New strategies for the synthesis of biologically important tetrapyrroles. The "B,C + D + A" approach to linear tetrapyrroles

Sustainable Synthesis of Indole-Substituted Densely Functionalized Pyrrole

A novel chromatography- and catalyst-free methodology has been developed for the synthesis of poly substituted pyrrole in good yields via a multicomponent reaction of arylglyoxal, 1,3-dicarbonyl, indole, and aromatic amine. This strategy provides various advantages such as simple experimental and workup procedures, mild reaction conditions, no added catalyst, use of green solvent, and simple purification procedure of pure product without using column chromatography. This green method offers a simple and highly effective strategy to synthesize a wide range of indole-pyrrole conjugates in a one-pot operation.

Transition-Metal-Free Alkylative Aromatization of Tetralone Using Alcohol/Amino Alcohol towards the Synthesis of Bioactive Naphthol and Benzo[e/g]indole Derivatives

Herein, we report alkylative aromatization of tetralone for the synthesis of bioactive naphthols and benzo[e/g]indole derivatives using alcohols in the presence of NaOH via an aerobic oxidative cross-coupling protocol. This is a general and transition-metal-free method, which uses an inexpensive base, avoids inert conditions, and furnishes water and hydrogen peroxide as the byproducts. Moreover, this method demonstrated with wide substrate scope and obtained exclusive regioselectivity.

One-pot synthesis of cyclohexylamine and N-aryl pyrroles via hydrogenation of nitroarenes over the Pd0.5Ru0.5-PVP catalyst

The direct synthesis of cyclohexylamine via hydrogenation of nitrobenzene over monometallic (Pd, Ru or Rh) and bimetallic (Pd_xRu_1−x) catalysts was studied.

Acid-Catalyzed Multicomponent Rearrangements via 2-((Quinoxalin-3(4H)-on-2-yl)(aryl)methylene)malononitriles, Generated In Situ, for Divergent Synthesis of Pyrroles with Different Substitution Patterns

New three-component domino reactions, providing divergent approaches to multifunctionalized pyrroles with different substitution patterns, have been established (47 examples). In this work, a new rearrangement of quinoxalinones with the participation of the in situ-generated 2-en-1-imine moiety of the substituent at C3 makes it possible to construct two new heterocyclic systems, namely, a benzimidazolone and a pyrrole, simultaneously under one-pot reaction conditions. The reaction is easy to perform simply by mixing three common reactants of acetic acid with heating. Secondary amines or primary alcohols as the third component of the reaction, along with quinoxalin-3(4H)-ones and malononitrile, not only initiate the rearrangement but also are responsible for the nature of substituents at position 5 of the pyrrole ring in the newly formed new biheterocyclic system. The reaction proceeds smoothly and can be finished within 7 h, which makes workup convenient to give up to 97% chemical yields.

Synthesis of Pyrroles through the CuH-Catalyzed Coupling of Enynes and Nitriles

Herein, we describe an efficient method to prepare polysubstituted pyrroles via a copper hydride (CuH)-catalyzed enyne-nitrile coupling reaction. This protocol accommodates both aromatic and aliphatic substituents and a broad range of functional groups, providing a variety of N-H pyrroles in good yields and with high regioselectivity. We propose that the Cu-based catalyst promotes both the initial reductive coupling and subsequent cyclization steps. Density functional theory (DFT) calculations were performed to elucidate the reaction mechanism.

Regiodivergent Synthesis of Penta-Substituted Pyrroles through a Cascade [3 + 2] Cyclization of C-Acylimines with Activated Alkynes and Aromatic Nucleophiles

Highly robust one-pot, four-component cascade cyclization reaction of α-keto aldehydes, anilines, activated alkynes, and aromatic nucleophiles is developed to synthesize a diverse range of pharmaceutically important penta-substituted pyrroles. The reaction proceeds through the cascade cyclization of acylimines (in situ formed) with activated alkynes and aromatic nucleophiles such as indoles, pyrroles, and naphthols at room temperature under calcium(II) catalysis with high yields and broad substrate diversity.

Hetero-bimetallic cooperative catalysis for the synthesis of heteroarenes

Review covering the synthesis of 5- and 6-membered as well as condensed heteroarenes, focussing on the combinations in cooperative catalytic systems in strategies used to achieve selectivity and also highlights the mode of action for the cooperative catalysis leading to the synthesis of commercially and biologically relevant heteroarenes.

Pink bacteria-Production of the pink chromophore phycoerythrobilin with Escherichia coli

Phycoerythrobilin (PEB) is an open-chain tetrapyrrole derived from heme and plays an important role as light-harvesting pigment in the phycobiliproteins of cyanobacteria and red algae. Furthermore, PEB can also function as an antioxidant with potential use as a natural acid stable food colorant. PEB is not commercially available and large, pure quantities can only be obtained by laborious methanolysis of red algae followed by liquid chromatography. Here we describe an improved method for high yield production and purification of PEB in Escherichia coli via heterologous expression where the two required enzymes heme oxygenase and PEB synthase subsequently convert the substrate heme provided by the host cell. Experiments in shaking flasks resulted in the highest product yield of 680.23 ± 42.75 μg PEB per g cell dry weight, by induction with 0.1 mM IPTG. Scale-up to batch-operated fermentation in a 2 L bioreactor reached product concentrations up to 5.02 mg PEB L^-1 by adjustment of aeration, induction time, media composition and supplementation of precursors. A further approach included separation of PEB from developed foam above the culture. This enabled continuous product collection during cultivation and simplified product purification. Produced PEB was validated via UV-vis spectroscopy, high pressure liquid chromatography and mass spectrometry.

One-pot synthesis of N-substituted pyrroles from nitro compounds and 2,5-hexadione over a heterogeneous cobalt catalyst

One-pot synthesis of N-substituted pyrroles with highly stable CoN_x catalysts and HCOOH as hydrogen donor.

Cascade bicyclizations of o-alkynyl aldehydes with thiazolium salts: a new access toward poly-functionalized indeno[2,1-b]pyrroles

A new cascade bicyclization of o-alkynyl aldehydes with thiazolium salts is described, in which 25 examples of densely functionalized indeno[2,1-b]pyrroles are achieved in a functional-group-compatible manner. Thiazole carbenes generated in situ from thiazolium salts play dual roles as reaction partners and as NHC catalysts. The synthetic utility of these bicyclization reactions results in subsequent C-C bond-forming events to rapidly build up molecular complexity.

Synthesis of two distinct pyrrole moiety-containing arenes from nitroanilines using Paal–Knorr followed by an indium-mediated reaction

Synthesis of arenes substituted with two different substituted-pyrrole moieties was investigated.

Catalytic activity of the nanoporous MCM-41 surface for the Paal–Knorr pyrrole cyclocondensation

The investigation of different oxide surfaces revealed that nanoporous silica (MCM-41) had the best catalytic activity for Paal–Knorr pyrrole synthesis. Despite the same composition, MCM-41 proved to be more effective than SiO2 itself, probably due to a significantly higher surface area of the SiO2 nanopores. The important features of this “clean” solvent-free protocol are the ease of recovery and the reuse of the catalyst for several cycles, operational simplicity, and easy product isolation and purification.

How green is green chemistry? Chlorophylls as a bioresource from biorefineries and their commercial potential in medicine and photovoltaics

Chlorophylls are the natural green pigments par excellence and offer potential as therapeutics and in energy generation. This perspective outlines the state-of-the-art, their possible applications and indicates future directions in the context of green chemistry and their production from biorefineries.

Functionalization of 3,5,8-trichlorinated BODIPY dyes

Catalytic hydrogenation of dibenzyl 5-dipyrroketone-2,9-dicarboxylates followed by decarboxylative iodination affords a 2,9-diiododipyrroketone which gives a 2,5,9-trichlorodipyrromethene hydrochloride after nucleophilic addition/elimination, with adventitious chloride to replace the two iodide groups. Treatment with BF3·Et2O gives a 3,5,8-trichloro-BODIPY that readily undergoes regioselective Stille coupling at the 8-position, or homo/mixed couplings at the 3,8- or 3,5- and 8-positions. Stepwise and controlled replacement of the 3,5- and 8-chlorine atoms using Stille reagents results in formation of a completely unsymmetrical trisubstituted BODIPY. Several examples of unsymmetrical BODIPYs were synthesized and characterized using this methodology. Structure features of new BODIPYs are discussed within the context of 14 new X-ray structures, and photophysical parameters of all new BODIPY compounds are reported and discussed.

Allylic amination and N-arylation-based domino reactions providing rapid three-component strategies to fused pyrroles with different substituted patterns

New three-component domino reaction providing divergent approaches to multifunctionalized fused pyrroles with different substituted patterns have been established (40 examples). The direct C(sp(3))-N bond formation was achieved through intermolecular allylic amination in a one-pot operation, and N-arylation of amines was realized by varying N-amino acid enaminones. The reaction is easy to perform simply by mixing three common reactants in acetic acid under microwave heating. The reaction proceeds at fast rates and can be finished within 30 min, which makes workup convenient to give good chemical yields.