CuI-Catalyzed Selective 3-Alkylation of Indoles with N -Tosylhydrazones and the I2 -Mediated Further Cyclization to Chromeno[2,3-b ]Indoles

Selective C-O Bond Forming Reactions at Indole-C2-Position toward Polycyclic Indolone or Indolinone Derivatives Tethered with Medium-Sized Rings

A methodology involving the chemoselective synthesis of tetracyclic [1,3]oxazino[3,2-a]indol-4-one or tetracyclic [1,3]oxazino[3,2-a]indoline-4-one tethered with a medium-sized ring by cross dehydrogenative coupling (CDC) or nucleophilic addition (NA) reaction has been developed. [1,3]Oxazino[3,2-a]indol-4-one compounds fused with a medium-sized ring were constructed through a CDC reaction in the presence of I₂ and K₂CO₃. Whereas, [1,3]oxazino[3,2-a]indoline-4-ones tethered with a medium-sized ring were obtained with a TfOH system by NA reaction.

Synthesis of Dihydrochromeno[2,3-b]indoles via an Fe(OTf)3-Catalyzed Cascade Reaction

A tandem cyclization between 2-(2-aminophenyl)acetonitriles and 2-(hydroxy(aryl)methyl)phenols for the preparation of dihydrochromeno[2,3-b]indole derivatives was successfully developed. The transformation proceeded smoothly in the presence of a catalytic environmentally benign iron salt, and various multisubstituted dihydrochromeno[2,3-b]indoles were synthesized in moderate to high yields.

Copper-assisted synthesis of dihydropyrano[2.3-b]indole-4-ones by domino cascade reaction

A novel copper-catalyzed cascade intermolecular and intramolecular oxidation/cyclization domino one-pot reaction process for the regioselective synthesis of dihydropyrano[2.3-b]indol-4(9H)-ones has been successfully developed. In this methodology, it is proposed for the first time that the 4-benzyloxy group of indole substrates can be used as a guiding group to promote cyclization under mild conditions. Meanwhile, reaction mechanism studies indicate that carbonyl oxygen in pyranoindole-4-ones came from water and the guiding group is critical to the progress of the reaction.

Divergent Syntheses of Indoles and Quinolines Involving N1-C2-C3 Bond Formation through Two Distinct Pd Catalyses

Pd-catalyzed annulative couplings of 2-alkenylanilines with aldehydes using alcohols as both the solvent and hydrogen source have been developed. These domino processes allow divergent syntheses of two significant N-heterocycles, indoles and quinolines, from the same substrate by tuning reaction parameters, which seems to invoke two distinct mechanisms. The nature of the ligand and alcoholic solvent had a profound influence on the selectivity and efficiency of these protocols. Particularly noteworthy is that indole formation was achieved by overcoming two significant challenges, regioselective hydropalladation of alkenes and subsequent reactions between the resulting Csp³-Pd species and less reactive imines.

Completely regioselective insertion of unsymmetrical alkynes into electron-deficient alkenes for the synthesis of new pentacyclic indoles

A Lewis acid catalyzed insertion of unsymmetrical alkynes into electron-deficient alkenes has been reported for the first time, leading to 34 examples of hitherto unreported pentacyclic benzo[5,6]chromeno[2,3-b]indoles.

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.