Thanks to chemical stabilization, aldehyde-assisted fractionation (AAF) of lignocellulosic biomass has recently emerged as a powerful tool for the production of largely uncondensed lignin. Depolymerization of AAF lignin via ether cleavage provides aromatic monomers at near theoretical yields based on ether cleavage and an oligomeric fraction that remains largely unexploited despite its unique material properties. Here, we present an in-depth analytical characterization of AAF oligomers derived from hardwood and softwood in order to elucidate their molecular structures. These bioaromatic oligomers surpass technical Kraft lignin in terms of purity, solubility, and functionality and thus cannot even be compared to this common feedstock directly for material production. Instead, we performed comparative experiments with Kraft oligomers of similar molecular weight (Mn ∼ 1000) obtained through solvent extraction. These oligomers were then formulated into polyurethane materials. Substantial differences in material properties were observed depending on the amount of lignin, the botanical origin, and the biorefining process (AAF vs Kraft), suggesting new design principles for lignin-derived biopolymers with tailored properties. These results highlight the surprising versatility of AAF oligomers towards the design of new biomaterials and further demonstrate that AAF can enable the conversion of all biomass fractions into value-added products.

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Enantiopure 2-thia-4-azabicyclo[3.1.0]hexanes, which can be considered constrained cysteines, have been obtained from l-cysteine by application of the "self-regeneration of chirality" concept. The key intermediates are homochiral thiazolines that can be prepared in multigram scale and react smoothly with a series of diazoalkanes providing Δ-pyrazolines except for phenyldiazomethane that yield the isomeric Δ-pyrazolines. Nitrogen extrusion in Δ-pyrazolines and further reduction of the sulfinyl group yielded the target compounds in good overall yield. Computational studies of the cycloaddition reaction were used for determining the polarity of the process and explaining the observed stereoselectivity. Additional topological studies were employed for determining the influence of noncovalent interactions in the stereochemical course of the reaction, which showed to be a highly asynchronous concerted process.

The behavior of homochiral 2,3-dihydrothiazoles, easily available from l-cysteine in Diels-Alder reaction with different dienes, "en route" to sterically constrained modified cystines, has been studied. The oxidation level of the sulfur atom of the heterocyclic ring was crucial for the course of the reaction. Whereas 2,3-dihydrothiazoles did not lead to Diels-Alder adducts, 1-oxide and 1,1-dioxide derivatives afforded the exo adduct enantiopurely in high yields and diastereoselectivities. Further elaboration of the resulting adducts provided conformationally restricted quaternary cystines. DFT calculations correctly predict both the reactivity and stereoselectivity observed experimentally.