Selectivity in the Addition of Electron Deficient Radicals to the C2 Position of Indoles

Isomeric Identification of the Nitroindole Chromophore in Indole + NO3 Organic Aerosol

Oxidation of indole by nitrate radical (NO3) was previously proposed to form nitroindole, largely responsible for the brown color of indole secondary organic aerosol (SOA). As there are seven known nitroindole isomers, we used chromatographic separation to show that a single nitroindole isomer is produced in the indole + NO3 reaction and definitively assigned it to 3-nitroindole by comparison with chromatograms of nitroindole standards. Mass spectra of aerosolized 3-nitroindole particles were recorded with an aerosol mass spectrometer and directly compared to mass spectra of SOA from smog chamber oxidation of indole by NO3 in order to help identify peaks unique to nitroindole (m/z 162, 132, and 116). Quantum chemical calculations were done to determine the energetics of hypothesized indole + NO3 intermediates and products. The combination of these data suggests a mechanism, wherein a hydrogen atom is first abstracted from the N-H bond in indole, followed by isomerization to a carbon-centered radical in the 3-position and followed by addition of NO2. Alternative mechanisms involving a direct abstraction of a H atom from a C-H bond or a NO3 addition to the ring are predicted to be energetically unfavorable from large barriers for the initial reaction steps.

Photoredox C(2)-Arylation of Indole- and Tryptophan-Containing Biomolecules

We introduce a novel and straightforward methodology for photoredox arylation of an indole scaffold using aryldiazonium salts under mild and metal-free conditions. Our approach enables the regioselective and chemoselective introduction of several aryl groups to the C(2) position of indoles and tryptophan, even in competition with other amino acids. This approach extends to the late-stage functionalization of peptides and lysozyme, heralding the unprecedented arylation of tryptophan residues in wild-type proteins and offering broad utility in chemical biology.

Chemoselective Late-Stage Functionalization of Peptides via Photocatalytic C2-Alkylation of Tryptophan

Across eukaryotic proteomes, tryptophan is the least abundant of the 20 canonical amino acids, which makes it an ideal chemical handle for the late-stage functionalization of peptide and protein scaffolds with minimal production of undesired isoforms. Herein, we report the photocatalytic C2-alkylation of tryptophan using bromodifluoroacetate/acetamide-derived radical precursors. This rapid visible-light-mediated reaction is additive-free, operationally simple, and tolerates diverse functionality. We demonstrate the late-stage modification of a variety of complex peptides, including examples of biological significance.

Regioselective Radical Alkylation of Arenes Using Evolved Photoenzymes

Substituted arenes are ubiquitous in molecules with medicinal functions, making their synthesis a critical consideration when designing synthetic routes. Regioselective C-H functionalization reactions are attractive for preparing alkylated arenes; however, the selectivity of existing methods is modest and primarily governed by the substrate's electronic properties. Here, we demonstrate a biocatalyst-controlled method for the regioselective alkylation of electron-rich and electron-deficient heteroarenes. Starting from an unselective "ene"-reductase (ERED) (GluER-T36A), we evolved a variant that selectively alkylates the C4 position of indole, an elusive position using prior technologies. Mechanistic studies across the evolutionary series indicate that changes to the protein active site alter the electronic character of the charge transfer (CT) complex responsible for radical formation. This resulted in a variant with a significant degree of ground-state CT in the CT complex. Mechanistic studies on a C2-selective ERED suggest that the evolution of GluER-T36A helps disfavor a competing mechanistic pathway. Additional protein engineering campaigns were carried out for a C8-selective quinoline alkylation. This study highlights the opportunity to use enzymes for regioselective radical reactions, where small molecule catalysts struggle to alter selectivity.

Regioselective Radical Alkylation of Arenes Using Evolved Photoenzymes

Substituted arenes are ubiquitous in molecules with medicinal functions, making their synthesis a critical consideration when designing synthetic routes. ^1,2 Regioselective C-H functionalization reactions are attractive for preparing alkylated arenes, ^3-5 however, the selectivity of existing methods is modest and primarily governed by substrate electronic properties. ^6,7 Here, we demonstrate a biocatalyst-controlled method for the regioselective alkylation of electron-rich and electron-deficient heteroarenes. Starting from an unselective 'ene'-reductase (ERED) (GluER-T36A), we evolved a variant that selectively alkylates the C4 position of indole, an elusive position using prior technologies. Mechanistic studies across the evolutionary series indicate that changes to the protein active site alter the electronic character of the charge transfer (CT) complex responsible for radical formation. This resulted in a variant with a significant degree of ground state change transfer in the CT complex. Mechanistic studies on a C2 selective ERED suggest that the evolution of GluER-T36A helps disfavor a competing mechanistic pathway. Additional protein engineering campaigns were carried out for a C8 selective quinoline alkylation. This study highlights the opportunity to use enzymes for regioselective reactions where small molecule catalysts struggle to alter selectivity.

A Unified Synthetic Strategy to Introduce Heteroatoms via Electrochemical Functionalization of Alkyl Organoboron Reagents

Based on systematic electrochemical analysis, an integrated synthetic platform of C(sp³)-based organoboron compounds was established for the introduction of heteroatoms. The electrochemically mediated bond-forming strategy was shown to be highly effective for the functionalization of sp³-hybridized carbon atoms with significant steric hindrance. Moreover, virtually all the nonmetallic heteroatoms could be utilized as reaction partners using one unified protocol. The observed reactivity stems from the two consecutive single-electron oxidations of the substrate, which eventually generates an extremely reactive carbocation as the key intermediate. The detailed reaction profile could be elucidated through multifaceted electrochemical studies. Ultimately, a new dimension in the activation strategies for organoboron compounds was accomplished through the electrochemically driven reaction development.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials

The dearomative dicarboxylation of stable heteroaromatics using CO₂ is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO₂ radical anion (E_1/2 of CO₂ = -2.2 V in DMF and -2.3 V in CH₃CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.

Total Synthesis of (-)-Strempeliopine

A total synthesis of (-)-strempeliopine is disclosed that enlists a powerful SmI₂-mediated and BF₃·OEt₂-initiated dearomative transannular radical cyclization onto an indole by an N-acyl α-aminoalkyl radical that is derived by single electron reduction of an in situ generated iminium ion for formation of a quaternary center and the strategic C19-C2 bond in its core structure.