Controllable Synthesis of N-Heterocycles via Hydride Transfer Strategy-Enabled Formal [5 + 1] and [5 + 2] Cyclizations

Substrate- and reagent-dependent selective annulation of amidines and 2-tert-amino benzaldehydes

Herein, we present an efficient and substrate/reagent-dependent annulation of amidines with 2-tert-amino benzaldehydes. When N-alkyl benzimidamides are employed, amidines undergo self-annulation to generate a hydrotriazine skeleton. In contrast, N-aryl benzimidamides yield quinazolines/hydroquinazolines under similar conditions. Additionally, the coupling of 2-tert-amino benzaldehydes with amidines under basic conditions yields 1,3,5-triazine derivatives.

Gold-Catalyzed Intermolecular Assembly of Donor and Acceptor Fragments for [1,5]-Hydride Migrations

The [1,n]-hydride migration reactions represent one of the most powerful techniques to functionalize C(sp3)-H bonds and to rapidly construct molecular complexities. However, their substrates usually required multiple steps to prepare in advance. Herein, we developed a gold-catalyzed intermolecular coupling of hydride-donor-containing nucleophiles (amines) and hydride-acceptor-containing alkynes (1,3-diynamides) for cascade [1,5]-hydride migrations. Control experiments revealed the role of the dichloroethane solvent and the unexpected involvement of protonated anilines in promoting [1,5]-hydride migrations.

Double Hydride Transfer Enabled Substitution of All Hydrogens at α,β-Positions of Cyclic Amines: Access to α,β-Unsaturated Lactams

A one-shot substitution of all hydrogens at the α,β-positions of saturated cyclic amines was achieved. The key feature of this reaction is the sequential involvement of intra- and intermolecular redox processes. When N,O-acetals obtained through an internal redox process were treated with a catalytic amount of Zn(OTf)2 and an excess amount of benzylidene barbiturates, three key transformations involving intermolecular redox process occurred successively to afford α,β-unsaturated lactams in moderate to good chemical yields.

tert-Butyl Hypochlorite: A Reagent for the Synthesis of Chlorinated Oxindole and Indole Derivatives

tert-Butyl hypochlorite was employed as a versatile reagent for chlorooxidation of indoles, chlorination of 2-oxindoles, and decarboxylative chlorination of the indole-2-carboxylic acids. Four types of products including 2-chloro-3-oxindoles, 2,2-dichloro-3-oxindoles, 3,3-dichloro-2-oxindoles, and 2,3-dichloroindoles could be selectively obtained in moderate to excellent yields by switching the substrates. Various synthetically useful functional groups, such as halogen atoms, cyano, nitro, and methoxycarbonyl groups, remain intact during the reactions. Notable features of the approach include the universality of the starting materials, the mild reaction conditions, and the experimental simplicity.

Copper-Catalyzed Aerobic Cross-Dehydrogenation Coupling of Indoles for Synthesis of 2,3'-Bisindoles

2,3'-Bisindoles with C-C linkages have attracted interest in medicinal chemistry, yet their synthesis is intricate with many steps. Notably, direct C-H/C-H cross-coupling of non-directed heteroaromatics remains challenging, often requiring precious metals and oxidants to enhance coupling efficiency. Herein, we present a copper-catalyzed C-H/C-H cross-coupling method for N-substituted indoles without directing groups, facilitated by molecular oxygen under gentle conditions. It showed reasonable functional group compatibility and provided one-pot access to a variety of 2,3'-bisindoles derivatives in moderate to good yields.

Direct α,β-C-H Difunctionalization of Piperidines for the Construction of the N,O-Acetal Skeleton via 1,5-Hydride Transfer

Herein, we describe an unprecedented Lewis acid-catalyzed annulation of phenols with o-aminobenzaldehydes via a cascade coupling/1,5-hydride transfer/cyclization sequence. The α- and β-positions of cyclic amines were functionalized utilizing enamines generated in situ. A series of complex N,O-acetal derivatives are synthesized in moderate to good yields in one step. The methodology features high atom and step economy, excellent diastereoselectivity, and water as the sole byproduct.

Synthesis and Structure of 5-Methyl-9-(trifluoromethyl)-12H-quino[3,4-b][1,4]benzothiazinium Chloride as Anticancer Agent

In this work, the synthesis, structural analysis and anticancer properties of 5-methyl-9-trifluoromethyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (3) are described. Compound 3 was synthesized by reacting 1-methyl-4-butylthio-3-(benzoylthio)quinolinium chloride with 4-(trifluoromethyl)aniline, respectively. The structure of the resulting product was determined using 1H-NMR and 13C-NMR spectroscopy as well as HR-MS spectrometry. The spatial geometry of agent 3 and the arrangement of molecules in the crystal (unit cell) were also confirmed using X-ray diffraction. The tetracyclic quinobenzothiazinium system is fairly planar because the dihedral angle between the planes formed by the benzene ring and the quinoline system is 173.47°. In order to obtain insight into the electronic charge distribution of the investigated molecule, electronic structure calculations employing the Density Functional Theory (DFT) were performed. Moreover, antiproliferative activity against a set of pancreatic cancer cell lines was tested, with compound 3 showing IC50 values against human primary pancreatic adenocarcinoma BxPC-3 and human epithelioid pancreatic carcinoma Panc-1 of 0.051 µM and 0.066 µM, respectively. The IC50 value of cytotoxicity/cell viability of the investigated compound assessed on normal human lung fibroblasts WI38 was 0.36 µM.

Diastereoselective hydride transfer enables a synthesis of chiral 1,5-carboxamido-trifluoromethylcarbinols

The deployment of fluorinated functional groups has become a widespread tool in medicinal chemistry due to the impact of fluorine on lipophilicity and metabolic stability. Among these compounds, enantiopure secondary trifluoromethylcarbinols are recurrent features in bioactive compounds. Herein, we present a diastereoselective redox-neutral process allowing the stereospecific synthesis of 1,5-carboxamido-trifluoromethylcarbinols through the formal reduction of a trifluoromethylketone into a trifluoromethylcarbinol. A combined experimental and computational investigation unveiled a network of interconnected equilibria leading to a key hydride transfer event.

Naturally Occurring Xanthones and Their Biological Implications

Xanthones are chemical substances in higher plants, marine organisms, and lower microorganisms. The most prevalent naturally occurring sources of xanthones are those belonging to the families Caryophyllaceae, Guttiferae, and Gentianaceae. Structurally, xanthones (9H xanthan-9-one) are heterocyclic compounds with oxygen and a γ-pyrone component. They are densely packed with a two-benzene ring structure. The carbons in xanthones are numbered from their nucleus and biosynthetic construct. They have mixed shikimate-acetate (higher plants) and acetate-malonate (lower organisms) biosynthetic origins, which influence their classification. Based on the level of oxidation of the C-ring, they are classified into monomers, dimers, and heterodimers. While based on the level of oxygenation or the type of ring residue, they can be categorized into mono-, di-, tri-, tetra-, penta- and hexa-oxygenated xanthones, bis-xanthones, prenylated and related xanthones, xanthonolignoids, and other miscellaneous xanthones. This structural diversity has made xanthones exhibit considerable biological properties as promising antioxidant, antifungal, antimicrobial, and anticancer agents. Structure-activity relationship studies suggest C-1, C-3, C-6, and C-8 as the key positions that influence the biological activity of xanthones. Furthermore, the presence of functional groups, such as prenyl, hydroxyl, glycosyl, furan, and pyran, at the key positions of xanthones, may contribute to their spectrum of biological activity. The unique chemical scaffolds of xanthones, their notable biological activities, and the structure-activity relationships of some lead molecules were discussed to identify lead molecules as possible drug candidates.

Electrochemical Dehydrogenative [3 + 2]/[5 + 2] Annulation of N-Arylacrylamides with γ,σ-Unsaturated Malonates via Direct C(sp3)-H/C(sp2)-H Functionalization

Herein, we introduce an electrochemical dehydrogenative [3 + 2]/[5 + 2] annulation of easily available N-arylacrylamides with γ,σ-unsaturated malonates through C(sp3)-H/C(sp2)-H functionalization. The employment of inexpensive ferrocene as the redox catalyst allows access to diverse benzo[b]azepin-2-ones in moderate to excellent yields without stoichiometric oxidants. This protocol features broad substrate scope and excellent selectivity, and mechanistic studies indicated that the reaction proceeded through the oxidation of a C(sp3)-H bond to generate an alkyl radical, radical addition across the C═C bond, [3 + 2]/[5 + 2] annulations, and C(sp2)-H functionalization cascades.

K2CO3-mediated annulation of 1,3-acetonedicarboxylates with 2-fluoro-1-nitroarenes: synthesis of indoles

The K2CO3-mediated one-pot reaction of 1,3-acetonedicarboxylates with 2 equiv. of substituted 2-fluoro-1-nitrobenzenes has been developed to synthesize various 2,3-dicarboxylate indoles via a tandem annulation pathway. In the effective reaction, one carbon-carbon double bond, one carbon-carbon single bond and one carbon-nitrogen single bond are formed under open-vessel conditions. DFT calculations are used to rationalize the plausible mechanisms.

Synthesis of 2,3-Dialkyl-5-hydroxybenzofurans via a One-pot, Three-step Reaction Sequence of 2-Monosubstituted 1,3-Diketones and 1,4-Benzoquinones

An economical one-pot, three-step reaction sequence of readily available 2-monosubstituted 1,3-diketones and 1,4-benzoquinones has been explored for the facile access of 2,3-dialkyl-5-hydroxybenzofurans. By using cheap K2CO3 and conc. HCl as the reaction promoters, the reaction occurs smoothly via sequential Michael addition, aromatization, retro-Claisen, deacylation, hemiketalization, and dehydration processes under mild conditions in a practical manner. Additionally, an interesting phenomenon was observed during the derivatization studies, where the dihydroquinoline was converted into tetrahydroquinoline and quinoline products, respectively, via a disproportionation process.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Diversity-oriented synthesis of indole-fused scaffolds and bis(indolyl)methane from tosyl-protected tryptamine

An efficient, diversity-oriented synthesis of indole-1,2-fused 1,4-benzodiazepines, tetrahydro-β-carbolines, and 2,2'-bis(indolyl)methanes was established starting from tosyl-protected tryptamine. These diverse privileged skeletons were controllably constructed by adjusting different hydride donors and Brønsted acids. A variety of indole-1,2-fused 1,4-benzodiazepines were facilely accessed using benzaldehydes bearing cyclic amines as hydride donors via a cascade N-alkylation/dehydration/[1,5]-hydride transfer/Friedel-Crafts alkylation sequence. The reaction site could be switched when benzaldehydes bearing an alkoxy moiety as hydride donors were used for the generation of tetrahydro-β-carbolines. On the other hand, the switchable synthesis of 2,2'-bis(indolyl)methanes could be achieved as well by applying p-TsOH·H2O as a catalyst. The reactions feature mild conditions, simple and practical operation, excellent efficiency and the use of EtOH as a green solvent. Using the concept of diversity-oriented, reagent-based synthesis, the inexpensive feedstock tryptamine was efficiently converted to three different types of privileged scaffolds, which facilitates rapid compound library synthesis for accelerating drug discovery.

Dearomatization of 3-Aminophenols for Synthesis of Spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones via Hydride Transfer Strategy-Enabled [5+1] Annulations

The Sc(OTf)3-catalyzed dearomative [5+1] annulations between readily available 3-aminophenols and O-alkyl ortho-oxybenzaldehydes were developed for synthesis of spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones. The "two-birds-with-one-stone" strategy was disclosed by the dearomatization of phenols and direct α-C(sp3)-H bond functionalization of oxygen through cascade condensation/[1,5]-hydride transfer/dearomative-cyclization process. In addition, the antifungal activity assay and derivatizations of products were conducted to further enrich the utility of the structure.