Stereoselective Late-Stage Transformations of Indolo[2,3-a]quinolizines Skeleta to Nature-Inspired Scaffolds.

The indolo[2,3-a]quinolizines, canthines, and arborescidines natural products exhibit a wide range of bioactivities including anticancer, antiviral, antibacterial, and anti-inflammatory, among others. Therefore, the development of modular and efficient strategies to access the core scaffolds of these classes of natural products is a remarkable achievement. The Complexity-to-Diversity (CtD) strategy has become a powerful tool that transforms natural products into skeletal and stereochemical diversity. However, many of the reactions that could be utilized in this process are limited by the type of functional groups present in the starting material, which restrict transformations into a variety of products to achieve the desired diversity. In the course of employing a (CtD) strategy en route to the synthesis of nature-inspired compounds, unexpected stereoelectronic-driven rearrangement reactions have been discovered. These reactions provided a rapid access to indolo[2,3-a]quinolizines-, canthines-, and arborescidines-inspired alkaloids in a modular and diastereoselective manner. The disclosed strategies will be widely applicable to other late-stage natural product transformation programs and drug discovery initiatives.

Stereocontrolled transformations of cyclohexadienone derivatives to access stereochemically rich and natural product-inspired architectures.

The last two decades or so have witnessed an upsurge in defining the art of designing complex natural products and nature-inspired molecules. Throughout these decades, fundamental insights into stereocontrolled, step-economic and atom-economical synthesis principles were achieved by the numerous synthetic accomplishments particularly in diversity-oriented synthesis (DOS). This has empowered the visualization of the third dimension in synthetic design and thus has resulted in a dramatic increase with today's diversity-oriented synthesis (DOS) at the forefront enabling access to diverse scaffolds with a high degree of stereochemical and skeletal complexity. To this end, a starting material-based approach is one of the powerful tools utilized in DOS that allows rapid access to molecular architectures with a high sp3 content. Skeletal and stereochemical diversity is often paramount for the selective modulation of the biological function of a complementary protein in the biological space. In this context, stereocontrolled transformation of cyclohexadienone scaffolds has positioned itself as a powerful platform for the rapid generation of stereochemically enriched and natural product-inspired compound collections. In this review, we cover multidirectional synthetic strategies that utilized cyclohexadienone derivatives as pluripotent building blocks en route for the construction of novel chemical space.

Synthesis of 2-arylamino-3-cyanoquinolines via a cascade reaction through a nitrilium intermediate.

A new method for the preparation of 2-amino-3-cyanoquinolines from readily available aryldiazonium salts, 2-aminoarylketones, and malononitrile via a cascade reaction is reported. This one-pot approach involves the in situ generation of an N-arylnitrilium intermediate from the direct reaction of aryldiazonium salts and malononitrile, which undergoes intermolecular amination, Knoevenagel condensation, and then aromatization to yield the desired compound in moderate to good yields. This methodology features a quick assembly of C2 and C3 functionalized quinolines.

Preparation of N-Arylquinazolinium Salts via a Cascade Approach.

An easy manipulation method for the preparation of N-arylquinazolinium salts is described from readily available aryldiazonium salts, nitriles, and 2-aminoarylketones in a one-pot operation. This method relies on the in situ generation of the N-arylnitrilium intermediate from the reaction of aryldiazonium salt with nitrile, which undergoes amination/cascade cyclization/aromatization, leading to N-arylquinazolinium salts in excellent yields. Nucleophilic addition of alkoxide to these N-arylquinazolinium salts provides functionalized dihydro- N-arylquinazoline.

Preparation of Quinazolinoquinazolinones via a Cascade Approach.

A one-pot synthesis of quinazolino[3,4- a]quinazolin-13-ones was realized from the direct reaction of o-(methoxycarbonyl)benzenediazonium salts, nitriles, and 2-cyanoanilines in moderate to good yields. This method utilizes the in situ generation of reactive N-arylnitrilium ion, which undergoes further amination/tandem cyclization/amidation to deliver the desired polycyclic scaffolds with consecutive formation of four N-C bonds. Flexibility in substitution patterns, mild reaction conditions, and operational simplicity are the salient features of this methodology.

Preparation of Ketimines from Aryldiazonium Salts, Arenes, and Nitriles via Intermolecular Arylation of N-Arylnitrilium Ions.

A transition-metal-free approach for the preparation of N-arylketimines has been developed from the direct reaction of aryldiazonium salts, arenes, and nitriles in a one-pot fashion with the consecutive formation of N-C and C-C bonds. This approach proceeds via an in situ generation of N-arylnitrilium intermediate, which then undergoes intermolecular arylation. This three-component strategy offers a step- and atom-efficient way to N-arylketimines from easily accessible reagents under mild reaction conditions. The characterization of stereochemistry of ketimine was achieved by X-ray crystallographic structure and theoretical calculation. Operational simplicity, shorter reaction time, excellent functional group compatibility, and scalability are the key features of this report.

Novel derivatives of 1,2,3-triazole, cannabinoid-1 receptor ligands modulate gastrointestinal motility in mice.

Cannabinoid-1 (CB1) receptors are broadly distributed in the central and peripheral nervous systems; among others, they are located in the enteric nervous system. In the gastrointestinal (GI) system, they participate in regulation of intestinal motility or ion transport. The aim of our study was to assess the effect of 1,2,3-triazole derivatives (compound 1: 2-[4,5-bis(2,4-dichlorophenyl)-2H-1,2,3-triazol-2-yl]-N-(2-fluorobenzyl)acetamide, compound 2: 2-[4,5-bis(2,4-dichlorophenyl)-2H-1,2,3-triazol-2-yl]-N-(4-fluorobenzyl)acetamide, compound 3: N-benzyl-2-[4-(4-chlorophenyl)-5-(2,4-dichlorophenyl)-2H-1,2,3-triazol-2-yl]acetamide]), characterized in vitro as CB1 antagonists with high CB1 over CB2 selectivity, in the mouse GI tract. The action of compounds 1-3 was assessed in vitro (electrical field stimulated smooth muscle contractility of the mouse ileum and colon) and in vivo (whole GI transit time). Compound 1 decreased ileal (10-6 M) and colonic (10-7-10-6 M) smooth muscles contractility. Moreover, it prolonged whole GI transit. Compound 2 (10-10-10-8 M) slightly increased the amplitude of muscle contractions in the ileum, but at a higher concentration (10-6 M), the amplitude was decreased. Compound 2 reduced colonic contractility but accelerated GI transit. Compound 3 decreased the amplitude of intestinal muscle contractions in the ileum (10-6 M) and colon (10-10-10-6 M). Moreover, it increased the GI transit time in vivo. Triazole derivatives possess easily modifiable structure and interesting pharmacological action in the GI tract; further, alterations may enhance their efficacy at CB receptors and provide low side effect profile in clinical conditions.

Preparation of 3-hydroxyquinolines from direct oxidation of dihydroquinolinium salts.

A series of functionalized 3,4-dihydroquinolinium salts were prepared from the reaction of aryldiazonium salt with alkene in a nitrile solution. Further oxidation yielding either 3-hydroxyquinoline or quinoline products was investigated. A one-pot process from aryldiazonium salts, alkenes and nitriles leading to 3-hydroxyquinolines was also developed. Furthermore, an intramolecular trapping of an N-arylnitrilium ion with a vinyl group at the ortho position leading to 2-substituted quinolines was revealed.

Synthesis of Substituted Quinazolin-4(3H)-imines From Aryldiazonium Salts, Nitriles and 2-Cyanoanilines via A Metal-Free Tandem Approach.

A transition metal-free synthesis of multisubstituted quinazolin-4(3H)-imines has been realized by the direct reaction of aryldiazonium salts, nitriles, and 2-cyanoanilines in a one-pot fashion. This strategy utilizes the in situ formation of reactive N-arylnitrilium intermediate, which undergoes further tandem cyclization with consecutive formation of N-C bonds. Broad functional group compatibility, mild conditions, shorter time, and operational simplicity are the notable features of this report.

Preparation of Quinazolines via a 2+2+2 Annulation from Aryldiazonium Salts and Nitriles.

A (2+2+2) modular synthesis of multisubstituted quinazolines has been realized by the direct reaction of aryldiazonium salts with two equivalent of nitriles. Reaction of aryldiazonium salt with a nitrile provides the initial formation of a reactive nitrilium ion, which is attacked by another molecule of nitrile followed by electrophilic cyclization to deliver the desired product. Notable flexibility in the substitution patterns, readily available substrates, short reaction time, transition metal-free, and gram-scale synthesis are the advantages of this method.

One pot synthesis of α-ketoamides from ethylarenes and amines: a metal free difunctionalization strategy.

One-pot and metal free synthesis of α-ketoamides has been described through in situ generation of aryl ketones from easily available ethylarenes followed by amidation with various amines. This multiple oxidation protocol involves catalytic I2-pyridine-TBHP (t-butyl hydroperoxide) mediated oxidative benzylic carbonylation and sequential NaI-TBHP mediated oxidative amidation without using any solvent.

One-Pot Reactions for Synthesis of 2,5-Substituted Tetrazoles from Aryldiazonium Salts and Amidines.

One-pot sequential reactions of aryldiazonium salts with amidines followed by the treatment of I2/KI under basic conditions provide 2,5-disubstituted tetrazoles in moderate to excellent yields. This one-pot synthesis has several advantages such as mild reaction conditions, short reaction time, convenient workup, and high yields, making this methodology practical.

Preparation of substituted phenanthridines from the coupling of aryldiazonium salts with nitriles: a metal free approach.

A transition metal free approach for the synthesis of substituted phenanthridines from the coupling reaction of aryldiazonium tetrafluoroborates with nitriles has been developed. This operationally simple protocol proceeds through a substitution of aryldiazonium with nitriles followed by an intramolecular arylation to provide the corresponding phenanthridines in moderate to excellent yields.

Synthesis of the decalin core of codinaeopsin via an intramolecular Diels-Alder reaction.

We describe the synthesis of the decalin core of codinaeopsin (1), a tryptophan-polyketide hybrid natural product with promising antimalarial activity (IC50 4.7 µM, against Plasmodium falciparum), via an intramolecular Diels-Alder (IMDA) reaction. A convergent synthesis was developed to prepare the precursors for the IMDA reaction in 10 steps. The exo cycloadducts were derived from thermal, IMDA reactions of the substrates containing a Weinreb amide or ester conjugated dienophile, and the endo adducts were from Lewis acid promoted reactions of the substrates with a formyl group. Both exo and endo products of the IMDA were exclusively isolated and characterized by NMR spectroscopy. One endo cycloadduct was further confirmed with X-ray crystallography. Theoretical calculations reveal the influence of the substituents of the decalin core on the IMDA process.

1,2,3-Triazole derivatives as new cannabinoid CB1 receptor antagonists.

This letter reports the new entry of novel 1,2,3-triazole derivatives as CB1 receptor antagonists. The design, synthesis and biological evaluation of N1 and N2 substituted 1,2,3-trizoles are described. The N2 substituted, symmetrical 1,2,3-triazoles are more potent ligands than the unsymmetrical analogues. The in vitro activity of these triazoles is further improved by inserting a methylene group between the central core and the carbonyl side chain. The most potent antagonists prepared in this series (IC(50)<20 nM) are the triazoles containing benzyl amides. These triazoles also show excellent selectivity between CB1 and CB2 receptors (IC(50)>10 microM for CB2; CB2/CB1>1000).