Dialling-In New Reactivity into the Shono-Type Anodic Oxidation Reaction

A General Method to Access Underexplored Ylideneamino Sulfates as Interrupted Beckmann-Type Rearrangement Intermediates

The Beckmann rearrangement of ketoximes to their corresponding amides, using a Brønsted acid-mediated fragmentation and migration sequence, has found wide-spread industrial application. We postulated that the development of a methodology to access ylideneamino sulfates using tributylsulfoammonium betaine (TBSAB) would afford isolable Beckmann-type intermediates and competent partners for subsequent rearrangement cascades. The ylideneamino sulfates generated, isolated as their tributylammonium salts, are sufficiently activated to undergo Beckmann rearrangement without additional reagent activation. The generation of sulfuric acid in situ from the ylideneamino sulfate giving rise to a routine Beckmann rearrangement and additional amide bond cleavage to the corresponding aniline was detrimental to reaction success. The screening of bases revealed inexpensive sodium bicarbonate to be an effective additive to prevent classic Brønsted acid-mediated fragmentation and achieve optimal conversions of up to 99%.

Tunable Electrochemical Peptide Modifications: Unlocking New Levels of Orthogonality for Side-Chain Functionalization

Electrochemical transformations provide enticing opportunities for programmable, residue-specific peptide modifications. Herein, we harness the potential of amidic side-chains as underutilized handles for late-stage modification through the development of an electroauxiliary-assisted oxidation of glutamine residues within unprotected peptides. Glutamine building blocks bearing electroactive side-chain N,S-acetals are incorporated into peptides using standard Fmoc-SPPS. Anodic oxidation of the electroauxiliary in the presence of diverse alcohol nucleophiles enables the installation of high-value N,O-acetal functionalities. Proof-of-principle for an electrochemical peptide stapling protocol, as well as the functionalization of dynorphin B, an endogenous opioid peptide, demonstrates the applicability of the method to intricate peptide systems. Finally, the site-selective and tunable electrochemical modification of a peptide bearing two discretely oxidizable sites is achieved.

Electrochemical Friedel–Crafts-type amidomethylation of arenes by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborate

Electrochemical Friedel–Crafts-type amidomethylation was successfully carried out by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborates, such as iPr₂NHEtBF₄. Constant current electrolysis of 1,3,5-trimethoxybenzene or indoles in DMA containing 0.1 M iPr₂NHEtBF₄ using an undivided cell equipped with a Pt plate cathode and a Pt wire anode (a quasi-divided cell) resulted in selective formation of N-acyliminium ions of DMA at the anode, which reacted with arenes to give the corresponding amidomethylated products in good to high yields.

Electrosynthesis of 2-Substituted Benzoxazoles via Intramolecular Shono-Type Oxidative Coupling of Glycine Derivatives

Herein, an atom-economical and eco-friendly electrochemical oxidation/cyclization of glycine derivatives through intramolecular Shono-type oxidative coupling is disclosed, leading to a variety of 2-substituted benzoxazoles in 51-85% yields. This oxidative cyclization proceeded in transition metal- and oxidant-free conditions and generated H₂ as only a byproduct. Additionally, gram-scale reactions and a broad substrate scope demonstrated the synthetic usefulness of this protocol.

Electrochemical Desaturative β-Acylation of Cyclic N-Aryl Amines

Herein, we disclose a straightforward, robust, and simple route to access β-substituted desaturated cyclic amines via an electrochemically driven desaturative β-functionalization of cyclic amines. This transformation is based on multiple single-electron oxidation processes using catalytic amounts of ferrocene. The reaction proceeds in the absence of stoichiometric amounts of electrolyte under mild conditions, affording the desired products with high chemo- and regioselectivity. The reaction was tolerant of a broad range of substrates and also enables late-stage β-C(sp³ )-H acylation of potentially valuable products. Preliminary mechanistic studies using cyclic voltammetry reveal the key role of ferrocene as a redox mediator in the reaction.