Biomimetic synthesis of the antimalarial flindersial alkaloids

A multicomponent reaction for modular assembly of indole-fused heterocycles

Indoles are privileged chemical entities in natural products and drug discovery. Indole-fused heterocycles, particularly seven-membered ones, have received increasing attention due to their distinctive chemical characteristics and wide spectrum of bioactivities. However, the synthetic access to these compounds is highly limited. Herein, we report a unique multicomponent reaction (MCR) for modular assembly of indole-fused seven-membered heterocycles. In this process, indole, formaldehyde and amino hydrochloride could assemble rapidly to yield indole-fused oxadiazepines, and another addition of sodium thiosulphate would furnish indole-fused thiadiazepines. The biological evaluation disclosed the promising anticancer activity of these compounds. Furthermore, this MCR could be applicable in the late-stage and selective modifications of peptides. Therefore, this work provides a powerful strategy for indole functionalization and valuable tool for construction of seven-membered heterocycles.

Ca(II)-Catalyzed Cascade Reaction of Tryptamines with Propargylic Alcohols: Temperature-Driven Ring Opening and Closing via the Allene Migration Pathway for the Synthesis of Pyrrolo[1,2-a]indoles

A temperature-dependent cascade of reactions between tryptamines and propargylic alcohols was developed to achieve selective formation of pyrroloindoline and pyrrolo[1,2-a]indole heterocycles by Ca(II) catalysis. The cascade consists of electrophilic addition of allene at the C3 carbon of indole followed by intramolecular cyclization at 60 °C to yield pyrroloindolines. Furthermore, simultaneous 1,2-allene migration and pyrrolidine ring opening were followed by intramolecular cyclization via C-N bond formation at reflux temperature to obtain pyrrolo[1,2-a]indole scaffolds. A wide range of substrates, a clean reaction profile, scalability, and good to excellent yields are the advantages of this protocol.

Visible-Light-Induced Cascade Cyclization of 1-Acryloyl-2-cyanoindole: Access of Difluoroalkylated Pyrrolo[1,2-a]indolediones

An effective method for accessing diverse difluoroalkylated pyrrolo[1,2-a]indolediones via visible-light-induced PhI(OAc)2-promoted cascade difluoroalkylation/cyclization reaction under mild conditions has been established. This method is noteworthy for its use of DMSO-H2O as a green medium at room temperature and avoidance of photocatalysts. The reactions are straightforward to execute and convenient to expand on, provide good to excellent yields, and have good functional group tolerance.

Discovery of a Hydroxylamine-Based Brain-Penetrant EGFR Inhibitor for Metastatic Non-Small-Cell Lung Cancer

Metastases to the brain remain a significant problem in lung cancer, as treatment by most small-molecule targeted therapies is severely limited by efflux transporters at the blood-brain barrier (BBB). Here, we report the discovery of a selective, orally bioavailable, epidermal growth factor receptor (EGFR) inhibitor, 9, that exhibits high brain penetration and potent activity in osimertinib-resistant cell lines bearing L858R/C797S and exon19del/C797S EGFR resistance mutations. In vivo, 9 induced tumor regression in an intracranial patient-derived xenograft (PDX) murine model suggesting it as a potential lead for the treatment of localized and metastatic non-small-cell lung cancer (NSCLC) driven by activating mutant bearing EGFR. Overall, we demonstrate that an underrepresented functional group in medicinal chemistry, the trisubstituted hydroxylamine moiety, can be incorporated into a drug scaffold without the toxicity commonly surmised to accompany these units, all while maintaining potent biological activity and without the molecular weight creep common to drug optimization campaigns.

Does thermotropic liquid crystalline self-assembly control biological activity in amphiphilic amino acids? - tyrosine ILCs as a case study

Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0-3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmA_d) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Col_h), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli ΔTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.

Synthesis of Polysubstituted 1,2-Dihydro-3H-pyrrolo[1,2-a]indol-3-ones through Domino Palladium-Catalyzed Reactions of Indol-2-ylmethyl Acetates with 1,3-Dicarbonyl Derivatives

A straightforward assembly of polysubstituted 1,2-dihydro-3H-pyrrolo[1,2-a]indol-3-ones through a domino palladium-catalyzed reaction of indol-2-ylmethyl acetates with 1,3-dicarbonyl derivatives is described. The key role of the features of the 1,3-dicarbonyls on the reaction outcome has been explored. The employment of 2-methylcyclohexan-1,3-dione as the dicarbonyl source could allow further challenging indole nucleus functionalizations.

1-Acryloyl-2-cyanoindole: A Skeleton for Visible-Light-Induced Cascade Annulation

1-Acryloyl-2-cyanoindoles were found to be novel and efficient skeletons in visible-light-induced persulfate-promoted cascade cyclization reactions. With this transition-metal-free photocatalytic procedure, various sulfonated/thiocyanated pyrrolo[1,2-a]indolediones were synthesized from 1-acryloyl-2-cyanoindoles with sulfonyl hydrazides/NH₄SCN at room temperature under mild reaction conditions.

K2S2O8-mediated acylarylation of unactivated alkenes via acyl radical addition/C–H annulation cascade of N-allyl-indoles with silver cocatalysis

A silver-catalyzed, K2S2O8-mediated protocol to access the regioselective acylarylation of unactivated alkenes was reported.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Highly Enantioselective Synthesis of Pyrroloindolones and Pyrroloquinolinones via an N-Heterocyclic Carbene-Catalyzed Cascade Reaction

In this work, the NHC-catalyzed Michael/Mannich/lactamization cascade reaction of enals with either indole-2-carboxaldehyde-derived aldimines or indole-7-carboxaldehyde-derived aldimines is described. This protocol enables the rapid assembly of optically active pyrroloindolones and pyrroloquinolinones derivatives under mild conditions with high yields, excellent enantioselectivities, and a broad substrate scope.

An insight in anti-malarial potential of indole scaffold: A review

Malaria is a major parasitic disease in tropical and sub-tropical regions. Pertaining to the sustaining resistance in malarial parasite against the available drugs, novel treatment options are the need of the hour. In this resolve recently, focus has shifted to finding the natural alternatives that possess anti-plasmodial activity for combatting malaria. Drawing on the text written in ancient scriptures and Ayurveda, natural compounds are now being screened for their therapeutic properties. Indole is one such natural compound, present in all living organisms, it displays a range of therapeutic activities including anticancer, anti-inflammatory, antimalarial etc. In this review, we have discussed various indole scaffold as well as the semi-synthetic drugs containing indole moiety that have been synthesized for malaria treatment.

Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to β-Indolyl Enones and β-Pyrrolyl Enones

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone β-position. For the former, the closer the heteroatom is to the enone β-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction. Thus, reaction rates are increased by the closer proximity of inductive electron-withdrawing elements, but if resonance effects are involved, then increased rates are observed with electron-donating ability. Evidence for these trends in isomeric substrates is presented, and the application of these insights has allowed for reaction conditions that provide improved reactivity with previously problematic substrates.

Indole: The After Next Scaffold of Antiplasmodial Agents?

Malaria remains a global public health problem due to the uphill fight against the causative Plasmodium parasites that are relentless in developing resistance. Indole-based antiplasmodial compounds are endowed with multiple modes of action, of which inhibition of hemozoin formation is the major mechanism of action reported for compounds such as cryptolepine, flinderoles, and isosungucine. Indole-based compounds exert their potent activity against chloroquine-resistant Plasmodium strains by inhibiting hemozoin formation in a mode of action different from that of chloroquine or through a novel mechanism of action. For example, dysregulating the sodium and osmotic homeostasis of Plasmodium through inhibition of PfATP4 is the novel mechanism of cipargamin. The potential of developing multi-targeted compounds through molecular hybridization ensures the existence of indole-based compounds in the antimalarial pipeline.

Fe-Catalyzed Sequential C(sp3)-H/N-H Annulation of 2-Methylindoles with Ethyl Trifluoropyruvate at Room Temperature: Construction of Pyrrolo[1,2-α]indoles

An efficient and benign iron-catalyzed room-temperature method was developed for direct sequential C(sp³)-H/N-H annulation to construct pyrroloindole scaffolds. This strategy features cheap and readily available raw materials and mild room-temperature reaction conditions and provides a green and practical method for the one-pot rapid synthesis of a wide range of diversely functionalized pyrrolo[1,2-α]indoles.

Silver-catalyzed acylative annulation of N-propargylated indoles with α-keto acids: access to acylated pyrrolo[1,2-a]indoles

A novel carbon-centered radical cyclization of N-propargyl indoles with α-keto acids to form acylated pyrrolo[1,2-a]indoles involving difunctionalisation of the alkyne moiety with concomitant annulation is disclosed.

Visible-light-driven metal-free aerobic synthesis of highly diastereoselective phosphinoylpyrroloindoles

A tandem C–P and C–C bond formation promoted by visible light at room temperature in a highly diastereoselective manner is described.

Visible light photocatalytic asymmetric synthesis of pyrrolo[1,2-a]indoles via intermolecular [3+2] cycloaddition

The asymmetric synthesis of pyrrolo[1,2-a]indoles is developed through a [3+2] cycloaddition between silyl-indole derivatives and α,β-unsaturated N-acyl oxazolidinones by merging photocatalysis and Lewis acid catalysis.

Amino acid/crown ether hybrid materials: how charge affects liquid crystalline self-assembly

To probe the influence of electrostatic interactions on the mesomorphic self-assembly and phase behaviour of hybrid liquid crystals a series of crown ether/tyrosine hybrid systems was prepared by Steglich esterification of alkyl N-(tert-butoxycarbonyl)-l-tyrosinates with 4-carboxybenzo[15]crown-5 and 4-carboxybenzo[18]crown-6. The obtained derivatives allowed further manipulations at the NH functional group and complexation of the crown ether unit with NaI to give neutral or charged hybrid materials. All compounds were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) measurements. Neither the variation of the N-protecting group, introduction of charge at the N-terminus nor anion exchange (Cl vs. I) resulted in mesomorphism. In contrast, N-Boc-protected and unprotected hybrids formed rectangular columnar (Col_r) mesophases with phase widths up to 35 K. NaI complexation switched these neutral thermotropic derivatives into ionic liquid crystals (ILCs) and induced a change of mesophase type from Col_r to smectic A (SmA). A comparison of experimentally obtained layer distances and theoretically calculated molecular lengths indicated bilayer SmA formation with interdigitated alkyl chains. Packing models for both mesophase types are proposed and discussed with respect to stabilizing interactions.

Biomimetic Total Synthesis of (±)-Griffipavixanthone via a Cationic Cycloaddition-Cyclization Cascade

We report the concise, biomimetic total synthesis of the dimeric, Diels-Alder natural product griffipavixanthone from a readily accessible prenylated xanthone monomer. The key step utilizes a novel intermolecular [4+2] cycloaddition-cyclization cascade between a vinyl p-quinone methide and an in situ generated isomeric diene promoted by either Lewis or Brønsted acids. Experimental and computational studies of the reaction pathway suggest that a stepwise, cationic Diels-Alder cycloaddition is operative.

[3+2]-Annulation of platinum-bound azomethine ylides with distal C[double bond, length as m-dash]C bonds of N-allenamides

A Pt-catalyzed, highly regioselective reaction between N-allenamides and imino-alkynes leading to pyrrolo[1,2-a]indoles is described. This represents the first example of [3+2]-annulation of Pt-bound azomethine ylides with the distal C[double bond, length as m-dash]C bond of N-allenamides. The mechanism of the reaction was established by computational studies.

Ruthenium-catalysed one-pot regio- and diastereoselective synthesis of pyrrolo[1,2-a]indoles via cascade C–H functionalization/annulation

Catalytic, regioselective and diastereoselective synthesis of pyrrolo[1,2-a]indoles through a cascade C–H activation and cyclization process.

Oxidative Furan-to-Indole Rearrangement. Synthesis of 2-(2-Acylvinyl)indoles and Flinderole C Analogues

Oxidative rearrangement of 2-(2-aminobenzyl)furans affording 2-(2-acylvinyl)indoles in a stereocontrolled manner in good-to-excellent yields has been developed. Thus, (2-aminobenzyl)furans with electron-releasing alkoxy substituents in the phenyl group form only E-isomers of 2-(2-acylvinyl)indoles. Conversely, substrates without such substituents produce target products as Z-isomers exclusively. A short diastereoselective method for the transformation of the obtained 2-(2-acylvinyl)indoles into antimalarial bisindole alkaloid flinderole A-C analogues has been developed.

Asymmetric copper-catalyzed C-N cross-couplings induced by visible light

Despite a well-developed and growing body of work in copper catalysis, the potential of copper to serve as a photocatalyst remains underexplored. Here we describe a photoinduced copper-catalyzed method for coupling readily available racemic tertiary alkyl chloride electrophiles with amines to generate fully substituted stereocenters with high enantioselectivity. The reaction proceeds at -40°C under excitation by a blue light-emitting diode and benefits from the use of a single, Earth-abundant transition metal acting as both the photocatalyst and the source of asymmetric induction. An enantioconvergent mechanism transforms the racemic starting material into a single product enantiomer.

Expedient synthesis of densely substituted pyrrolo[1,2-a]indoles

The strategy was further applied for the synthesis of optically active pyrrolo[1,2-a]indoles in >97% de using the chiral auxiliary based approach.

A Novel Pd-Catalysed Annulation Reaction for the Syntheses of Pyrroloindoles and Pyrroloquinolines

Pd-catalysed annulation reactions between indole derivatives and internal alkyne esters leading to various pyrrolo[1,2-a]indoles and pyrroloquinolines have been developed. The strategy involves an intermolecular addition of the indole nitrogen on to the internal alkyne ester followed by an intramolecular insertion of a vinyl-palladium complex into the carbonyl group. This method offers a facile and practical approach to pyrrolo[1,2-a]indoles and pyrroloquinolines.

Efficient synthesis of some new antiproliferative N-fused indoles and isoquinolines via 1,3-dipolar cycloaddition reaction in an ionic liquid

Triethylammonium acetate (TEAA) as a new, recyclable and environmentally friendly reaction medium for the intramolecular[3+2] cycloaddition reaction.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Facilitating Biomimetic Syntheses of Borrerine Derived Alkaloids by Means of Flow-Chemical Methods

Flow chemistry is widely used nowadays in synthetic chemistry and has increasingly been applied to complex natural product synthesis. However, to date flow chemistry has not found a place in the area of biomimetic synthesis. Here we show the syntheses of borrerine derived alkaloids, indicating that we can use biomimetic principles in flow to prepare complex architectures in a single step.

Enantioselective hydroacylation of N-vinylindole-2-carboxaldehydes

We report rhodium-catalyzed hydroacylations of N-vinylindole-2-carboxaldehydes that form chiral, non-racemic 2,3-dihydro-1H-pyrrolo[1,2-a]indol-1-ones in high yields with excellent enantioselectivities.