On water: silver-catalyzed domino approach for the synthesis of benzoxazine/oxazine-fused isoquinolines and naphthyridines from o-alkynyl aldehydes

o-Alkynylaryl 2-Cyanoacrylates: A Synthon for the Construction of Diverse Isoquinolones/Napthyridinones

o-Alkynylaryl 2-cyanoacrylates have been disclosed as a new synthon for the regioselective synthesis of isoquinolones and naphthyridinones under mild reaction conditions. The attractive feature of this investigation includes carbon-carbon double bond cleavage under metal-free conditions by an intramolecular SN2 reaction. Incorporating two distinct C-I bonds in the resulting products provides facile opportunity for structural elaboration. The viability of the present protocol was unveiled by postfunctionalization with drug analogues and gram-scale synthesis.

Urea-Promoted Neat Synthesis of Fused Dihydroisoquinolines and Disubstituted Pyridines: A Mechanistic Observation with Molecular-Sensing Studies

An efficient and short synthesis of fused dihydroisoquinolines, diaryl substituted pyridine derivatives in good to high yields has been established by using an environmentally safe, solvent-metal-oxidant-free tandem approach. In this article, we discuss how the electrocyclic reaction is more pronounced in the solid phase in the presence of urea, whereas the typical aza-Michael addition is more prominent in presence of arylamine in the solution phase for 3-(2-formylcycloalkenyl)acrylic ester derivative substrates. The wide range of substrates and urea-promoted neat synthesis made our approach more significant in medical and also analytical science. Moreover, an isoquinoline alkaloid decumbenine B analogue has been synthesized by using our newly developed neat methodology. We have also investigated the photophysical properties of the synthesized fused dihydroisoquinoline derivatives. One of the synthesized molecules was used as a sensor for the selective detection of toxic picric acid. Therefore, the effective neat synthesis and molecular sensing applications of these compounds made our approach more exciting in the field of heterocyclic chemistry.

Environmentally friendly Nafion-catalyzed synthesis of 3-substituted isoquinoline by using hexamethyldisilazane as a nitrogen source under microwave irradiation

This study developed an eco-friendly method to synthesize 3-arylisoquinoline from 2-alkynylbenzaldehydes using Nafion® NR50 as an acidic catalyst and hexamethyldisilazane (HMDS) as a nitrogen source. The reaction proceeded via a 6-exo-dig cyclization under microwave irradiation, giving the corresponding isoquinolines in excellent yields. The advantages of this protocol include: (1) the use of recyclable acid catalysts, (2) transition-metal-free catalysis, and (3) the effective formation of the target product. These features make this methodology a promising approach for the sustainable and efficient synthesis of 3-arylisoquinoline. Some structures were also confirmed by single-crystal X-ray diffraction analysis.

Silver-Catalyzed Cascade Cyclization of Amino-NH-1,2,3-Triazoles with 2-Alkynylbenzaldehydes: An Access to Pentacyclic Fused Triazoles

An operationally simple Ag(I)-catalyzed approach for the synthesis of isoquinoline and quinazoline fused 1,2,3-triazoles was developed by a condensation and amination cyclization cascade of amino-NH-1,2,3-triazoles with 2-alkynylbenzaldehydes involving three new C-N bond formations in one manipulation, in which the group of -NH of the triazole ring serves as a nucleophile to form the quinazoline skeleton. The efficient protocol can be applied to a variety of substrates containing a range of functional groups, delivering novel pentacyclic fused 1,2,3-triazoles in good-to-excellent yields.

Synthetic Methods and Pharmacological Potential of Different Oxazine Analogs

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Oxazine analog is a vital class of heterocyclic compounds and has attracted synthetic interest owing to their wide range of biological activities. Oxazine analogs are valuable in medicinal organic chemistry and exhibit different varieties of biological activities such as antimicrobial, anticancer, antimalarial, antitubercular, sedative, anticonvulsant, analgesic, anti-inflammatory, antipyretic, etc. Oxazine can be derived from benzene by appropriate substitution of carbon atoms of the ring by nitrogen and oxygen atoms. Nowadays, the development of drug resistance is a key problem, and to defeat this problem, it is crucial to synthesize novel compounds. So, novel oxazine analogs may play a crucial role to overcome these problems. Oxazine analogs are prepared by reaction of chalcone derivatives with thiourea in the presence of alcohol and sodium hydroxide. The present aims of this review are to give an outline of some different synthetic methods and different types of biological activities of oxazine analogs. We hope that this review will be motivating for researchers concerned with oxazine analogs.

Sodium sulphide promoted synthesis of fused quinoline at room temperature

A novel, simple and eco-friendly strategy for the synthesis of thiopyrano[4,3-b]quinolin-1-ones and pyrrolo[3,4-b]quinolin-1-ones from 2-alkynylquinoline-3-carbonitriles and sodium sulphide (Na₂S·9H₂O) under catalyst-free conditions at room temperature has been described. In this reaction, a readily available inorganic salt (Na₂S·9H₂O) serves as the sulphur source and leads to the generation of diverse functionalized thiopyrano[4,3-b]quinolin-1-ones and pyrrolo[3,4-b]quinolin-1-ones in moderate to excellent yields through sulfuration, annulation, and aerial oxidation.

Synthesis of cyclopentaquinolinone and cyclopentapyridinone from ortho-alkynyl-N-arylaldehyde via superbase-promoted C-N, C-O and C-C bond formation

An environmentally benign, transition metal-free, superbase-mediated intramolecular annulation of o-alkynylaldehydes with primary amines forms highly functionalized amino-substituted cyclopentaquinolinones and cyclopentapyridinones via C-N, C-C, and C[double bond, length as m-dash]O bond formation. Contrary to the traditional approaches of ring closures, a different mode of annulation is disclosed. The protocol involves the in situ generations of imine intermediate followed by potassium hydroxide-promoted intramolecular cyclization and subsequent dimethyl sulfoxide induced dehydrogenation leads to the formation of N-heterocycles. X-ray crystallographic studies support the assigned structures of the amino-fused N-heterocycles.

Tandem Construction of Indole-Fused Phthalazines from (2-Alkynylbenzylidene)hydrazines under Metal-Free Conditions

An efficient approach to invent diversely substituted indole-fused phthalazines from in situ formed (2-alkynylbenzylidene)hydrazines under metal-free conditions via selective radical cyclization has been developed. Notably, this 6-exo-dig addition-cyclization tandem procedure proceeds under air atmosphere and shows a broad substrate suitability, as well as avoids harmful byproducts, which complies with the concept of green synthesis.

Dual function of carbon tetrachloride: synthesis of chlorinated heterocycles

An efficient tandem approach to invent highly chlorinated and structurally diversified dihydropyridine fused heterocycles from easily accessible ortho-alkynylaldehydes and primary amines under metal-free conditions via four sequential bond formations is described. The proposed tandem route proceeds via a carbon tetrachloride-mediated 6-endo-dig ring closure followed by nucleophilic attack of the trichloromethyl anion.

Cu(I)-Catalyzed Oxygen and Nitrogen Nucleophiles Triggered Regioselective Synthesis of Furo/Pyrrolo-Annulated Quinolines

Cu(I)-catalyzed intramolecular annulation of o-ethynylquinoline-3-carbaldehydes leads to the synthesis of alkoxy/imidazole-substituted 1,3-dihydrofuro[3,4-b]quinolines via C-O and C-N bond formation. The scope of the reaction was further extended to o-ethynylquinoline-3-carbonitriles for the synthesis of alkoxy-substituted 3H-pyrrolo[3,4-b]quinolines using alcohols as nucleophiles. These reactions are regioselectively favoring the 5-exo-dig cyclizations in all the annulation processes.

On Water: Metal-Free Synthesis of Highly Functionalized Benzothiazolylidene from ortho-Haloanilines

An environmentally benign, transition-metal-free organic base promoted one-pot cascade synthesis of highly functionalized benzo[ d]thiazol-2(3 H)-ylidene benzamide in the presence of water was accomplished by three-component reaction of ortho-iodoanilines, acrylates, and aroyl isothiocyanates. The protocol involves the in situ generation of thiourea intermediate followed by triethylamine induced intramolecular SN_Ar displacement reaction and subsequent Michael addition onto acrylate leads to the formation of benzo[ d]thiazol-2(3 H)-ylidene benzamide. Benzo[ b]thiazole is also generated in good yields using amidation and intramolecular aromatic nucleophilic substitution chemistry. The control experiments support the proposed mechanistic pathway. Further X-ray crystallographic studies confirmed the assigned structures of the fused benzamide.

Synthesis of 4-(1H-isochromen-1-yl)isoquinolines through the silver-catalysed homodimerization of ortho-alkynylarylaldehydes and subsequent condensation of the 1,5-dicarbonyl motif with NH3

4-(1H-Isochromen-1-yl)isoquinoline derivatives were synthesized in high yields via the AgBF₄-catalyzed self-reaction of ortho-alkynylarylaldehydes to give isochromene intermediates, followed by the dehydration of the 1,5-dicarbonyl motif with NH₃. Compared with electron-rich aromatic substituents, this strategy can provide the desired isochromene products with an electron-deficient isoquinoline unit. The reactions feature simple experimental operations, mild reaction conditions and high product yields.

We demonstrate a strategy that can provide isochromenes with an electron-deficient isoquinoline unit. The reactions feature simple operations, mild conditions and high yields.

Palladium meets copper: one-pot tandem synthesis of pyrido fused heterocycles via Sonogashira conjoined electrophilic cyclization

An efficient step-economical tandem approach for the direct synthesis of pyrido fused indole, quinoline, benzofuran and benzothiophene derivatives using a bimetallic Pd/Cu catalytic system has been described. The three component reaction of o-halo aldehydes, alkynes and tert-butylamine leads to the synthesis of biologically active polyheterocycles. The present strategy involves the dual role of tert-butylamine and copper(i) in Sonogashira coupling followed by electrophilic cyclization through imine formation. The chemistry has been validated by X-ray crystallographic studies.

Silver-catalyzed synthesis of pyrrolopiperazine fused with oxazine/imidazole via a domino approach: evaluation of anti-cancer activity

Ag-Catalyzed synthesis of pyrrolopiperazine fused with oxazine/imidazole by the reaction of δ-alkynyl aldehydes and nucleophilic amines was performed. Several of these compounds were found to exhibit anti-cancer activity against cancer cell lines.

Pd-Catalyzed One-Pot Stepwise Synthesis of Benzo[b][1,6]naphthyridines from 2-Chloroquinoline-3-carbonitriles Using Sulfur and Amines As Nucleophiles

A palladium-catalyzed one-pot stepwise coupling-annulation reaction of 2-chloroqunoline-3-carbonitriles enabled the direct synthesis of sulfur-substituted benzo[b][1,6]naphthyridines via multiple bond formation. The reaction provided an unusual mode for cyclization as sodium sulfide, a soft nucleophile, preferred to attack on the carbon of the nitrile group rather than on the C-C triple bond. The developed chemistry was extended with the secondary amines as nucleophiles to afford nitrogen-substituted benzo[b][1,6]naphythyridines while primary amines afforded hydroamination products . The hydromination products were transformed to benzo[b][1,6]naphthyridones via a base-mediated cyclization reaction. The  developed protocol features inexpensive and easily synthesizable starting materials, easy operations, and a high efficiency and tolerance to a broad range of substrates.

Recent Developments on Ultrasound-Assisted Synthesis of Bioactive N-Heterocycles at Ambient Temperature

N-Heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. The last decade has seen a tremendous practice to carry out reactions at ambient temperature avoiding harsh reaction conditions. By applying ultrasonic radiation in organic synthesis we can make synthetic protocols more sustainable and can carry out reactions at room temperature avoiding the traditional thermal harsh reaction conditions. Therefore the synthesis of biologically relevant N-heterocycles at room temperature under the influence of ultrasonic irradiation is one of the advancing areas in the 21st century among organic chemists. The present review summarises the latest developments on ultrasound-assisted synthesis of biologically relevant N-heterocycles at ambient temperature.

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.

An expedient approach to 1,2-dihydroisoquinoline derivatives via cobalt catalysed 6-endo dig cyclization followed by Mannich condensation of o-alkynylarylaldimines

A highly effective 6-endo dig cyclisation of o-alkynylaldimines to 1,2-dihydroisoquinolines has been described via direct and nitro Mannich condensation using inexpensive and readily available cobalt chloride as catalyst. This strategy provides an effective procedure for the synthesis of substituted 1,2-dihydroisoquinolines derivatives in moderate to high yields. An addition of pronucleophiles, such as nitromethane, acetone and α-hydroxyacetone, to o-alkynylarylaldimines has been achieved via isoquinolinium intermediate.

Silver-catalysed reactions of alkynes: recent advances

Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon-carbon triple bond makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010-2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e., C-C, C-N, C-O, C-Halo, C-P and C-B). Compared with other coinage metals such as Au and Cu, silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions.