Regioselective intramolecular reactions of 2-indolylacyl radicals with pyridines: a direct synthetic entry to ellipticine quinones

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Design, synthesis, in vitro antiproliferative activity and apoptosis-inducing studies of 1-(3',4',5'-trimethoxyphenyl)-3-(2'-alkoxycarbonylindolyl)-2-propen-1-one derivatives obtained by a molecular hybridisation approach

Inhibition of microtubule function using tubulin targeting agents has received growing attention in the last several decades. The indole scaffold has been recognized as an important scaffold in the design of novel compounds acting as antimitotic agents. Indole-based chalcones, in which one of the aryl rings was replaced by an indole, have been explored in the last few years for their anticancer potential in different cancer cell lines. Eighteen novel (3′,4′,5′-trimethoxyphenyl)-indolyl-propenone derivatives with general structure 9 were synthesized and evaluated for their antiproliferative activity against a panel of four different human cancer cell lines. The highest IC₅₀ values were obtained against the human promyelocytic leukemia HL-60 cell line. This series of chalcone derivatives was characterized by the presence of a 2-alkoxycarbonyl indole ring as the second aryl system attached at the carbonyl of the 3-position of the 1-(3′,4′,5′-trimethoxyphenyl)-2-propen-1-one framework. The structure–activity relationship (SAR) of the indole-based chalcone derivatives was investigated by varying the position of the methoxy group, by the introduction of different substituents (hydrogen, methyl, ethyl or benzyl) at the N-1 position and by the activity differences between methoxycarbonyl and ethoxycarbonyl moieties at the 2-position of the indole nucleus. The antiproliferative activity data of the novel synthesized compounds revealed that generally N-substituted indole analogues exhibited considerably reduced potency as compared with their parent N-unsubstituted counterparts, demonstrating that the presence of a hydrogen on the indole nitrogen plays a decisive role in increasing antiproliferative activity. The results also revealed that the position of the methoxy group on the indole ring is a critical determinant of biological activity. Among the synthesized derivatives, compound 9e, containing the 2-methoxycarbonyl-6-methoxy-N-1H-indole moiety exhibited the highest antiproliferative activity, with IC₅₀ values of 0.37, 0.16 and 0.17 μM against HeLa, HT29 and MCF-7 cancer cell lines, respectively, and with considerably lower activity against HL-60 cells (IC₅₀: 18 μM). This derivative also displayed cytotoxic properties (IC₅₀ values ∼1 μM) in the human myeloid leukemia U-937 cell line overexpressing human Bcl-2 (U-937/Bcl-2) via cell cycle progression arrest at the G₂-M phase and induction of apoptosis. The results obtained also demonstrated that the antiproliferative activity of this molecule is related to inhibition of tubulin polymerisation. The presence of a methoxy group at the C5- or C6-position of the indole nucleus, as well as the absence of substituents at the N-1-indole position, contributed to the optimal activity of the indole-propenone-3′,4′,5′-trimethoxyphenyl scaffold.

Synthesis and characterization of fused imidazole heterocyclic selenoesters and their application for chemical detoxification of HgCl2

Selenoester derivatives of imidazo[1,2-a]pyridine/imidazo[1,2-a]pyrimidine has been synthesized by the reaction of sodium selenocarboxylates with 2-(chloromethyl)imidazo[1,2-a]pyridine/pyrimidine.

Total synthesis of ellipticine quinones, olivacine, and calothrixin B

A direct route to the synthesis of biologically active ellipticine quinones, olivacine, and calothrixin B is described. The prominent key steps involved are Friedel-Crafts hydroxyalkylation followed by oxidation and directed ortho-lithiation reactions of readily available indole-2-carboxylate esters with appropriately substituted pyridine and quinoline carboxaldehydes.

Evaluation of thieno[3,2-b]pyrrole[3,2-d]pyridazinones as activators of the tumor cell specific M2 isoform of pyruvate kinase

Cancer cells have distinct metabolic needs that are different from normal cells and can be exploited for development of anti-cancer therapeutics. Activation of the tumor specific M2 form of pyruvate kinase (PKM2) is a potential strategy for returning cancer cells to a metabolic state characteristic of normal cells. Here, we describe activators of PKM2 based upon a substituted thieno[3,2-b]pyrrole[3,2-d]pyridazinone scaffold. The synthesis of these agents, structure-activity relationships, analysis of activity at related targets (PKM1, PKR and PKL) and examination of aqueous solubility are investigated. These agents represent the second reported chemotype for activation of PKM2.

Attempted Synthesis of Spongidines by a Radical Cascade Terminating onto a Pyridine Ring

Mn(III)-based oxidative free-radical cyclization of an unsaturated beta-keto ester containing a pyridine ring as radical trap has been studied. This intramolecular reaction of nucleophilic carbon-centered radicals with the pyridine ring leads to the stereospecific construction of a tetracyclic compound in which five chiral centers are created in one pot. This synthetic approach represents the first attempt to prepare the anti-inflammatory pyridinium alkaloids spongidine A, B, and D.

SmI2-Promoted Radical Addition Reactions with N-(2-Indolylacyl)oxazolidinones: Synthesis of Bisindole Compounds

The treatment of N-acyl oxazolidinones of N-benzyl 2-indolecarboxylic acids varying in the substitution pattern of the indole ring with samarium diiodide at -78 degrees C led to the formation of two indole dimer products. The major product isolated in yields from 55 to 59% represents an unsymmetrical dimer arising from 1,4-addition to the 2-indolecarboxylic acid derivative of a possible ketyl-type radical anion intermediate originating from the reduction of the exocyclic carbonyl group of the N-acyl oxazolidinone. The minor dimer, represented by a symmetrical diketone, was produced in yields ranging from 11 to 23%. Even in the presence of an alpha,beta-unsaturated amide, dimerization was the preferred pathway rather than the formation of a gamma-keto amide. Upon treatment with acid, the unsymmetrical indole dimer cyclized to form a diindolequinone. Finally, the N-acyl oxazolidinones of pyrrole-2-carboxylic acid and 3-indolecarboxylic acid preferred in both cases to undergo C-C bond formation with an acrylamide in the presence of SmI2 rather than dimerization.

Radical reactions with 3H-quinazolin-4-ones: synthesis of deoxyvasicinone, mackinazolinone, luotonin A, rutaecarpine and tryptanthrin

Alkyl, aryl, heteroaryl and acyl radicals have been cyclised onto the 2-position of 3H-quinazolin-4-one. The side chains containing the radical precursors were attached to the nitrogen atom in the 3-position. The cyclisations take place by aromatic homolytic substitution hence retain the aromaticity of the 3H-quinazolin-4-one ring. The highest yields were obtained using hexamethylditin to facilitate cyclisation rather than reduction without cyclisation. The alkaloids deoxyvasicinone , mackinazolinone , tryptanthrin , luotonin A and rutaecarpine were synthesised by radical cyclisation onto 3H-quinazolin-4-one.