The first aza Diels-Alder reaction involving an alpha,beta-unsaturated hydrazone as the dienophile: stereoselective synthesis of C-4 functionalized 1,2,3,4-tetrahydroquinolines containing a quaternary stereocenter

First Diels–Alder reaction of a N-nitrogen-substituted iminium ion as dienophile furnishing N-amino-substituted 1,2,5,6-tetrahydropyridines

The exploratory and preliminary work on the reaction of 1-(methoxydiphenylmethyl)-2-methyldiazene with 1,3-dienes reveals a remarkable dichotomy furnishing different heterocyclic Diels–Alder products depending on the presence or absence of water. In contrast to the acid-induced hydrolysis of the N,O-ketal starting material with 1,3-dienes furnishing 1-methyl-1,2,3,6-tetrahydropyridazines (preceding communication), the same acid-promoted reaction albeit under exclusion of water affords N-(methyleneamino)-1,2,5,6-tetrahydropyridines. The serendipitous discovery of the novel 1,2,5,6-tetrahydropyridines was corroborated by an alternative and independent synthesis providing the in situ generated formaldehyde benzophenone azine and its conjugate acid, 2-(diphenylmethylene)-1-methylenehydrazin-1-ium ion. The protonated azine serving as the putative dienophile resembles the first example of an N-nitrogen-substituted iminium ion employed as a dienophile in a bona fide normal electron-demand Diels–Alder reaction.

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Recent advances in chemistry and therapeutic potential of functionalized quinoline motifs – a review

Quinoline, which consists of benzene fused with N-heterocyclic pyridine, has received considerable attention as a core template in drug design because of its broad spectrum of bioactivity. This review aims to present the recent advances in chemistry, medicinal potential and pharmacological applications of quinoline motifs to unveil their substantial efficacies for future drug development. Essential information in all the current and available literature used was accessed and retrieved using different search engines and databases, including Scopus, ISI Web of Knowledge, Google and PUBMED. Numerous derivatives of the bioactive quinolines have been harnessed via expeditious synthetic approaches, as highlighted herein. This review reveals that quinoline is an indisputable pharmacophore due to its tremendous benefits in medicinal chemistry research and other valuable areas of human endeavour. The recent in vivo and in vitro screening reported by scientists is highlighted herein, which may pave the way for novel drug development. Owing to the array of information available and highlighted herein on the medicinal potential of quinoline and its functionalized derivatives, a new window of opportunity may be opened to medicinal chemists to access more biomolecular quinolines for future drug development.

Quinoline, which consists of benzene fused with N-heterocyclic pyridine, has received considerable attention as a core template in drug design because of its broad spectrum of bioactivity.

(2S*,4S*)-4-[(E)-(2,2-Dimethylhydrazono)methyl]-6-methoxy-4-methyl-2-[(E)-styryl]-1,2,3,4-tetrahydroquinoline

The Povarov reaction of p-anisidine, cinnamaldehyde and methacrolein dimethylhydrazone afforded a 1,2,3,4-tetrahydroquinoline derivative bearing 2-styryl, 4-methyl and 4-dimethylhydrazono substituents in a fully diastereoselective fashion. This is the first example of the combination of a type I aza-vinylogous Povarov reaction and a type II vinylogous Povarov reaction in the same process.

Mechanochemical Aza-Vinylogous Povarov Reactions for the Synthesis of Highly Functionalized 1,2,3,4-Tetrahydroquinolines and 1,2,3,4-Tetrahydro-1,5-Naphthyridines

The aza-vinylogous Povarov reaction between aromatic amines, α-ketoaldehydes or α-formylesters and α,β-unsaturated dimethylhydrazones was carried out in a sequential three-component fashion under mechanochemical conditions. Following extensive optimization work, the reaction was performed on a vibratory ball mill operating at 20 Hz and using zirconium oxide balls and milling jar, and afforded 1,2,3,4-tetrahydroquinolines and 1,2,3,4-tetrahydro- 1,5-naphthyridines functionalized at C-2, C-4 and also at C-6, in the latter case. This protocol generally afforded the target compounds in good to excellent yields and diastereoselectivities. A comparison of representative examples with the results obtained under conventional conditions revealed that the mechanochemical protocol affords faster Povarov reactions in comparable yields using a solvent-less environment.

Synthesis of Pyrazoles Utilizing the Ambiphilic Reactivity of Hydrazones

A Brønsted acid-mediated synthesis of pyrazoles from conjugated hydrazones through a β-protonation/nucleophilic addition/cyclization/aromatization sequence was developed. This protocol utilizing the ambiphilic reactivity of hydrazones enables not only self-condensation but also cross-condensation, affording multisubstituted pyrazoles in high yields, with a broad substrate scope. This sequential reaction proceeds under mild conditions via a simple operation. Moreover, the method can be applied to the synthesis of a nonsteroidal anti-inflammatory drug, Lonazolac.

Antiaromaticity Gain Activates Tropone and Nonbenzenoid Aromatics as Normal-Electron-Demand Diels-Alder Dienes

We propose a carbonyl umpolung strategy for activating tropone as a normal-electron-demand Diels-Alder diene. Tropone has low reactivity for Diels-Alder reactions because of its [4n+2] π-aromaticity. Conversion of the carbonyl group into a hydrazone ion (═N-NR-) reverses the polarity of the exocyclic double bond, increases the [4n] ring π-antiaromaticity, and raises the HOMO energy. Computed gas-phase activation free energies for a Diels-Alder reaction with maleimide suggest a billion-fold rate increase when the tropone C═O is replaced by ═N-NR- (R = H or SO2CH3). Other nonbenzenoid aromatics can be activated as normal-electron-demand Diels-Alder dienes in the same way.

InCl3: A Versatile Catalyst for Synthesizing a Broad Spectrum of Heterocycles

This review deals with the recent applications of the indium trichloride (InCl₃) catalyst in the synthesis of a broad spectrum of heterocyclic compounds. Over the years, a number of reviews on the applications of InCl₃-catalyzed organic synthesis have appeared in the literature. It is evident that InCl₃ has emerged as a valuable catalyst for a wide range of organic transformations due to its stability when exposed to moisture and also in an aqueous medium. The most attractive feature of this review is the application of the InCl₃ catalyst for synthesizing bioactive heterocyclic compounds. The study of InCl₃-catalyzed organic reactions has high potential and better intriguing aspects, which are anticipated to originate from this field of research.

Inverse-Electron-Demand oxa-Diels-Alder Reactions of α-Keto-β,γ-unsaturated Esters and α,β-Unsaturated Hydrazones

A concise synthetic method for dihydropyrans has been developed by inverse-electron-demand oxa-Diels-Alder reaction of α-keto-β,γ-unsaturated esters with α,β-unsaturated hydrazones as electron-rich olefins. This reaction is catalyzed by Eu(hfc)₃ and proceeds in an endo-selective manner. This umpolung cycloaddition affords a variety of substituted dihydropyrans stereoselectively in high yields. In addition, indirect synthesis of formyl-substituted dihydropyran was achieved by dehydrazonation of the cycloadduct. This method is expected to be useful for the synthesis of dihydropyrans and tetrahydropyrans with unusual substitution patterns.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Inverse-electron-demand Diels-Alder reactions of α,β-unsaturated hydrazones with 3-methoxycarbonyl α-pyrones

Inverse-electron-demand Diels-Alder reactions of 3-electron-withdrawing group substituted α-pyrones with α,β-unsaturated hydrazones as electron-rich counterparts are catalyzed by Eu(hfc)3 to afford bicyclic lactone cycloadducts. This is an example of umpolung cycloaddition based on functional transformation of carbonyls to hydrazones. A subsequent dehydrazonation reaction enables indirect synthesis of carbonyl group-containing bicyclic lactones, which cannot be easily obtained by the cycloaddition of α-pyrones and enals.

Catalytic enantioselective ene-type reactions of vinylogous hydrazone: construction of α-methylene-γ-butyrolactone derivatives

Catalytic asymmetric ene-type reactions of vinylogous hydrazone with isatins, α-ketoester, imines and aldehydes were accomplished which gave an environmentally friendly, straightforward approach to afford bioactive chiral α-methylene-γ-butyrolactone derivatives.

Expanding the tetrahydroquinoline pharmacophore

Tetrahydroquinoline is a privileged scaffold with a large number of biological applications. The tetrahydroquinoline pharmacophore has been expanded to yield 34 compounds. Biological screening of these compounds led to the identification of tetrahydroquinoline as neurotropic agents not reported earlier.

Recent developments in multicomponent synthesis of structurally diversified tetrahydropyridines

Tetrahydropyridines (THPDs) have recognized as a major constituent of naturally occurring alkaloids and exhibits various biological and pharmacological profiles. For their synthesis, MCRs are employed as an excellent tool to generate vast libraries.

Exceptional effect of nitro substituent on the phosphonation of imines: the first report on phosphonation of imines to α-iminophosphonates and α-(N-phosphorylamino)phosphonates

A novel chemoselective method for the simple phosphonation of imines withH-phosphonate diethyl ester and study of the electronic and orientation effects of the substituents on phosphorylation reaction.

Asymmetric N-Heterocyclic Carbene Catalyzed Annulation of 2-Alkenylbenzothiazoles with α-Chloro Aldehydes

Diastereo- and enantioselective N-heterocyclic carbene catalyzed 1-azadiene Diels-Alder reactions of (E)-2-styrylbenzothiazoles with α-chloro aldehydes are reported. This annulation strategy provides an efficient access to medicinally important dihydrobenzothiazolopyridin-1-ones in good to excellent yields (44-97%) with very good to excellent stereoselectivities (up to 9:1 dr, 98% ee) and tolerates quite a range of substituents.

New types of reactivity of α,β-unsaturated N,N-dimethylhydrazones: chemodivergent diastereoselective synthesis of functionalized tetrahydroquinolines and hexahydropyrrolo[3,2-b]indoles

The indium trichloride-catalyzed reaction between aromatic imines and α,β-unsaturated N,N-dimethylhydrazones in acetonitrile afforded 1,2,3,4-tetrahydroquinolines bearing a hydrazone function at C4 through a one-pot diastereoselective domino process that involves the formation of two C-C bonds and the controlled generation of two stereocenters, one of which is quaternary. This reaction constitutes the first example of an α,β-unsaturated dimethylhydrazone that behaves as a dienophile in a hetero Diels-Alder reaction. The related reaction between anilines, aromatic aldehydes, and methacrolein dimethylhydrazone in CHCl(3) with BF(3)⋅Et(2)O as catalyst afforded polysubstituted 1,2,3,3a,4,8b-hexahydropyrrolo[3,2-b]indoles as major products through a fully diastereoselective ABB'C four-component domino process that generates two cycles, three stereocenters, two C-C bonds, and two C-N bonds in a single operation.

Povarov reactions involving 3-aminocoumarins: synthesis of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins and pyrido[2,3-c]coumarins

Condensation of 3-aminocoumarin (5) with 4-nitrobenzaldehyde (8) afforded a 2-azadiene (9), which reacted with various electron-rich alkenes (10 examples) in the presence of Yb(OTf)3 to afford 1,2,3,4-tetrahydropyrido[2,3-c]coumarins. Yields were generally good, but the diastereomeric ratios were highly variable. The products arose through a formal [4 + 2] cycloaddition (inverse electron demand Diels-Alder reaction) followed by tautomerization. As such, these are examples of the Povarov reaction. A range of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins was then synthesized using a three-component version of this reaction, which involves in situ formation of the 2-azadiene component. Some of these products were converted into the corresponding pyrido[2,3-c]coumarins upon treatment with various oxidants, the most effective of which proved to be nitrous gases.

1-Azadienes in cycloaddition and multicomponent reactions towards N-heterocycles

1-Azadienes are versatile building blocks for the efficient construction of various N-heterocycles. Depending on the substitution pattern and reaction partner, they may participate in a range of different reactions. An overview of recent methods for the generation of 1-azadienes is presented, as well as their application in cycloaddition, electrocyclization, and multicomponent reactions. Considering the broad range of reactivities and resulting heterocyclic scaffold structures, 1-azadienes are very useful reactive intermediates for the development of modular reaction sequences in diversity-oriented synthesis.