation. DESs are cheap, biodegradable and easy to handle and interest in using these solvents for biomass fractionation is growing in the last years. The development of processes for the valorization of lignin in the fractionation solvent is crucial and at the moment just very few example of valorization of lignin in DES are known. DES systems are proposed as an alternative to NaOH and ILs as electrolyte for the lignin electrochemical depolymerisation processes and a proof of principle for lignin electrochemical depolymerisation is showed. The process can be performed in pure or diluted DES and a subsequent liquid-liquid extraction of the products can be successfully performed. Molecular weight of lignin decreases due to the electrochemical process producing low molecular weight products, as shown in the size exclusion chromatography (SEC) measurement. We present successful extraction of the products and their characterization by SEC and GC-MS, proving that a successful depolymerisation of lignin was achieved. GC-MS identified vanillin and guaiacol as the most abundant produced phenolic compounds, with relative yields up to 38 % for guaiacol and 37 % for vanillin. Optimization of the process parameters, such as electrode material and stability, applied potentials and DES recycling are the next important steps that need to be taken into account for the electrochemical depolymerisation of lignin in this new class of solvents.
Progress and obstacles in the production and application of recombinant lignin-degrading peroxidases
Camilla Lambertz, Selin Ece, Rainer Fischer & Ulrich Commandeur
Lignin is 1 of the 3 major components of lignocellulose. Its polymeric structure includes aromatic subunits that can be converted into high-value-added products, but this potential cannot yet been fully exploited because lignin is highly recalcitrant to degradation. Different approaches for the depolymerization of lignin have been tested, including pyrolysis, chemical oxidation, and hydrolysis under supercritical conditions. An additional strategy is the use of lignin-degrading enzymes, which imitates the natural degradation process. A versatile set of enzymes for lignin degradation has been identified, and research has focused on the production of recombinant enzymes in sufficient amounts to characterize their structure and reaction mechanisms. Enzymes have been analyzed individually and in combinations using artificial substrates, lignin model compounds, lignin and lignocellulose. Here we consider progress in the production of recombinant lignin-degrading peroxidases, the advantages and disadvantages of different expression hosts, and obstacles that must be overcome before such enzymes can be characterized and used for the industrial processing of lignin.