Indole synthesis: a review and proposed classification

An Asymmetric Variant of the Bischler–Möhlau Indole Synthesis

Enantiomerically enriched planar chiral indoles were prepared by an asymmetric variant of the Bischler–Möhlau synthesis. Cinchonine was used as a ‘traceless’ resolving reagent, allowing the formation of readily separable quaternary ammonium salts that were the key intermediates in the indole synthesis. The condensation of the ammonium salt and various substituted anilines was achieved under microwave irradiation; this reduced reaction times and minimised racemisation. The utility of the indoles was demonstrated by their transformation into monophosphane ligands that were capable of coupling challenging substrates in the Suzuki–Miyaura reaction.

Unusual 1,2-aryl migration in Pd(ii)-catalyzed aza-Wacker-type cyclization of 2-alkenylanilines

The discovery of an unprecedented 1,2-aryl migratory process in Pd(ii)-catalyzed aza-Wacker-type cyclization of 2-alkenylanilines that led to a novel synthesis of C3-substituted indoles is demonstrated.

Synthesis of N-vinylindoles through copper catalyzed cyclization reaction of N-(2-alkynylphenyl)imine

A Cu(ii)-catalyzed cyclization reaction of N-(2-alkynylphenyl)imine to synthesize substituted N-vinylindoles is developed.

Palladium-catalyzed synthesis of 2-alkenyl-3-arylindoles via a dual α-arylation strategy: formal synthesis of the antilipemic drug fluvastatin

A new approach has been developed for the synthesis of substituted 2-alkenyl-3-arylindoles utilizing a palladium-catalyzed dual α-arylation of TES-enol ethers of enones as the key step.

Synthesis of functionalized tryptamines by Brønsted acid catalysed cascade reactions

A new solvent-free Brønsted acid catalysed cascade reaction has been established, allowing access to highly substituted tryptamines under mild conditions.

Synthesis of biologically important, fluorescence active 5-hydroxy benzo[g]indoles through four-component domino condensations and their fluorescence “Turn-off” sensing of Fe(iii) ions

The efficiently synthesized fluorescence active 5-hydroxy benzo[g]indoles show excellent fluorescence "Turn-off" sensing of Fe³⁺ ions.

Metal-free nitrative cyclization of N-aryl imines with tert-butyl nitrite: dehydrogenative access to 3-nitroindoles

A novel metal-free oxidative C–H functionalization and nitrative cyclization access to 3-nitroindoles from N-aryl imines is described.

Biosynthetically inspired divergent approach to monoterpene indole alkaloids: total synthesis of mersicarpine, leuconodines B and D, leuconoxine, melodinine E, leuconolam, and rhazinilam

Inspired by their potential biosynthesis, we have developed divergent total syntheses of seven monoterpene indole alkaloids including mersicarpine, leuconodines B and D, leuconoxine, melodinine E, leuconolam, and rhazinilam, and one unnatural analogue with an unprecedented structural skeleton. The key steps involve a Witkop-Winterfeldt oxidative indole cleavage followed by transannular cyclization. The transannular cyclization product was then converted to the corresponding structural skeletons by pairing its functional groups into different reaction modes.

A practical route to substituted 7-aminoindoles from pyrrole-3-carboxaldehydes

Among privileged structures, indoles occupy a central place in medicinal chemistry and alkaloid research. Here we report a flexible and efficient conversion of pyrrole-3-carboxaldehydes to substituted 7-amino-5-cyanoindoles. Phosphine addition to fumaronitrile proceeds with prototropic rearrangement of the initially formed zwitterion to the thermodynamically favored phosphonium ylide, which is poised for in situ Wittig olefination. The predominantly E-alkene product positions the allylic nitrile for facile intramolecular Hoeben–Hoesch reaction in the presence of BF₃·OEt₂. Syntheses of 2,5- and 3,5-disubstituted 7-aminoindoles are illustrated. Additionally, dianion alkylation of the allylic nitrile is demonstrated to furnish, after cyclization, 5,6-disubstituted 7-aminoindoles to further exemplify this scalable and high-yielding method.

Pyrazoloquinazolines: Synthetic strategies and bioactivities

Numerous N-heterocycles are indisputably evidenced to exhibit myriad biological activities. In the recent past, attempts made to condense the various heterocycles have resulted in derivatives possessing better bioactivities. Among many such condensed heterocycles, pyrazoloquinazolines have managed to hold the attention of many researchers, owing to the broad spectrum of activities they portray. This review is the first of its kind to congregate the various pyrazoloquinazolines reported until now and categorizes these structurally isomeric classes into eleven different groups based on the fusion pattern of the ring such as [1,5-c], [5,1-b], [4,3-h], etc. Furthermore, this review is a concerted effort to highlight design, synthetic strategies as well as biological activities of each class of this condensed heterocycle. Structure-activity relationship studies and in silico approaches wherever reported have also been discussed. In addition, manuscript also offers scope for design, synthesis and generation of libraries of unreported classes of pyrazoloquinazolines for the biological evaluation.

A highly concise and convergent synthesis of HCV polymerase inhibitor Deleobuvir (BI 207127): application of a one-pot borylation-Suzuki coupling reaction

A highly concise and convergent synthesis of HCV polymerase inhibitor Deleobuvir (BI 207127, 1) was achieved, featuring efficient Pd-catalyzed one-pot borylation-Suzuki coupling where TFP was identified as the unique ligand effective for these transformations.

N-heterocyclic-carbene-catalyzed synthesis of 2-aryl indoles

A convergent and efficient transition-metal-free catalytic synthesis of 2-aryl-indoles has been developed. The interception of a highly reactive and transient aza-ortho-quinone methide by an acyl anion equivalent generated through N-hetereocyclic carbene catalysis is central to this successful strategy. High yields and a wide scope as well as the streamlined synthesis of a kinase inhibitor are reported.

Copper-Catalyzed Synthesis of N-Aryl and N-Sulfonyl Indoles from 2-Vinylanilines with O2 as Terminal Oxidant and TEMPO as Co-Catalyst

A copper-catalyzed intramolecular alkene oxidative amination that utilizes TEMPO as co-catalyst and O₂ as the terminal oxidant has been developed. The method furnishes N-aryl and N-sulfonyl indoles from N-aryl and N-sulfonyl 2-vinylanilines, respectively. Additionally, sequential copper-catalyzed reactions where initial Chan-Lam coupling of 2-vinylanilines with arylboronic acids is followed by oxidative amination of the alkene can generate N-aryl indoles in one pot.

UV-photoelectron spectroscopy of BN indoles: experimental and computational electronic structure analysis

We present a comprehensive electronic structure analysis of two BN isosteres of indole using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of external BN indole I and fused BN indole II have been recorded, assessed by density functional theory calculations, and compared with natural indole. The first ionization energies of these indoles are natural indole (7.9 eV), external BN indole I (7.9 eV), and fused BN indole II (8.05 eV). The computationally determined molecular dipole moments are in the order: natural indole (2.177 D) > fused BN indole II (1.512 D) > external BN indole I (0.543 D). The λ_max in the UV–vis absorption spectra are in the order: fused BN indole II (292 nm) > external BN indole I (282 nm) > natural indole (270 nm). The observed relative electrophilic aromatic substitution reactivity of the investigated indoles with dimethyliminium chloride as the electrophile is as follows: fused BN indole II > natural indole > external BN indole I, and this trend correlates with the π-orbital coefficient at the 3-position. Nucleus-independent chemical shifts calculations show that the introduction of boron into an aromatic 6π-electron system leads to a reduction in aromaticity, presumably due to a stronger bond localization. Trends and conclusions from BN isosteres of simple monocyclic aromatic systems such as benzene and toluene are not necessarily translated to the bicyclic indole core. Thus, electronic structure consequences resulting from BN/CC isosterism will need to be evaluated individually from system to system.

A practical synthesis of indoles via a Pd-catalyzed C-N ring formation

A method for the synthesis of N-functionalized C2-/C3-substituted indoles via Pd-catalyzed C-N bond coupling of halo-aryl enamines is described. The general strategy utilizes a variety of amines and β-keto esters which are elaborated into halo-aryl enamines as latent precursors to indoles. The preferred conditions comprising the RuPhos precatalyst and RuPhos in the presence of NaOMe in 1,4-dioxane tolerate a variety of substituents and are scalable for the construction of indoles in multigram quantities.

Efficient synthesis of 3H-indoles enabled by the lead-mediated α-arylation of β-ketoesters or γ-lactams using aryl azides

The development of a lead-mediated α-arylation reaction between aryl azides and β-ketoesters or γ-lactams that facilitates the formation of 3H-indoles is disclosed. Twenty-five examples are included which demonstrate the generality of this reaction to access aryl azides bearing tetrasubstituted o-alkyl substituents. When paired with a Staudinger reduction, this reaction streamlines the synthesis of functionalized 3H-indoles.