3‐Aminoindole Synthesis from 2‐Nitrochalcones and Ammonia or Primary Amines

A Two-Step Synthesis of Unprotected 3-Aminoindoles via Post Functionalization with Nitrostyrene

A novel, low-cost method for the preparation of not easily accessible free 3-aminoindoles has been developed. This approach is based on a well-established reaction between indoles and nitrostyrene in the presence of phosphorous acid, which results in the formation of 4'-phenyl-4'H-spiro[indole-3,5'-isoxazoles]. The latter could be transformed to corresponding aminated indoles by reaction with hydrazine hydrate in good or excellent yields upon microwave-assisted heating.

Synthesis of 2-carboxyaniline-substituted maleimides from 2'-nitrochalcones

A practical, one-pot approach to 3-anilino-4-(het)arylmaleimides by simple heating of aqueous DMSO solution of 2'-nitrochalcones with potassium cyanide in the presence of formic acid has been developed. This new reaction provides effective access to a variety of β-substituted α-aminomaleimides which have recently become a subject of growing interest as small, easily modified and environmentally responsive fluorescent probes.

PPh3-Mediated Cascade Reaction of 2-Alkynylnitrobenzenes and Thioureas for the Construction of Imidazo[4,5-b]indole-2-thiones

A simple method for the preparation of imidazo[4,5-b]indole-2-thiones from 2-alkynylnitrobenzenes and thioureas is described. In the reaction, a Wittig-like process was triggered by PPh₃ and followed by a cyclization step. The products were afforded in yields of 70-98% under mild conditions. Additionally, the 2-alkynylnitrobenzenes were stable and could be prepared via a simple coupling step.

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reduction of the stable nitro group is a fundamental and widely used transformation for the construction of complex and functionalized heterocyclic architectures. The unfolding of the reactivity of the nitro group in the 2-nitrostyrene moiety not only triggers the formation of carbon-nitrogen bonds, but also offers the opportunity for annulation and heteroannulation, thereby providing a cascade process for the synthesis of highly conjugated natural and unnatural molecules. In this review, we comprehensively discuss the use of 2-nitrostyrene motifs in the synthesis of various N-heterocycles. We offer readers an overview of the synthetic achievements achieved to date, highlighting their important features, reactivities, and mechanisms.

Cu(I)-Catalyzed and Base-Promoted [5 + 2 + 1] Cascade Cyclization of 2-Nitrochalcones with Aliphatic Primary Amines to 5H-Pyrimido[5,4-b]indole Frameworks

An unprecedented [5 + 2 + 1] cascade cyclization to the preparation of 5H-pyrimido[5,4-b]indole derivatives has been disclosed. The novel protocol of 2-nitrochalcones reacted with aliphatic primary amines catalyzed by CuI and promoted by Cs₂CO₃, which underwent a critical intermediate 2,3-disubstituted indole, providing structurally diverse 5H-pyrimido[5,4-b]indoles in generally high yields (77-90%) and broad substrate scopes (34 examples).

Cobalt-Catalyzed Chemoselective Transfer Hydrogenative Cyclization Cascade of Enone-Tethered Aldehydes

The ligand-free Co-catalyzed chemoselective reductive cyclization cascade of enone-tethered aldehydes with i-PrOH as the environmentally benign hydrogen surrogate is developed by this study. Mechanistic studies disclosed that such a protocol is initiated by an ortho-enone-assisted Co(I)-catalyzed reduction of the aldehyde functionality with i-PrOH. Meanwhile, the selectivity from the Michael-Aldol cycloreduction cascade to the oxa-Michael cascade is feasible and readily adjusted by the addition of steric Lewis bases, such as TEMPO and DABCO, delivering substituted 1H-indenes and dihydroisobenzofurans, respectively.

N2H4-H2O Enabled Umpolung Cyclization of o-Nitro Chalcones for the Construction of Quinoline N-Oxides

Umpolung is a unique strategy which converts the property of an atom into the opposite one. An efficient and general method for the construction of quinoline N-oxides via umpolung of carbonyl groups was developed from ortho-nitro chalcones and hydrazine in basic conditions. The strategy is transition-metal free and has good functional group tolerance, environmental friendliness, as well as mild reaction conditions with nitrogen gas as the byproduct.

Rh(iii)-Catalyzed sulfonylamination of α-indolyl alcohols via Csp2–Csp3 bond cleavage

A Rh(iii)-catalyzed beta-carbon amination of α-aryl alcohols with sulfonyl azides has been developed. This transformation features unstrained Csp²–Csp³ σ bond amination via C–C bond cleavage, and provides a direct approach to complex 2-aminoindoles.