Green Synthesis of Bioactive Pyrrole Derivatives via Heterogeneous Catalysts Since 2010

Pyrrole derivatives are known as building blocks for the synthesis of biological compounds and pharmaceutical drugs. Several processes were employed to synthesize pyrroles, including Hantzsch, Paal-Knorr, and cycloaddition of dicarbonyl compounds reaction. Using catalysts like nanoparticles, metal salts, and heterogeneous ones was necessary to obtain the targeted pyrrole structure. Also, to afford more active pyrrole compounds, heterocyclic molecules such as imidazole or other rings were used in the synthesis as amines. This review presents heterogeneous catalysts since 2010 for the green synthesis of bioactive pyrroles in a one-pot multi-component reaction. Additionally, each synthetic method included a demonstration of the suggested mechanisms. Diakylacetylenedicarboxylate, dicarbonyl group, amines, furans, and acetylene group are consolidated to yield biological pyrroles through the heterogeneous catalysts. Finally, various pyrrole-performed activities were displayed, such as antibacterial, anti-inflammatory, analgesic, and other significant activities.

Photocatalytic furan-to-pyrrole conversion

The identity of a heteroatom within an aromatic ring influences the chemical properties of that heterocyclic compound. Systematically evaluating the effect of a single atom, however, poses synthetic challenges, primarily as a result of thermodynamic mismatches in atomic exchange processes. We present a photocatalytic strategy that swaps an oxygen atom of furan with a nitrogen group, directly converting the furan into a pyrrole analog in a single intermolecular reaction. High compatibility was observed with various furan derivatives and nitrogen nucleophiles commonly used in drug discovery, and the late-stage functionalization furnished otherwise difficult-to-access pyrroles from naturally occurring furans of high molecular complexity. Mechanistic analysis suggested that polarity inversion through single electron transfer initiates the redox-neutral atom exchange processes at room temperature.

Unveiling Orthogonal Reactivity of Substituted 2-(2-Azidostyryl)furans: Thermolysis and Photolysis versus Catalysis

A systematic investigation of the decomposition of substituted 2-(2-azidostyryl)furans has been reported. The products of catalytic decomposition align with predictable patterns, consistent with established literature data. In contrast, photolysis and thermolysis lead to the formation of unexpected products. In this case, generated nitrenes surprisingly exhibited an affinity for the furan core, deviating from the anticipated attack on the olefin moiety.

Interrupted Furan-Yne Cyclization: Access to Unsaturated Dicarbonyl Compounds and Their Subsequent Transformation into Functionalized Pyridazines

The key carbenoid intermediate of transition-metal-catalyzed furan-yne cyclization in Hashmi phenol synthesis could be efficiently intercepted with water under the developed reaction conditions in order to provide access to functionalized unsaturated dicarbonyl compounds that might serve as convenient precursors for the straightforward synthesis of annulated pyridazines.

Oxidative Ring-Opening Transformation of 5-Acyl-4-pyrones as an Approach for the Tunable Synthesis of Hydroxylated Pyrones and Furans

A selective and tunable approach for oxidation of 4-pyrones has been developed via ring-opening transformations leading to various hydroxylated oxaheterocycles. The first step of the strategy includes the base-catalyzed epoxidation of 5-acyl-4-pyrones in the presence of hydrogen peroxide for the effective synthesis of pyrone epoxides in high yields. The epoxides bearing the CO2Et group are reactive molecules that can undergo both pyrone and oxirane ring-opening via deformylation to produce hydroxylated 2-pyrones or 4-pyrones. The acid-promoted transformation led to 3-hydroxy-4-pyrones (24-76% yields), whereas the K2CO3-catalyzed ring-opening process of 2-carbethoxy-4-pyrone epoxides proceeded as an attack of alcohol at the C-3 position bearing the CO2Et group to give functionalized 6-acyl-5-hydroxy-2-pyrones (27-87% yields). The base-catalyzed reaction of 2-aryl-4-pyrone epoxides was followed by ring contraction and the dearoylation process to produce 3-hydroxyfuran-2-carbaldehydes in 42-80% yields. The transformation of 3-aroylchromone epoxides led to flavonols and 3-hydroxybenzofuran-2-carbaldehyde in the acidic and basic conditions, respectively. The prepared hydroxylated heterocycles demonstrated high reactivity for further transformations and low cytotoxicity and are promising fluorophores or UV filters.