Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Visible-Light-Induced Highly Site-Selective Direct C-H Phosphorylation of Pyrrolo[2,3-d]pyrimidine Derivatives with H-Phosphine Oxides

An efficient and highly regioselective C6-phosphorylation protocol for pyrrolo[2,3-d]pyrimidine (7-DAP) derivatives with various H-phosphine oxides induced by visible light at room temperature is described for the first time. This protocol has been successfully achieved by the combination of Na2-eosin Y as a photocatalyst and LPO as an oxidant under transition metal- and additive-free conditions. The broad substrate scope, good functional group tolerance, excellent regioselectivity, and air tolerant conditions make this process favorable for the functional modification of pyrrolo[2,3-d]pyrimidine scaffold and enrich the phosphorylated 7-DAP compounds for further biological evaluation.

Regioselective C3-H Alkylation of Imidazopyridines with Donor-Acceptor Cyclopropanes

Alkylated imidazopyridines are crucial structures for medicinal chemistry. Here, an efficient method for the C3-H alkylation of imidazopyridines was devised. Under Lewis acid-catalyzed conditions, reactions of imidazopyridines with different donor-acceptor cyclopropanes were carried out. Finally, 1,3-bisfunctionalizaton of donor-acceptor cyclopropanes was performed. In addition, synthesized C3-alkylated imidazopyridines are amenable for diverse synthetic applications.

Palladium-Catalyzed Regioselective C3-Allylic Alkylation of 2-Aryl Imidazopyridines with MBH Carbonates

Imidazopyridine is an important framework that constitutes several pharmaceutical drugs and biologically active molecules. Herein, we present the palladium-catalyzed regioselective C3-allylic alkylation of 2-aryl imidazopyridines with MBH carbonates. This strategy furnishes a broad spectrum of C3-allylated imidazopyridines, and their structures have been unequivocally established using X-ray analysis. Besides, the reaction can be easily scaled up on a gram scale, and the ensuing product can be smoothly manipulated into synthetically useful entities.

Functionalization of imidazo[1,2-a]pyridines via radical reactions

The recent advances in radical reactions for the direct functionalization of imidazo[1,2-a]pyridines are reviewed.

Shining Light on the Light-Bearing Element: A Brief Review of Photomediated C–H Phosphorylation Reactions

Organophosphorus compounds have numerous useful applications, from versatile ligands and nucleophiles in the case of trivalent organophosphorus species to therapeutics, agrochemicals and material additives for pentavalent species. Although phosphorus chemistry is a fairly mature field, the construction of C–P(V) bonds relies heavily on either prefunctionalized substrates such as alkyl or aryl halides, or requires previously oxidized bonds such as C=N or C=O, leading to potential sustainability issues when looking at the overall synthetic route. In light of the recent advances in photochemistry, using photons as a reagent can provide better alternatives for phosphorylations by unlocking radical mechanisms and providing interesting redox pathways. This review will showcase the different photomediated phosphorylation procedures available for converting C–H bonds into C–P(V) bonds.

1 Introduction

1.1 Organophosphorus Compounds

1.2 Phosphorylation: Construction of C–P(V) Bonds

1.3 Photochemistry as an Alternative to Classical Phosphorylations

2 Ionic Mechanisms Involving Nucleophilic Additions

3 Mechanisms Involving Radical Intermediates

3.1 Mechanisms Involving Reactive Carbon Radicals

3.2 Mechanisms Involving Phosphorus Radicals

3.2.1 Photoredox: Direct Creation of Phosphorus Radicals

3.2.2 Photoredox: Indirect Creation of Phosphorus Radicals

3.2.3 Dual Catalysis

3.3 Photolytic Cleavage

4 Conclusion and Outlook

Diversity-Oriented Synthesis toward Aryl- and Phosphoryl-Functionalized Imidazo[1,2-a]pyridines

We report herein an efficient synthesis of diversely polysubstituted imidazo[1,2-a]pyridines, a family of aza-heterocycles endowed with numerous biological properties, through a sequence involving two consecutive palladium-catalyzed cross-coupling reactions. First, we demonstrated that a Hirao coupling occurred straightforwardly in high yields at positions 3, 5, and 6 of imidazopyridine derivatives, giving access to a wide variety of substituted phosphonates, phosphinates, and phosphine oxides. In a second step, direct CH-arylation of phosphorylimidazopyridines with aryl halides was found to be effective and fully selective, leading to 3-aryl-substituted imidazopyridines in moderate to high yields depending on steric hindrance.

Visible-Light-Induced Phosphorylation of Imidazo-Fused Heterocycles under Metal-Free Conditions

A metal-free and base-free procedure for the phosphorylation of imidazo[1,2-a]pyridines with phosphine oxides under the irradiation of visible light at room temperature in green solvent was reported, featuring mild and sustainable conditions, convenient operation, as well as good functional group compatibility.

Oxidative sulfonamidomethylation of imidazopyridines utilizing methanol as the main C1 source

An efficient one pot, three component synthesis of C3 sulfonamidomethylated imidazopyridines has been disclosed under metal-free conditions, which utilized the commercially available and renewable reagent methanol as the main methylene source. A wide range of substituted imidazopyridines and sulfamides/amines were well tolerated to afford the corresponding products in up to 92% yield. In the isotopic labelling experiment, it was found that a minor part of the methylene also originated from DTBP. Moreover, the radical scavenger reactions were conducted, which suggested that a free-radical mechanism was probably not involved. The current methodology featured several advantages, including broad substrate scope, good functional group tolerance and high reaction efficiency.

Exogenous-oxidant-free electrochemical oxidative C-H phosphonylation with hydrogen evolution

We herein report a versatile and environmentally friendly electrochemical oxidative C-H phosphonylation protocol. This protocol features a broad substrate scope; not only C(sp2)-H phosphonylation, but also C(sp3)-H phosphonylation is tolerated well under exogenous-oxidant-free and metal catalyst-free electrochemical oxidation conditions.

The regioselective synthesis of 2-phosphinoylindoles via Rh(iii)-catalyzed C–H activation

A step-economical route for the synthesis of 2-phosphinoylindoles via the Rh(iii)-catalyzed redox-neutral annulation of N-nitrosoanilines with 1-alkynylphosphine oxides was developed.

Recent progress in the synthesis of phosphorus-containing indole derivatives

Phosphorus-containing indole derivatives represent a special class of phosphorus-containing nitrogen heterocycles. This review summarizes the recent progress in the synthesis of such compounds, briefly discusses the reaction mechanisms and challenges, and outlines the synthetic opportunities still open.

Copper-catalyzed C–N bond formation with imidazo[1,2-a]pyridines

An efficient Cu-catalyzed C–N bond formation with imidazo[1,2-a]pyridines on the C-3 position via a radical pathway is reported.

CuCl/air-mediated oxidative coupling reaction of imidazoheterocycles with N-aryl glycine esters

A novel CuCl/air-mediated oxidative cross-coupling of imidazoheterocycles with glycine esters has been developed.

Visible light-induced C3-sulfonamidation of imidazopyridines with sulfamides

A visible light-induced regioselective sulfonamidation of imidazo[1,2-a]pyridines was developed using sulfamides as the nitrogen sources and aqueous NaClO solution as the oxidant under mild conditions.

Silver(i)-promoted C5–H phosphonation of 8-aminoquinoline amides with H-phosphonates

An efficient protocol for silver(i)-promoted C5–H phosphonation of 8-aminoquinoline amides with dialkyl H-phosphonates was developed.