Multicomponent assembly strategies for the synthesis of diverse tetrahydroisoquinoline scaffolds

An ultrasound assisted, ionic liquid-molecular iodine synergy driven efficient green synthesis of pyrrolobenzodiazepine-triazole hybrids as potential anticancer agents

Herein, we report an efficient and eco-friendly, ultrasound assisted synthetic strategy for the construction of diversified pyrrolobenzodiazepine-triazole hybrids, which are potentially pharmaceutically important scaffolds, via a domino reaction involving intermolecular electrophilic substitution followed by intramolecular Huisgen 1,3-dipolar azide-alkyne cycloaddition. The USP of the reported protocol is the use of benign and inexpensive, recyclable molecular iodine-ionic liquid synergistic catalytic system cum reaction media for achieving the synthesis. The other salient features of this method are the use of mild reaction conditions, high yield and atom economy, operational simplicity, broad substrate scope and easy workup and purification. All the synthesized compounds were evaluated for in vitro anti-proliferative activity against various cancer cell lines. From among the synthesized title compounds, 9,9-dimethyl-8-phenyl-9H-benzo [b]pyrrolo [1,2-d][1,2,3]triazolo[5,1-g][1,4]diazepine (7) was found most to be the most active compound exhibiting IC₅₀ value of 6.60, 5.45, 7.85, 11.21, 12.24, 10.12, and 11.32 µM against MCF-7, MDA-MB-231, HeLa, SKOV-3, A549, HCT-116 and DLD-1 cell lines, respectively. Further the compounds were found to be non-toxic against normal human embryonic kidney (HEK-293) cell line.

Advances in the Synthesis of Fused 1,2,3-Triazoles via a MCR-Intramolecular Azide-Alkyne Cycloaddition Approach

The present review narrates several reports which deal with the synthesis of fused 1,2,3-triazole containing scaffolds following a sequential multicomponent reaction (MCR)-intramolecular azide-alkyne cycloaddition (IAAC) approach. The reviewed reactions were cleverly designed so as to incorporate azide and alkyne functionalities in the MCR product which was then subjected to IAAC. The review is divided into two sections based on the number of components in the multicomponent reaction. We have aimed at a critical discussion and also have highlighted either advantages or disadvantages of each methodology.

Synthesis and biological evaluation of the new ring system benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepine and its cycloalkane and cycloalkene condensed analogues

Derivatives of the new ring system benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepinone and its cycloalkane and cycloalkene condensed analogues have been conveniently synthesized through a three-step reaction sequence. An atom-economical, one-pot, three-step cascade process engaging five reactive centers (amide, amine, carbonyl, azide, and alkyne) has been performed for the synthesis of alicyclic derivatives of quinazolinotriazolobenzodiazepine using cyclohexane, cyclohexene, and norbornene β-amino amides. The stereochemistry and relative configurations of the synthesized compounds were determined by 1D and 2D NMR spectroscopy and X-ray crystallography. The reaction was also performed using enantiomeric starting materials leading to enantiomeric quinazolinotriazolobenzodiazepine with an ee of 95%. The synthesis of 9H-benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepinone, a new heterocyclic system, was achieved in a good yield using a retro Diels-Alder (RDA) procedure. Some compounds were tested for antiproliferative activities against five human cancer cell lines of gynecological.

Design, Synthesis, and Evaluation of Novel Anti-Trypanosomal Compounds

Human African trypanosomiasis (HAT) is a deadly neglected tropical disease caused by the protozoan parasite Trypanosoma brucei. During the course of screening a collection of diverse nitrogenous heterocycles, we discovered two novel compounds that contain the tetracyclic core of the Yohimbine and Corynanthe alkaloids, were potent inhibitors of T. brucei proliferation and T. brucei methionyl-tRNA synthetase (TbMetRS) activity. Inspired by these key findings, we prepared several novel series of hydroxyalkyl δ-lactam, δ-lactam, and piperidine analogs and tested their anti-trypanosomal activity. A number of inhibitors are more potent against T. brucei than these initial hits with one hydroxyalkyl δ-lactam derivative being 25-fold more effective in our assay. Surprisingly, most of these active compounds failed to inhibit TbMetRS. This work underscores the importance of verifying, irrespective of close structural similarities, that new compounds designed from a lead with a known biological target engage the putative binding site.

Allosteric GABAA Receptor Modulators-A Review on the Most Recent Heterocyclic Chemotypes and Their Synthetic Accessibility

GABAA receptor modulators are structurally almost as diverse as their target protein. A plethora of heterocyclic scaffolds has been described as modulating this extremely important receptor family. Some made it into clinical trials and, even on the market, some were dismissed. This review focuses on the synthetic accessibility and potential for library synthesis of GABAA receptor modulators containing at least one heterocyclic scaffold, which were disclosed within the last 10 years.

Recent advances in the synthesis of bridgehead (or ring-junction) nitrogen heterocycles via transition metal-catalyzed C–H bond activation and functionalization

An update on recent advances in the synthesis of bridgehead nitrogen fused heterocycles via transition metal-catalyzed C–H activation and functionalization is reported.

TFA/TBHP-promoted oxidative cyclisation for the construction of tetracyclic quinazolinones and rutaecarpine

An efficient TFA/TBHP-promoted oxidative cyclisation of readily available isatins with 1,2,3,4-tetrahydroisoquinolines has been firstly developed. The potential utility of this strategy was demonstrated by one-step synthesis of a natural alkaloid Rutaecarpin.

Stereoselective synthesis of 1,3-disubstituted dihydroisoquinolines vial-phenylalanine-derived dihydroisoquinoline N-oxides

The preparation of chiral pool-derived nitrone 3 and its use in the protecting-group free, stereoselective synthesis of a range of 1,3-disubstituted tetrahydroisoquinolines is described. Grignard reagent additions to nitrone 3 yielded trans-1,3-disubstituted N-hydroxytetrahydroisoquinolines 6 with good levels of selectivity, while 1,3-dipolar cycloadditions to this nitrone provided access to 3-(2-hydroxyalkyl)isoquinolines 12 as single diastereomers.

Modular One-Step Three-Component Synthesis of Tetrahydroisoquinolines Using a Catellani Strategy

Reported is a modular one-step three-component synthesis of tetrahydroisoquinolines using a Catellani strategy. This process exploits aziridines as the alkylating reagents, through palladium/norbornene cooperative catalysis, to enable a Catellani/Heck/aza-Michael addition cascade. This mild, chemoselective, and scalable protocol has broad substrate scope (43 examples, up to 90 % yield). The most striking feature of this protocol is the excellent regioselectivity and diastereoselectivity observed for 2-alkyl- and 2-aryl-substituted aziridines to access 1,3-cis-substituted and 1,4-cis-substituted tetrahydroisoquinolines, respectively. Moreover, this is a versatile process with high step and atom economy.

Multicomponent Synthesis of Tetrahydroisoquinolines

A multicomponent synthesis of tetrahydroisoquinolines from carboxylic acids, alkynyl ethers, and dihydroisoquinolines is described. This process features readily available starting materials, simple experimental procedures for achievement of molecule complexity, and structural diversity. The preliminary control experiment and crossover reaction provide important insight into the reaction mechanism. The formed tetrahydroisoquinolines could be transformed to an array of compounds.

Norbenzomorphan Framework as a Novel Scaffold for Generating Sigma 2 Receptor/PGRMC1 Subtype-Selective Ligands

A novel structural class with high affinity and subtype selectivity for the sigma 2 receptor has been discovered. Preliminary structure-affinity relationship data are presented showing that 8-substituted 1,3,4,5-tetrahydro-1,5-methanobenzazepine (norbenzomorphan) derivatives elicit modest to high selectivity for the sigma 2 over the sigma 1 receptor subtype. Indeed, piperazine analogue 8-(4-(3-ethoxy-3-oxopropyl)piperazin-1-yl)-1,3,4,5-tetrahydro-1,5-methanobenzazepine-2-carboxylate (SAS-1121) is 574-fold selective for the sigma 2 over the sigma 1 receptor, thereby establishing it as one of the more subtype-selective sigma 2 binding ligands reported to date. Emerging evidence has implicated the sigma 2 receptor in multiple health disorders, so the drug-like characteristics of many of the selective sigma 2 receptor ligands disclosed herein, coupled with their structural similarity to frameworks found in known drugs, suggest that norbenzomorphan analogues may be promising candidates for further development into drug leads.

Condensation of anthranilic acids with pyridines to furnish pyridoquinazolones via pyridine dearomatization

The unprecedented carbodiimide-mediated condensation between pyridines and anthranilic acids at room temperature has been developed for pyridoquinazolones.

Applications of Ring Closing Metathesis. Total Synthesis of (±)-Pseudotabersonine

A novel approach to the Aspidosperma family of alkaloids was developed and applied to a concise total synthesis of (±)-pseudotabersonine that was accomplished in 11 steps. Key transformations include a stepwise variant of a Mannich-like multicomponent assembly process, a double ring-closing metathesis sequence, and a one-pot deprotection/cyclization reaction.

Progress in Studies on Rutaecarpine. II.--Synthesis and Structure-Biological Activity Relationships

Rutaecarpine is a pentacyclic indolopyridoquinazolinone alkaloid found in Evodia rutaecarpa and other related herbs. It has a variety of intriguing biological properties, which continue to attract the academic and industrial interest. Studies on rutaecarpine have included isolation from new natural sources, development of new synthetic methods for its total synthesis, the discovery of new biological activities, metabolism, toxicology, and establishment of analytical methods for determining rutaecarpine content. The present review focuses on the synthesis, biological activities, and structure-activity relationships of rutaecarpine derivatives, with respect to their antiplatelet, vasodilatory, cytotoxic, and anticholinesterase activities.

Palladium-Catalyzed Cascade Reaction of 2-Amino-N'-arylbenzohydrazides with Triethyl Orthobenzoates To Construct Indazolo[3,2-b]quinazolinones

A palladium-catalyzed sequential cyclization/C-H activation cascade reaction of 2-amino-N'-arylbenzohydrazides with triethyl orthobenzoates has been developed, providing indazolo[3,2-b]quinazolinones in good to high yields. Two key intermediates of the reaction, 2-phenyl-3-(phenylamino)quinazolinone and C-H insertion palladacycle, were isolated, and their structures were unambiguously confirmed by X-ray crystallography. This method represents an unprecedented example of a halogen-free protocol to access indazolo[3,2-b]quinazolinones. Moreover, this chemistry also provides a useful tool for the discovery of fluorescent materials.

Evolution of a strategy for preparing bioactive small molecules by sequential multicomponent assembly processes, cyclizations, and diversification

A strategy for generating diverse collections of small molecules has been developed that features a multicomponent assembly process (MCAP) to efficiently construct a variety of intermediates possessing an aryl aminomethyl subunit. These key compounds are then transformed via selective ring-forming reactions into heterocyclic scaffolds, each of which possesses suitable functional handles for further derivatizations and palladium-catalyzed cross coupling reactions. The modular nature of this approach enables the facile construction of libraries of polycyclic compounds bearing a broad range of substituents and substitution patterns for biological evaluation. Screening of several compound libraries thus produced has revealed a large subset of compounds that exhibit a broad spectrum of medicinally-relevant activities.

Multicomponent, Mannich-type assembly process for generating novel, biologically-active 2-arylpiperidines and derivatives

A multicomponent, Mannich-type assembly process commencing with commercially available bromobenzaldehydes was sequenced with [3+2] dipolar cycloaddition reactions involving nitrones and azomethine ylides to generate collections of fused, bicyclic scaffolds based on the 2-arylpiperidine subunit. Use of the 4-pentenoyl group, which served both as an activator in the Mannich-type reaction and a readily-cleaved amine protecting group, allowed sub-libraries to be prepared through piperidine N-functionalization and cross-coupling of the aryl bromide. A number of these derivatives displayed biological activities that had not previously been associated with this substructure. Methods were also developed that allowed rapid conversion of these scaffolds to novel, polycyclic dihydroquinazolin-2-ones, 2-imino-1,3-benzothiazinanes, dihydroisoquinolin-3-ones and bridged tetrahydroquinolines.

Direct imine acylation for molecular diversity in heterocyclic synthesis

Imines and carboxylic acids have been directly coupled using propylphosphonic acid anhydride and NEt(i-Pr)2 to give N-acyliminium ions, which were intramolecularly trapped with oxygen, nitrogen, sulfur, and carbon nucleophiles to provide a wide range of structurally diverse heterocycles.

Multicomponent assembly processes for the synthesis of diverse yohimbine and corynanthe alkaloid analogues

A strategy involving a Mannich-type multicomponent assembly process followed by a 1,3-dipolar cycloaddition has been developed for the rapid and efficient construction of parent heterocyclic scaffolds bearing indole and isoxazolidine rings. These key intermediates were then readily elaborated using well-established protocols for refunctionalization and cross-coupling to access a diverse 180-member library of novel pentacyclic and tetracyclic compounds related to the Yohimbine and Corynanthe alkaloids. Several other new multicomponent assembly processes were developed to access dihydro-β-carboline-fused benzodiazepines, pyrimidinediones, and rutaecarpine derivatives.

Selective copper(II) acetate and potassium iodide catalyzed oxidation of aminals to dihydroquinazoline and quinazolinone alkaloids

Copper(II) acetate/acetic acid/O2 and potassium iodide/tert-butylhydroperoxide systems are shown to affect the selective oxidation of ring-fused aminals to dihydroquinazolines and quinazolinones, respectively. These methods enable the facile preparation of a number of quinazoline alkaloid natural products and their analogues.

Expedient synthesis of norbenzomorphan library via multicomponent assembly process coupled with ring-closing reactions

A 124-member norbenzomorphan library has been prepared utilizing a novel multicomponent assembly process (MCAP) followed by a variety of ring-closing reactions to generate norbenzomorphan scaffolds that were readily derivatized via a series of aryl halide cross-coupling and nitrogen functionalization reactions. Biological screening has revealed some novel activities that have not been previously associated with this class of compounds.

One-pot, two-step cascade synthesis of quinazolinotriazolobenzodiazepines

An operationally simple, one-pot, two-step cascade method has been developed to afford quinazolino[1,2,3]triazolo[1,4]benzodiazepines. This unique, atom-economical transformation engages five reactive centers (amide, aniline, carbonyl, azide, and alkyne) and employs environmentally benign iodine as a catalyst. The method proceeds via sequential quinazolinone-forming condensation and intramolecular azide-alkyne 1,3-dipolar cycloaddition reactions. Substrate scope, multicomponent examples, and mechanistic insights are discussed.

Application of a sequential multicomponent assembly process/huisgen cycloaddition strategy to the preparation of libraries of 1,2,3-triazole-fused 1,4-benzodiazepines

A strategy featuring a multicomponent assembly process followed by an intramolecular azide-alkyne dipolar (Huisgen) cycloaddition was implemented for the facile synthesis of three different 1,2,3-triazolo-1,4-benzodiazepine scaffolds. A diverse library of 170 compounds derived from these scaffolds was then created through N-functionalizations, palladium-catalyzed cross-coupling reactions, and several applications of α-aminonitrile chemistry.

Diversity Oriented Synthesis: Concise Entry to Novel Derivatives of Yohimbine and Corynanthe Alkaloids

A novel MCAP-cycloaddition sequence has been applied to the facile synthesis of β-carboline intermediates to gain rapid access to novel derivatives of yohimbine-like and corynanthe-like compounds that may be easily diversified by cross-coupling reactions and N-derivatizations to generate small compound libraries.

Libraries of 2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-amine derivatives via a multicomponent assembly process/1,3-dipolar cycloaddition strategy

A Mannich-type multicomponent assembly process/1,3-dipolar cycloaddition strategy has been developed for the rapid and efficient construction of a parent tetrahydroisoquinoline fused isoxazolidine scaffold, which was subsequently functionalized using well-established protocols to access a diverse 70-membered library of novel 2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline-2-amine derivatives.

APPLICATIONS OF MULTICOMPONENT ASSEMBLY PROCESSES TO THE FACILE SYNTHESES OF DIVERSELY FUNCTIONALIZED NITROGEN HETEROCYCLES

Several multicomponent assembly processes have been developed for the synthesis of intermediates that may be elaborated by a variety of cyclizations to generate a diverse array of highly functionalized heterocycles from readily-available starting materials. The overall approach enables the efficient preparation of libraries of small molecules derived from fused, privileged scaffolds.