Convenient synthesis of 10 H -indolo[3,2- b ]quinolines and 6 H -indolo[2,3- b ]quinolines by sequential chemoselective Suzuki reaction followed by double C-N coupling

Indoloquinoline Alkaloids as Antimalarials: Advances, Challenges, and Opportunities

Malaria infections affect almost half of the world's population, with over 200 million cases reported annually. Cryptolepis sanguinolenta, a plant native to West Africa, has long been used across various regions of Africa for malaria treatment. Chemical analysis has revealed that the plant is abundant in indoloquinolines, which have been shown to possess antimalarial properties. Cryptolepine, neocryptolepine, and isocryptolepine are well-studied indoloquinoline alkaloids known for their potent antimalarial activity. However, their structural rigidity and associated cellular toxicity are major drawbacks for preclinical development. This review focuses on the potential of indoloquinoline alkaloids (cryptolepine, neocryptolepine, and isocryptolepine) as scaffolds in drug discovery. The article delves into their antimalarial effects in vitro and in vivo, as well as their proposed mechanisms of action and structure-activity relationship studies. Several studies aim to improve these leads by reducing cytotoxicity while preserving or enhancing antimalarial activity and gaining insights into their mechanisms of action. These investigations highlight the potential of indoloquinolines as a scaffold for developing new antimalarial drugs.

Cu(OTf)2 Enhanced Intramolecular Nucleophilic N-Arylation of 2-Amino-3-arylquinolines

In the presence of Cu(OTf)₂ (5 mol %) and KO^tBu, a synergistic effect of the N-arylation process on 2-amino-3-arylquinolines is observed. Within 4 h, this method provides a wide variety of norneocryptolepine analogues with good to excellent yields. Overall, a double heteroannulation strategy for the synthesis of indoloquinoline alkaloids from nonheterocyclic precursors is demonstrated. Mechanistic investigations establish that the reaction proceeds via the S_NAr pathway. Despite moderate yields, the one-pot, two-step double heteroannulation illustrates that this procedure is highly atom-efficient. Neocryptolepine, a natural product, is also synthesized from indoloquinoline. A brief study of the photophysical properties of selected norneocryptolepine analogues is also discussed.

Palladium-Catalyzed Synthesis of Heterocyclic Ring Systems by Combination of Regioselective C–C with Twofold C–N Couplings

The combination of regioselective palladium-catalyzed C–C with twofold C–N couplings allows for the synthesis of a variety of heterocyclic ring systems. Starting materials include thiophenes and benzothiophenes, pyrroles and indoles, furans and benzofurans, pyridines, quinolines and quinoxalines, complex heterocyclic systems and benzophenone derivatives. The products are in many cases complex polyheterocyclic systems, which are not readily available by other methods or, in a number of cases, were not described in the literature before. They are of pharmacological relevance or interesting in the field of material science. Products include thieno[3,2-b:4,5-b′]diindoles, thieno[3,2-b]indoles, thieno[3,4-b]indoles, 5,10-dihydroindolo[3,2-b]indoles, furo[3,2-b:4,5-b′]diindoles, benzo[4,5]furo[3,2-b]indoles, 5,7-dihydropyrido[3,2-b:5,6-b′]diindoles, 5,7-dihydropyrido[2,3-b:6,5-b´]diindoles, α-, β-, γ- and δ-carbolines, indolo[3,2-b]quinolines, indolo[2,3-b]quinolines, indolo[3,2-c]quinolines, indoloquinoxalines, pyrido[2′,1′:2,3]imidazo[4,5-b]indoles, thiadiazolo[2′,3′:2,3]imidazo[4,5-b]indoles, benzo[b]carbazolediones, acridones and thieno[3,2-b]quinolones.

Contents

1 Introduction

2 Thiophenes and Benzothiophenes

3 Pyrroles and Indoles

4 Furans and Benzofurans

5 Pyridines

6 Quinolines and Quinoxalines

7 Complex Heterocyclic Systems

8 2,3-Dibromonaphthoquinone

9 Benzophenone Derivatives

10 Conclusions

Rhodium-catalyzed coupling-cyclization of o-alkynyl/propargyl arylazides or o-azidoaryl acetylenic ketones with arylisocyanides: synthesis of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines

The developed rhodium-catalyzed coupling-cyclization provides a new strategy for the assembly of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines.

Triflic acid-mediated N-heteroannulation of β-anilino-β-(methylthio)acrylonitriles: a facile synthesis of 4-amino-2-(methylthio)quinolines

Various functionalised 4-amino-2-(methylthio)quinolines are synthesised through triflic acid-mediated N-heteroannulation of α-functionalized-β-anilino-β-(methylthio)acrylonitriles for the first time. The N-heteroannulation process is highly chemoselective and has mild reaction conditions. However, this process fails in the absence of the β-methylthio group in the acrylonitriles. In addition, a new double N-heteroannulation process is demonstrated to synthesise indolo[3,2-c]quinolines from non-heterocyclic precursors. Natural product isocryptolepine is synthesised in four steps from an acyclic precursor.

Synthesis of thieno[2,3-h]-/[3,2-h]quinolines and thieno[2,3-f] quinolines by Brønsted acid mediated cycloisomerisation

Several thieno[2,3-h]-/[3,2-h]- and [2,3-f]quinolines have been synthesised from 2,3-dihalogenated pyridines or -quinolines by site-selective Pd catalysed cross-coupling reactions and Brønsted acid mediated cycloisomerisations as the final key step. This newly developed synthetic strategy is used in a modular way to synthesize diverse regioisomeric derivatives, tolerates various functional groups, and proceeds with high selectivity, and the desired final products have been isolated in high overall yields.

Advancements in the synthesis of fused tetracyclic quinoline derivatives

Fused tetracyclic systems containing a quinoline nucleus represent an important class of heterocyclic bioactive natural products and pharmaceuticals because of their significant and wide-spectrum biological properties. Several of these compounds have been obtained with diverse pharmacological and biological activities, such as antiplasmodial, antifungal, antibacterial, potent antiparasitic, antiproliferative, anti-tumor and anti-inflammatory activities. This information will be beneficial for medicinal chemists in the field of drug discovery to design and synthesize new fused tetracyclic quinolines as potent therapeutical agents. This review article provides a comprehensive report regarding the methods developed for the synthesis of fused tetracyclic quinolines reported so far (till October 2019). The article includes synthesis by one-pot domino reaction, microwave synthesis using a catalyst, using ionic liquids, photocatalytic synthesis (UV radiation), Pfitzinger reaction, I2-catalyzed cyclization reaction, Wittig reaction, cascade reaction, imino Diels-Alder reaction, Friedel-Crafts reaction, CDC reaction, solvent-free reactions and using small chiral organic molecules as catalysts. To the best of our knowledge, this is the first review focused on the synthesis of fused tetracyclic quinolines along with mechanistic aspects.

Recent Advances in the Hofmann Rearrangement and Its Application to Natural Product Synthesis

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C-N bond formation reactions are the most important transformations in (bio)organic chemistry because of the widespread occurrence of amines in pharmaceuticals, natural products, and biologically active compounds. The Hofmann rearrangement is a well-known method used for the preparation of primary amines from amides. But, the traditional version of the Hofmann rearrangement often gave relatively poor yields due to over-oxidation or due to the poor solubility of some amides in aqueous base, and created an enormous amount of waste products. Developments over the last two decades, in particular, have focused on refining both of these factors affecting the reaction. This review covers both the description of recent advances (2000-2019) in the Hofmann rearrangements and its applications in the synthesis of heterocycles, natural products and complex molecules of biological interest. It is revealed that organo-catalytic systems especially hypervalent iodine-based catalysts have been developed for the green and environmentally friendly conversion of carboxamides to primary amines and carbamates.

Efficient two-step synthesis of structurally diverse indolo[2,3-b]quinoline derivatives

A general and efficient synthesis of tetracyclic indolo[2,3-b]quinoline motifs with diverse functional groups is achieved through a simple but conceptually different annulation strategy in excellent yields.