PhI(OAc)2-mediated intramolecular oxidative aryl-aldehyde Csp(2)-Csp(2) bond formation: metal-free synthesis of acridone derivatives

Synthesis of acridones via Ir(III)-catalyzed amination annulation of oxazoles with anthranils

An unprecedented Ir(III)-catalyzed C-H activation/amination/annulation of 2-phenyloxazoles with anthranils for the highly selective preparation of acridone derivatives in one-pot under controlled conditions is reported. This protocol is characterized by atom economy and high regioselectivity. A wide range of anthranils with 2-phenyloxazoles were well tolerated and afforded the desired products in moderate to good yields, in which the anthranil serves as a convenient amination reagent.

Palladium-Catalyzed Dual C-H Carbonylation of Diarylamines Leading to Diversified Acridones under CO-Free Conditions

A Pd-catalyzed dual C-H carbonylation of commercially available diarylamines using Co2(CO)8 as a safe CO source has been developed. This methodology provides a facile approach for the synthesis of diversified acridones in moderate to good yields. The protocol features good functional group compatibility, operational safety, easy scale-up, and versatile transformations.

K2CO3-Catalyzed Dual C-C-Coupled Cyclization to 3-Amino-4-benzoylbiphenyls and In Situ I2-Catalyzed C-N Bond Forming Annulation: A Metal-Free Synthesis of Arylacridones

Unprecedented metal-free cyclization catalysis reactions are developed in a highly regioselective fashion to synthesize 3-amino-4-benzoyl biphenyls and arylacridones with high atom economy. Catalytic K2CO3 is utilized as the only reagent for the unusual rapid dual C-C-coupled cyclization between β-keto enamines and cinnamaldehydes to furnish the functionalized biphenyls. Its C(sp2)-H functionalized C-N bond-forming cyclization was performed in situ using molecular I2 as a catalyst to furnish valuable arylacridones. Plausible mechanisms for the new cyclization reactions are predicted by conducting various control experiments and ESI-MS analyses.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Base-Controlled Synthesis of Fluorescent Acridone Derivatives via Formal (4 + 2) Cycloaddition

A transition-metal-free formal (4 + 2) cycloaddition for the direct assembly of acridone derivatives has been developed from simple and easily accessible o-aminobenzamides and 2-(trimethylsilyl)aryl triflates. The base played an important role in the selective controlled synthesis of N-H and N-aryl acridones. A preliminary study on the fluorescence properties of N-aryl acridones demonstrated that they could be used as fluorescent materials with a broad emission range.

Double substitution leads to a highly polymorphic system in 5-methyl-2-m-tolylamino-benzoic acid

Substitution on both aromatic rings of fenamic acid led to a highly polymorphic system in 5-methyl-2-m-tolylamino-benzoic acid, in contrast to the two mono-substituted analogs, which had only one or two forms obtained.

Synthesis of highly substituted fluorenones via metal-free TBHP-promoted oxidative cyclization of 2-(aminomethyl)biphenyls. Application to the total synthesis of nobilone

Highly substituted fluorenones are readily prepared in mostly fair to good yields via metal- and additive-free TBHP-promoted cross-dehydrogenative coupling (CDC) of readily accessible N-methyl-2-(aminomethyl)biphenyls and 2-(aminomethyl)biphenyls. This methodology is compatible with numerous functional groups (methoxy, cyano, nitro, chloro, and SEM and TBS-protective groups for phenols) and was further utilized in the first total synthesis of the natural product nobilone.

Access to acridones by tandem copper(I)-catalyzed electrophilic amination/Ag(I)-mediated oxidative annulation of anthranils with arylboronic acids

An efficient and practical approach for the synthesis of medicinally important acridones was developed from anthranils and commercially available arylboronic acids by a tandem copper(I)-catalyzed electrophilic amination/Ag(I)-mediated oxidative annulation strategy. This new and straightforward protocol displayed a broad substrate scope (25 examples) and high functional group tolerance. What's more, a possible mechanistic proposal was also presented.

Transition-metal-free oxidative cyclization reaction of enynals to access pyrane-2-one derivatives

A novel and efficient metal-free C-H functionalization of enynals is developed to synthesize α-pyrone derivatives via the formation of two C-O bonds. In this project, K₂S₂O₈ has been introduced as an efficient oxygen source and C-H functionalization agent in regioselective oxidative cyclization reaction with a relatively broad substrate scope.

[1,5]-Hydride Shift-Cyclization versus C(sp2)-H Functionalization in the Knoevenagel-Cyclization Domino Reactions of 1,4- and 1,5-Benzoxazepines

Domino cyclization reactions of N-aryl-1,4- and 1,5-benzoxazepine derivatives involving [1,5]-hydride shift or C(sp2)-H functionalization were investigated. Neuroprotective and acetylcholinesterase activities of the products were studied. Domino Knoevenagel-[1,5]-hydride shift-cyclization reaction of N-aryl-1,4-benzoxazepine derivatives with 1,3-dicarbonyl reagents having active methylene group afforded the 1,2,8,9-tetrahydro-7bH-quinolino [1,2-d][1,4]benzoxazepine scaffold with different substitution pattern. The C(sp3)-H activation step of the tertiary amine moiety occurred with complete regioselectivity and the 6-endo cyclization took place in a complete diastereoselective manner. In two cases, the enantiomers of the chiral condensed new 1,4-benzoxazepine systems were separated by chiral HPLC, HPLC-ECD spectra were recorded, and absolute configurations were determined by time-dependent density functional theory- electronic circular dichroism (TDDFT-ECD) calculations. In contrast, the analogue reaction of the regioisomeric N-aryl-1,5-benzoxazepine derivative did not follow the above mechanism but instead the Knoevenagel intermediate reacted in an SEAr reaction [C(sp2)-H functionalization] resulting in a condensed acridane derivative. The AChE inhibitory assays of the new derivatives revealed that the acridane derivative had a 6.98 μM IC50 value.

Practical N-Hydroxyphthalimide-Mediated Oxidation of Sulfonamides to N-Sulfonylimines

A new method to prepare sulfonylimines through the oxidation of sulfonamides mediated by N-hydroxyphthalimide under mild conditions has been developed. Compared to reported oxidation methods, broader substrates scope and milder conditions were achieved in our method. Importantly, this oxidation method can afford N-sulfonyl enaminones using Mannich products as starting materials. Additionally, the one-pot Friedel-Crafts arylation reaction of unseparated N-sulfonylimine formed in our system with 1,3,5-trimethoxybenzene was successful without any additional catalyst.

Copper-catalyzed acylation of pyrazolones with aldehydes to afford 4-acylpyrazolones

Copper-catalyzed direct acylation of the alkenyl C-H bond in 1,2-dihydro-3H-pyrazol-3-ones has been developed, affording a series of 4-acylpyrazolones in moderate to good yields. Notably, this protocol involves readily accessible substrates and reagents, which have good functional group tolerance leading to pyrazolone derivatives under mild reaction conditions.

Boron Trifluoride Etherate Promoted Microwave-Assisted Synthesis of Antimalarial Acridones

A microwave-assisted, rapid and efficient method using boron trifluoride etherate (BF₃.Et₂O) for the synthesis of acridones, via an intramolecular acylation of N-phenylanthranilic acid derivatives, has been developed. The reaction proceeds under solvent-free conditions, tolerates a wide range of functional groups, and provides rapid access to a range of acridones in good to excellent yields. Several of the synthesized acridones exhibited potent antimalarial activities against CQ sensitive and multi-drug resistant (MDR) parasites.

An Efficient Buchwald-Hartwig/Reductive Cyclization for the Scaffold Diversification of Halogenated Phenazines: Potent Antibacterial Targeting, Biofilm Eradication, and Prodrug Exploration

Bacterial biofilms are surface-attached communities comprised of nonreplicating persister cells housed within a protective extracellular matrix. Biofilms display tolerance toward conventional antibiotics, occur in ∼80% of infections, and lead to >500000 deaths annually. We recently identified halogenated phenazine (HP) analogues which demonstrate biofilm-eradicating activities against priority pathogens; however, the synthesis of phenazines presents limitations. Herein, we report a refined HP synthesis which expedited the identification of improved biofilm-eradicating agents. 1-Methoxyphenazine scaffolds were generated through a Buchwald-Hartwig cross-coupling (70% average yield) and subsequent reductive cyclization (68% average yield), expediting the discovery of potent biofilm-eradicating HPs (e.g., 61: MRSA BAA-1707 MBEC = 4.69 μM). We also developed bacterial-selective prodrugs (reductively activated quinone-alkyloxycarbonyloxymethyl moiety) to afford HP 87, which demonstrated excellent antibacterial and biofilm eradication activities against MRSA BAA-1707 (MIC = 0.15 μM, MBEC = 12.5 μM). Furthermore, active HPs herein exhibit negligible cytotoxic or hemolytic effects, highlighting their potential to target biofilms.

Synthesis of 2-aminobenzophenones through acylation of anilines with α-oxocarboxylic acids assisted by tert-butyl nitrite

A highly facile all-in-one-pot synthesis of 2-aminobenzophenones directly from anilines, tert-butyl nitrite and α-oxocarboxylic acids under the catalysis of Pd(OAc)₂ is presented.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Catalytic double hydroarylation of alkynes to 9,9-disubstituted 9,10-dihydroacridine derivatives by an electrophilic phenoxyphosphonium dication

An electrophilic pyridinium-substituted phenoxyphosphonium dicationic salt catalyzes the double hydroarylation of alkynes with diarylamines to give dihydroacridine derivatives.

Metal-free intermolecular cyclopropanation between alkenes and iodonium ylides mediated by PhI(OAc)2·Bu4NI

A simple and highly effective synthesis of poly substituted cyclopropanes was developed. This metal-free intermolecular reaction between iodonium ylides and alkenes is mediated by PhI(OAc)₂ and Bu₄NI.

Copper-catalyzed, hypervalent iodine mediated CC bond activation of enaminones for the synthesis of α-keto amides

An unprecedented approach toward the general synthesis of α-keto amides has been established by tailoring the CC double bond of enaminones in the presence of CuI and hypervalent iodine.

Copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes: an efficient approach to functionalized quinazolines

The first copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes was described.

Metal-Free Oxidative C-C Bond Formation through C-H Bond Functionalization

The formation of C-C bonds embodies the core of organic chemistry because of its fundamental application in generation of molecular diversity and complexity. C-C bond-forming reactions are well-known challenges. To achieve this goal through direct functionalization of C-H bonds in both of the coupling partners represents the state-of-the-art in organic synthesis. Oxidative C-C bond formation obviates the need for prefunctionalization of both substrates. This Minireview is dedicated to the field of C-C bond-forming reactions through direct C-H bond functionalization under completely metal-free oxidative conditions. Selected important developments in this area have been summarized with representative examples and discussions on their reaction mechanisms.

Green method for the synthesis of chromeno[2,3-c]pyrazol-4(1H)-ones through ionic liquid promoted directed annulation of 5-(aryloxy)-1H-pyrazole-4-carbaldehydes in aqueous media

The first classical heterocyclic ionic liquid (IL) promoted C-H bond oxidant cross-coupling reaction for the intramolecular annulation of 5-(aryloxy)-1H-pyrazole-4-carbaldehydes to chromeno[2,3-c]pyrazol-4(1H)-ones has been disclosed. The promoter 1,3-dibutyl-1H-benzo[d][1,2,3]triazol-3-ium bromide can be easily recycled and reused with the same efficacies for at least five cycles in aqueous medium. The strategy works smoothly and provides an applicable protocol to construct a wide range of products.

Oxidative Heterocycle Formation Using Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) reagents have been widely exploited in a diverse array of synthetic transformations. This chapter focuses on the general application of hypervalent iodine(III) reagents in the de novo synthesis and in the late stage functionalization of heterocyclic compounds.

NIS-mediated intramolecular oxidative α-functionalization of tertiary amines: transition metal-free synthesis of 1,2-dihydro-(4H)-3,1-benzoxazin-4-one derivatives

A novel method for direct α-functionalization of tertiary amines via NIS-mediated oxidative C–O bond formation, where NIS serves as both an oxidant and an iodination reagent, has been developed.