Late-Stage Photoredox C-H Amidation of N-Unprotected Indole Derivatives: Access to N-(Indol-2-yl)amides

The late-stage functionalization of N-unprotected indoles can be useful for modifying low-molecular-weight drugs and bioactive peptides. Whereas indole carboxamides are valuable in pharmaceutical applications, the preparation N-(indol-2-yl)amides with similar structures continues to be challenging. Herein we report on visible-light-induced late-stage photoredox C-H amidation with N-unprotected indoles and tryptophan-containing peptides, leading to the formation of N-(indol-2-yl)amide derivatives. N-Unprotected indoles and aryloxyamides that contain an electron-withdrawing group could be coupled directly to eosin Y as the photocatalyst by irradiation with a green light-emitting diode at room temperature. Mechanistic studies and density functional theory calculations indicate that the transformation might proceed through the oxidative C-H functionalization of indole with a PS* to PS^•- cycle. This protocol provides a new toolkit for the late-stage modification labeling and peptide-drug conjugation of N-unprotected indole derivatives.

Chemoselective Cu-catalyzed synthesis of diverse N-arylindole carboxamides, β-oxo amides and N-arylindole-3-carbonitriles using diaryliodonium salts

Chemoselective copper-catalyzed synthesis of diverse N-arylindole-3-carboxamides, β-oxo amides and N-arylindole-3-carbonitriles from readily accessible indole-3-carbonitriles, α-cyano ketones and diaryliodonium salts has been developed. Diverse N-arylindole-3-carboxamides and β-oxo amides were successfully achieved in high yields under copper-catalyzed neutral reaction conditions, and the addition of an organic base (DIPEA) resulted in a completely different selectivity pattern to produce N-arylindole-3-carbonitriles. Moreover, the importance of the developed methodology was realized by the synthesis of indoloquinolones and N-((1H-indol-3-yl)methyl)aniline and by a single-step gram-scale synthesis of the naturally occurring cephalandole A analogue.

Carboxylic Acid-Promoted Single-Step Indole Construction from Simple Anilines and Ketones via Aerobic Cross-Dehydrogenative Coupling

The cross-dehydrogenative coupling (CDC) reaction is an efficient strategy for indole synthesis. However, most CDC methods require special substrates, and the presence of inherent groups limits the versatility for further transformation. A carboxylic acid-promoted aerobic catalytic system is developed herein for a single-step synthesis of indoles from simple anilines and ketones. This versatile system is featured by the broad substrate scope and the use of ambient oxygen as an oxidant and is convenient and economical for both laboratory and industry applications. The existence of the labile hydrogen at C-3 and the highly transformable carbonyl at C-2 makes the indoles versatile building blocks for organic synthesis in different contexts. Computational studies based on the density functional theory (DFT) suggest that the rate-determining step is carboxylic acid-assisted condensation of the substrates, rather than the functionalization of aryl C-H. Accordingly, a pathway via imine intermediates is deemed to be the preferred mechanism. In contrast to the general deduction, the in situ formed imine, instead of its enamine isomer, is believed to be involved in the first ligand exchange and later carbopalladation of the α-Me, which shed new light on this indolization mechanism.

The Curtius Rearrangement: Applications in Modern Drug Discovery and Medicinal Chemistry

The Curtius rearrangement is the thermal decomposition of an acyl azide derived from carboxylic acid to produce an isocyanate as the initial product. The isocyanate can undergo further reactions to provide amines and their derivatives. Due to its tolerance for a large variety of functional groups and complete retention of stereochemistry during rearrangement, the Curtius rearrangement has been used in the synthesis of a wide variety of medicinal agents with amines and amine-derived functional groups such as ureas and urethanes. The current review outlines various applications of the Curtius rearrangement in drug discovery and medicinal chemistry. In particular, the review highlights some widely used rearrangement methods, syntheses of some key agents for popular drug targets and FDA-approved drugs. In addition, the review highlights applications of the Curtius rearrangement in continuous-flow protocols for the scale-up of active pharmaceutical ingredients.