A TBADT photocatalyst-enabled radical-induced cyclization pathway to access functionalized dihydrobenzofurans.

Herein, this work describes a photoinduced visible light-mediated radical cyclization for constructing dihydrobenzofuran (DHB) scaffolds. Notably, this cascade photochemical process is tolerable with various aromatic aldehydes and diverse alkynyl aryl ethers and proceeds via an intramolecular 1,5-hydrogen atom transfer (HAT) pathway. Significantly, acyl C-H activation under mild conditions has been achieved without the use of additives or reagents. The photocatalyst, tetrabutylammonium decatungstate (TBADT), plays an important role in the present strategy by facilitating the well-known hydrogen atom transfer during the course of the reaction.

Electrochemical Oxidative Annulation of Inactivated Alkynes for the Synthesis of Sulfonated 2H-Chromene Derivatives.

A unique, facile, and straightforward electrochemical oxidative annulation of inactivated propargyl aryl ethers with sulfonyl hydrazides leading to 3-sulfonated 2H-chromenes has been achieved. Significantly, this protocol involves a green approach that works under mild reaction conditions using a constant current in an undivided cell and is devoid of oxidants and catalysts. Notably, the process exhibited a broad scope and functional group tolerance to deliver 2H-chromenes and would represent an alternative and sustainable strategy versus conventional chromene synthesis.

Ester-directed orthogonal dual C-H activation and ortho aryl C-H alkenylation via distal weak coordination.

An unprecedented orthogonal cross-coupling between aromatic C(sp2) and aliphatic olefinic C(sp2) carbons of two same molecules via dual C-H bond activation in an intermolecular fashion has been developed using a distal ester-directing group. This new coupling reaction led to the synthesis of the highly functionalized 1,3-diaryl molecular architecture in very good yields and with high chemo- and regioselectivities. In addition, using ester as the distal directing group, ortho C-H olefination of α-methyl aryl acrylates and cinnamic esters with various alkenes has been achieved in very good yields and with a wide range of substrate scope.

Azomethine ylide cycloaddition of 1,3-dienyl esters: highly regio- and diastereoselective synthesis of functionalized pyrrolidinochromenes.

An efficient protocol for the synthesis of tricyclic pyrrolidinochromenes has been developed via an intramolecular 1,3-dipolar cycloaddition of azomethine ylides generated in situ from 1,3-dienyl ester tethered O-hydroxyarylaldehyde and glycine esters. The reaction is highly regio- and diastereoselective in nature and provided the potentially bioactive pyrrolidine fused tricyclic cycloadducts in excellent yields with wide substrate scope. Interestingly this reaction constructs two rings and four contiguous stereogenic centers in which one of them is an all carbon quaternary center in a unique fashion.

Photocatalytic Visible-Light-Induced Nitrogen Insertion via Dual C(sp3)-H and C(sp2)-H Bond Functionalization: Access to Privileged Imidazole-based Scaffolds.

Here we have demonstrated a visible-light-mediated metal-free organic-dye-catalyzed dehydrogenative N-insertion leading to highly substituted imidazoles and privileged dihydroisoquinoline-based imidazole derivatives via C(sp3)-H and C(sp2)-H bond functionalization. A sustainable, convenient, metal-free azidation/C-H aminative cyclization approach in the absence of stoichiometric oxidants is presented. This protocol involves a rare photoinduced iminyl radical as a key intermediate for the "N" insertion.

Visible light-mediated photocatalytic oxidative cleavage of activated alkynes via hydroamination: a direct approach to oxamates.

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature. In this reaction, the single electron oxidation of an in situ formed enamine followed by radical coupling with an oxidant finally delivers the oxamate. The key features of this photocatalytic reaction are the mild reaction conditions, metal-free organic dye as a photocatalyst, and TBHP playing a dual role as "O" source and for the regeneration of the photocatalyst.

Ruthenium-catalyzed, site-selective C-H activation: access to C5-substituted azaflavanone.

A site-selective ruthenium-catalyzed keto group assisted C-H bond activation of 2-aryl tetrahydroquinoline (azaflavanone) derivatives has been achieved with a variety of alkenes for the first time. A wide range of substrates was utilized for the synthesis of a wide variety of alkenylated azaflavanones. This simple and efficient protocol provides the C5-substituted azaflavanone derivatives in high yields with a broad range of functional group tolerance. Further, the C5-alkenylated products were converted into substituted 2-aryl quinoline derivatives in good yields.

Exocyclic N-Acyliminium Ion (NAI) Cyclization: Access to Fully Substituted Oxazoles and Furocoumarins.

A concise, one-pot route to oxazoles and furocoumarins has been reported. The key step in this transformation involves in situ generation of N-acyliminium ion (NAI) precursor under catalyst and solvent-free conditions and their further transformations promoted by superacid in the same pot. We have also presented the experimental evidence for the involvement of proto-solvated novel exocyclic N-acyliminium ion. Further, the UV-visible and fluorescence studies revealed that few of the compounds reported here exhibited emission of blue light upon irradiation in EtOH in the region of 404-422 nm.

Regioselective C3-H Trifluoromethylation of 2 H-Indazole under Transition-Metal-Free Photoredox Catalysis.

Trifluoromethyl-substituted heteroarenes are biologically active compounds and useful building blocks. In this sequence, we have developed a visible-light-promoted regioselective C3-H trifluoromethylation of 2 H-indazole under metal-free conditions, which proceeds via a radical mechanism. The combination of photocatalysis and hypervalent iodine reagent provides a practical approach to a library of trifluoromethylated indazoles in 35-83% yields.

Two step, one-pot sequential synthesis of functionalized hybrid polyheterocyclic scaffolds via a solid state melt reaction (SSMR).

A new one pot assembly of highly functionalized benzo[a]phenazinone fused chromene/bicyclic scaffolds via a domino Knoevenagel intramolecular hetero-Diels-Alder (IMHDA) strategy using a solid state melt reaction (SSMR) of 2-hydroxynaphthalene 1,4-dione, o-phenylenediamine, O-allyl salicylaldehyde/O-vinyl salicylaldehyde derivatives is reported. The formation of five new bonds (two C-C bonds and three C-O bonds), three six-membered rings, and three stereogenic centers in a one-pot manner is very attractive. Ease of reaction with short time, good yields with water as the only byproduct and work up free procedure are some of the excellent features of the present protocol.

A novel multicomponent quadruple/double quadruple domino reaction: highly efficient synthesis of polyheterocyclic architectures.

A novel multicomponent quadruple domino reaction (MCQDR) for the assembly of structurally complex molecular architectures via the formation of three rings and three contiguous stereogenic centers has been accomplished with high regio- and diastereoselectivity. Solvents, catalysts and work-up were not required to obtain the target molecules. In addition, this new protocol is also extended for the multicomponent double quadruple domino reaction (MCDQDR) to create novel polyheterocyclic architectures in an orthogonal manner.

Solvent and catalyst free ring expansion of indoles: a simple synthesis of highly functionalized benzazepines.

Highly functionalized benzazepines have been synthesized by using various substituted indoles and dialkylacetylene dicarboxylates under thermal and open air conditions. The reaction was carried out in a solvent and under catalyst free conditions and the products are formed in very good yields. The reaction proceeds in a concerted fashion thus providing potentially bioactive benzazepines by ring expansion of a five-membered indole ring. It is also described that the ring contraction of seven-membered benzazepines in the presence of KOtBu base at room temperature smoothly afforded the corresponding 3-alkenylated indole derivatives in very good yields.

Iron promoted C3-H nitration of 2H-indazole: direct access to 3-nitro-2H-indazoles.

An efficient C3-H functionalization of indazole has been demonstrated. Notably, this method involves chelation-free radical C-H nitration on 2H-indazole. The radical mechanism was confirmed by control experiments and quantum chemical calculations. The synthetic utility has been proven by the synthesis of bio-relevant benzimidazoindazoles via reductive cyclization.

A Direct Cycloaminative Approach to Imidazole Derivatives via Dual C-H Functionalization.

Organoiodine(III)-promoted C(sp3)-H azidation was a key step for the cycloaminative process. An unprecedented method for metal-free dehydrogenative N-incorporation into C(sp3)-H and C(sp2)-H bonds for the synthesis of diverse imidazoles has been disclosed. The overall transformation involves the construction of four C-N bonds through hydroamination-azidation-cyclization sequence. The reaction can be easily handled and proceeds under mild conditions. Further, the potential of the present strategy is revealed by the practical synthesis of N-heterocyclic carbene (NHC) precursors.

A robust synthesis of functionalized 2H-indazoles via solid state melt reaction (SSMR) and their anti-tubercular activity.

A facile and convenient approach has been developed for the synthesis of functionalized indazoles via solid state melt reaction using easily accessible starting materials under catalyst-free conditions. This transformation involves electrocyclization via a conjugated nitrene intermediate obtained under thermal conditions. Further anti-tubercular activity screening of the molecules was undertaken, among the compounds 3a-3x screened for in vitro antimycobacterial activity against Mycobacterium tuberculosis H37Rv, compound 3u (MIC: 4.20µM) was found to be most active and are superior over existing standard drugs ciprofloxacin and ethambutol. Compounds 3c and 3x were found to equally potent as ethambutol. Among most potent compounds in the series, four compounds (3n, 3o, 3p and 3u) showed lower cytotoxicity which could be promising drug candidates for further development.

Nickel-Catalyzed Aerobic Oxidative Isocyanide Insertion: Access to Benzimidazoquinazoline Derivatives via a Sequential Double Annulation Cascade (SDAC) Strategy.

An efficient protocol for the synthesis of quinazoline derivatives through nickel-catalyzed ligand-/base-free oxidative isocyanide insertion under aerobic conditions with intramolecular bis-amine nucleophiles has been developed. A one-pot sequential double annulation cascade (SDAC) strategy involving an opening of isatoic anhydride and annulation to benzimidazole and further nickel-catalyzed intramolecular isocyanide insertion has also been demonstrated. The method is operationally simple to implement with a wide variety of substrates and represents a new approach for multiple C-N bond formations. The methodology has been successfully applied to the syntheses of hitherto unreported imidazo-fused benzimidazoquinazolines via a deprotection-GBB reaction sequence. Further, a florescence study reveals the potential of the present strategy for the discovery of highly fluorescent probes.

A metal-free cyclic iminium induced one-pot double annulation cascade: access to dihydroisoquinolinium (DHIQ) salts.

A reactive cyclic iminium induced one-pot Groebke-Blackburn-Bienayme (GBB) double annulation cascade (DAC) for the synthesis of skeletally diverse DHIQ salts has been described. The key features of this protocol are transition-metal and solvent-free, mild reaction conditions, robust method, one-step construction of two privileged heterocyclic rings, clean reaction profile and operational simplicity.

Scaffold Diversity through a Branching Double-Annulation Cascade Strategy: Iminium-Induced One-Pot Synthesis of Diverse Fused Tetrahydroisoquinoline Scaffolds.

A branching double-annulation cascade (BDAC) strategy for diverse and complex fused THIQ scaffolds via a highly reactive iminium-induced one-pot double-cyclization sequence involving Pictect-Spengler-type cyclization has been developed for the first time. The salient features of this protocol are that it allows direct and rapid access to unprecedented diverse fused THIQ skeletons, is metal/catalyst free, has a cleaner reaction profile, provides good to excellent yields, and is a convenient approach. This catalyst-free domino process facilitates the double annulation with a variety of scaffold building agents via two C-N and one C-X (X = C, N, O) bond formation in a single step under uniform reaction conditions. Furthermore, we reveal an unusual dual BDAC sequence leading to N-N-linked isoquinoline dimer.

Correction: Hypervalent iodine(iii)-promoted N-incorporation into N-aryl vinylogous carbamates to quinoxaline diesters: access to 1,4,5,8-tetraazaphenanthrene.

Correction for 'Hypervalent iodine(iii)-promoted N-incorporation into N-aryl vinylogous carbamates to quinoxaline diesters: access to 1,4,5,8-tetraazaphenanthrene' by A. Sagar, et al., Org. Biomol. Chem., 2016, DOI: 10.1039/c6ob00447d.

Hypervalent iodine(iii)-promoted N-incorporation into N-aryl vinylogous carbamates to quinoxaline diesters: access to 1,4,5,8-tetraazaphenanthrene.

A novel oxidative N-incorporation strategy for synthesis of quinoxaline diesters under metal-free conditions is described for the first time. The mild reaction conditions allow for this transformation via the formation of two C(sp(2))-N bonds utilizing cheaply available NaN3 as the N-atom source. N-Aryl vinylogous carbamates in this study undergo azidation at enamino C(sp(2))-H selectively. The robustness of this strategy is further demonstrated by the synthesis of a valuable 1,4,5,8-tetraazaphenanthrene derivative using a mild and convenient approach.