Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH₄OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.

Cu(OAc)2/DABCO-mediated domino reaction of vinyl malononitriles with cyclic sulfamidate imines: access to 6-hydroxyaryl-2-aminonicotinonitriles

A novel Cu(II)-salt/DABCO-mediated one-pot access to a myriad of highly substituted biologically relevant 2-aminonicotinonitriles possessing a resourceful phenolic moiety with satisfactory yields is reported. This method involves cyclic sulfamidate imines as 1C1N sources and different kinds of acyclic/cyclic vinyl malononitriles as 4C sources for pyridine synthesis via a vinylogous Mannich-cycloaromatization sequence process, creating two new C-N bonds under mild conditions. Importantly, this de novo strategy is applicable to gram-scale syntheses, underlining the method's practicability and allowing for a wide range of substrates with excellent functional group tolerance.

Total syntheses and antiproliferative activities of prenostodione and its analogues

A high-yielding total synthesis of the indole alkaloid prenostodione was completed in 4 steps and 44% overall yield from 1H-indole-3-carboxylic acid. The expedient syntheses of prenostodiones containing distinct substituents at the para position of the phenyl frame underscored the scope of this methodology. The cytotoxic activities of the tert-butyl esters of prenostodione analogues were tested using six tumor cell lines. Preliminary structure-activity studies revealed the importance of the identity of the aromatic substituent at the C-4 position for cytotoxic activity. The IC₅₀ values of these compounds were found to compare satisfactorily with those of the commercially available drugs etoposide and cisplatin. Furthermore, the compounds with, respectively, -OMe (14d) and -NO₂ (14f) groups at C-4 were more selective than these control compounds in PC-3, K-562, and MCF-7 cells. Also, computational studies were carried out to determine the ADMET profiles and passive membrane permeabilities of the compounds. The results suggested the promise of 14d and 14f as hit compounds for the development of new anticancer agents.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

γ-Carboline synthesis enabled by Rh(iii)-catalysed regioselective C-H annulation

A redox-neutral Rh(iii)-catalyzed C-H annulation of indolyl oximes was developed. Relying on the use of various alkynyl silanes as the terminal alkyne surrogates, the reaction exhibited a reverse regioselectivity, thus giving an exclusive and easy way for the synthesis of a wide range of substituent free γ-carbolines at C3 position with high efficiency. Deuterium-labelling experiments and kinetic analysis have preliminarily shed light on the working mode of this catalytic system.

Metal-Free Carbonyl-Assisted Regioselective Hydration of Alkynes: An Access to Dicarbonyls

Metal-free regioselective hydration of o-alkynylaldehydes with the assistance of neighboring carbonyl oxygen is disclosed. The developed protocol provides a facile route to synthesize a series of multisubstituted carbonyl containing scaffolds that enable the potential application toward the synthesis of highly diversified 5-azaindoles. γ-Carbolines and 2,8-diazacarbazoles can also be accessed directly without isolating the dicarbonyl compounds. The developed methodology is operationally simple and environment-friendly, tolerates a wide variety of functional groups, and is applicable toward large scale synthesis.

A Unified Strategy for the Synthesis of β-Carbolines, γ-Carbolines, and Other Fused Azaheteroaromatics under Mild, Metal-Free Conditions

An efficient, unified approach for the synthesis of β-carbolines, γ-carbolines, and other fused azaheteroaromatics has been realized under metal-free conditions, from propargylic amines and (hetero)aromatic aldehydes. This unified strategy provides β- and γ-carbolines as well as a range of fused azaheteroaromatics with a broad substrate scope and excellent functional group compatibility. The formal synthesis of oxopropalines D and G has been achieved on gram scale (3a), in a one-pot reaction from commercially available materials (previous shortest reported route to 3a was 5 steps). NMR studies of the conversion of imine intermediate 3aa to β-carboline 3a were conducted and revealed that the reaction proceeded through an allene intermediate.

Recent developments on synthesis and biological activities of γ-carboline

γ-Carboline alkaloids are a family of natural and synthetic agents that have diverse bioactivities including antiviral, antibacterial, antifungal, antiparasitic, antitumor, anti-inflammatory, neuropharmacological activities and so on. They constitute an important class of pharmacologically active scaffolds that exhibit biological activity via diverse mechanisms. This review provides an update on the recent developments (2010-2017) in the synthesis and biological activities of these compounds. In cases where sufficient information is available, the mechanism and the structure-activity relationship (SAR) of biological activity are presented, and based on our expertise in the field and careful analysis of the recent literature, for the potential of γ-carboline alkaloids as medicinal drugs is proposed.

γ-Carboline Synthesis by Heterocyclization of TosMIC Derivatives

A new method for the synthesis of γ-carbolines by a heterocyclization that involves α-indol-2-ylmethyl TosMIC derivatives and different electrophiles has been developed. This methodology has been successfully applied to the synthesis of several highly substituted γ-carbolines.