Enamine catalysis is a fundamental activation mode in organocatalysis and can be successfully combined with other catalytic methods, e.g., photocatalysis. Recently, the elusive enamine intermediates were detected, and their stabilization modes were revealed. However, the formation pathway of this central organocatalytic intermediate is still a matter of dispute, and several mechanisms involving iminium and/or oxazolidinone are proposed. Here, the first experimentally determined rate constants and rates of enamine formation are presented using 1D selective exchange spectroscopy (EXSY) buildup curves and initial rate approximation. The trends of the enamine formation rates from exo-oxazolidinones and endo-oxazolidinones upon variation of the proline and water concentrations as well as the nucelophilic/basic properties of additives are investigated together with isomerization rates of the oxazolidinones. These first kinetic data of enamine formations in combination with theoretical calculations reveal the deprotonation of iminium intermediates as the dominant pathway in dimethyl sulfoxide (DMSO). The dominant enamine formation pathway varies according to the experimental conditions, e.g., the presence and strength of basic additives. The enamine formation is zero-order in proline and oxazolidinones, which excludes the direct deprotonation of oxazolidinones via E2 mechanism. The nucleophilicity of the additives influences only the isomerization rates of the oxazolidinones and not the enamine formation rates, which excludes a nucleophile-assisted anti elimination of oxazolidinones as a major enamine formation pathway.

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Conjugated enynes as well as cyclic nitronates are crucial building blocks for numerous natural products and pharmaceuticals. However, so far, no common and metal‐free synthetic route to both conjugated enynes and cyclic nitronates has been reported. Herein, in situ NMR, labelling studies and theoretical calculations were combined to investigate the mechanism of the unusual triple bond formation towards conjugated enynes. Starting from nitroalkene dimers, first an isoxazolidine‐2,5‐diol derivative is formed as central intermediate. From this, enynes were generated by a combination of oxidation, dehydration, and retro 1,3‐dipolar cycloaddition, whereas for nitronates a base induced intramolecular reorganization is proposed. While the product distribution could be controlled and high yields of nitronate were achieved, only medium to good yields for enynes were obtained due to polymerization losses. Nevertheless, we hope that these mechanistic investigations may provide a basis for further developments of organocatalytic or metal‐free preparations of conjugated enynes and nitronates.

The accurate description of cis/trans peptide structures is of fundamental relevance for the field of protein modeling and protein structure determination. A comprehensive conformational analysis of dipeptide model Ace-Gly-NMe (1) has been carried out by using a combination of theoretical calculations and experimental ((1) H and (13) C NMR and NOESY) spectroscopic measurements to assess the relevance of cis-peptide conformers. NMR measurements in dimethyl sulfoxide (DMSO) solution and calculations employing a continuum solvation model both point to the extended trans,trans conformer C5_tt as the global minimum. The cis-peptide structures C5_ct and C5_tc, with the N- or C-terminal amide group in cis-conformation, are observed separately and located 13.0±2 kJ mol(-1) higher in energy. This is in close agreement with the theoretical prediction of around 12 kJ mol(-1) in DMSO. The ability of common protein force fields to reproduce the energies of the cis-amide conformers C5_ct and C5_tc in 1 is limited, making these methods unsuitable for the description of cis-peptide structures in protein simulations.

The Cover Feature shows a machine that generates conjugated enynes and cyclic nitronates within a one‐pot reaction from simple nitroalkenes. The reaction towards both products shares a common pathway that deviates towards enynes or nitronates depending on the reaction conditions. By adjusting additive and base, the selectivity between enynes and nitronates can be controlled. NMR reaction monitoring in combination with theoretical calculations helped to elucidate the mechanism towards both products. This machine stands as a symbol for reaction flasks or NMR‐tubes in which the reactions were carried out. More information can be found in the https://doi.org/10.1002/ejoc.201801153