Efficient methods to introduce bioorthogonal groups, such as terminal alkynes, into biomolecules are important tools for chemical biology. State-of-the-art approaches are based on the introduction of a linker between the targeted amino acid and the alkyne, and still present limitations of either reactivity, selectivity or adduct stability. Herein, we present an ethynylation method of cysteine residues based on the use of ethynylbenziodoxolone (EBX) reagents. In contrast to other approaches, the acetylene group is directly introduced onto the thiol group of cysteine and can be used in onepot in a copper-catalyzed alkyne-azide cycloaddition (CuAAC) for further functionalization. Labeling proceeded with reaction rates comparable or higher than the most often used iodoacetamide on peptides or maleimide on the antibody trastuzumab. Under optimized conditions, high cysteine selectivity was observed. The reagents were also used in living cells for cysteine proteomic profiling and displayed an improved coverage of the cysteinome compared to previously reported iodoacetamide or hypervalent iodine-reagent based probes. Fine-tuning of the EBX reagents allowed optimization of their reactivity and physical properties for the desired application. Scheme 1. Well-established alkyne-linker approach for cysteine functionalization.

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Herein, we report a mechanistic exploration of the unusual FeCl-mediated hydroarylation of N-Ac indoles. Electron density topology analysis of a crystal, in situ IR monitoring, Hammett and Taft studies as well as DFT computations allowed us to determine that activation of acetyl with FeCl and of the C2[double bond, length as m-dash]C3 bond with a proton is involved.