Organocatalytic Approach for C(sp3)–H Bond Arylation, Alkylation, and Amidation of Isochromans under Facile Conditions

High atomic utilization conversion of ethers into furancarbaldehydes via an ether oxidation iminium-ion activation cascade strategy

A novel organocatalytic one-pot cascade ether oxidation iminium-ion activation strategy for the synthesis of naphtho[2,1-b]furan-1-carbaldehyde and benzofuran-3-carbaldehyde from high atomic utilization transformation of aryl allyl ethers has been developed. Its synthetic application will provide a new ether oxidation iminium-ion activation cascade tool for the efficient synthesis of complex molecules.

Synthesis of functionalised isochromans: epoxides as aldehyde surrogates in hexafluoroisopropanol

The oxa-Pictet-Spengler reaction is arguably the most straightforward and modular way to construct the privileged isochroman motif, but its scope is largely limited to benzaldehyde derivatives and to electron-rich β-phenylethanols that lack substitution along the aliphatic chain. Here we describe a variant of this reaction starting from an epoxide, rather than an aldehyde, that greatly expands the scope and rate of the reaction (<1 h, 20 °C). Besides facilitating the initial Meinwald rearrangement, the use of hexafluoroisopropanol (HFIP) as a solvent expands the electrophile scope to include partners equivalent to ketones, aliphatic aldehydes, and phenylacetyl aldehydes, and the nucleophile scope to include modestly electronically deactivated and highly substituted β-phenylethanols. The products could be easily further derivatised in the same pot by subsequent ring-opening, reductions, and intra- and intermolecular Friedel-Crafts reactions, also in HFIP. Finally, owing to the high pharmacological relevance of the isochroman motif, the synthesis of drug analogues was demonstrated.

Metal-free regioselective mono- and poly-halogenation of 2-substituted indazoles

An unprecedented metal-free regioselective halogenation of 2H-indazoles has been revealed, which not only realized the highly selective synthesis of mono-halogenated products, but also completed poly-halogenations by fine tuning the reaction conditions. Various mono-/poly-/hetero-halogenated indazoles were obtained in moderate to excellent yields. Notably, this approach features environmentally friendly solvents, mild reaction conditions, simple execution and short reaction time.

Electrochemical α-C(sp3)-H/O-H cross-coupling of isochromans and alcohols assisted by benzoic acid

Isochroman moieties are present in a wide variety of biologically active molecules, but converting isochromans to α-substituted derivatives under mild conditions is challenging. Herein, we report a mild, convenient protocol for synthesis of α-alkoxy isochroman derivatives by means of electrochemical α-C(sp³)-H/O-H cross-coupling reactions of isochromans and alcohols in the presence of benzoic acid, which facilitated the electro-oxidation process and increased the product yield. Various alcohols and isochromans, as well as other structurally similar substrates, gave moderate to high yields of the desired coupling products, and the reaction could be carried out on a gram scale.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Diastereoselective synthesis of glycopyrans 1,2-annulated with dioxazinanes from 1,2-anhydrosugars and N-substituted nitrones

1,2-Annulated pyranose sugars fused with six membered rings have emerged as an important class of carbohydrates with wide biological and synthetic utility. We now describe zinc chloride catalyzed one pot diastereoselective synthesis of sugar fused dioxazinanes from 1,2-anhydro sugars and N-substituted aromatic nitrones. Various aromatic nitrones with different substituents undergo the reaction smoothly. The developed strategy works well with both ester and ether protection on the sugar and proceeds under mild reaction conditions. The mechanism seems to involve activation of the anhydrosugar by ZnCl₂ for nucleophilic attack by the nitrone followed by cyclization.

The synthesis of indeno[de]isochromene derivatives from arylvinyl epoxides and carbonyl compounds via tandem Nazarov and oxa-Pictet–Spengler cyclizations

The conversion of aryl vinyl epoxides and carbonyl compounds in the presence of a catalytic amount of BF3·OEt2 to indeno[de]isochromenes via tandem Nazarov and oxa-Pictet–Spengler cyclizations is described.

Recent advances in the functionalization of polyfluoro(aza)aromatics via C-C coupling strategies

Polyfluoro(aza)aromatic compounds are of interest in various fields of practical applications, such as medicinal and agrochemistry, materials science and advanced technologies. The C-C coupling reactions are known to be a promising synthetic tool to create challenging fluorinated molecules of diverse architectures. In this review, we have summarized the recent advances in the functionalization of polyfluoro(aza)aromatics via both transition metal-catalyzed and metal-free C-C coupling reactions for the period from 2006 to the beginning of 2021. Also, mechanistic features for chemical transformations of fluoroarene scaffolds and new opportunities for practical applications of the designed fluorinated molecules have been highlighted.

From Isocyanides to Iminonitriles via Silver-mediated Sequential Insertion of C(sp3)-H Bond

Heterocycles are prevalent constituents of many marketing drugs and biologically active molecules to meet modern medical challenges. Isocyanide insertion into C(sp³)–H bonds is challenging especially for the construction of quaternary carbon centers. Herein, we describe an efficient strategy for the synthesis of α-iminonitrile substituted isochromans and tetrahydroisoquinolines (THIQs) with quaternary carbon centers through silver-triflate-mediated sequential isocyanide insertion of C(sp³)–H bonds, where isocyanide acts as the crucial “CN” and “imine” sources. The produced α-iminonitriles have extensive applications as valuable synthetic building blocks for pharmacologically interesting heterocycles. This protocol could be further applied for the synthesis of iminonitrile-decorated phenanthridines and azapyrene. Interestingly, a remarkable aggregation-induced emission (AIE) effect was first observed for an iminonitrile-decorated pyrene derivative, which may open a particular area for iminonitrile applications in materials science.

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Iminonitrile formation via sequential C(sp³)-H bond isocyanide insertion

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Construction of quaternary center

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Isocyanide as both "imine" and "CN" sources

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Valuable synthetic building blocks and novel AIEgen

Direct functionalization of alkyl ethers to construct hemiaminal ether skeletons (HESs)

Hemiaminal ether skeletons (HESs) are important backbones generally found in organic compounds. Compared to traditional functional group transformations, strategies for the C-H functionalization of alkyl ethers to construct HESs have drawn much attention in the past decade. In this review, we focus on the recent progress in developing various pathways to construct HESs via the direct functionalization of alkyl ethers, including: (a) the transition-metal-catalyzed nitrene insertion pathway, (b) the transition-metal-free organonitrenoid insertion pathway,

Replacement of Stoichiometric DDQ with a Low Potential o-Quinone Catalyst Enabling Aerobic Dehydrogenation of Tertiary Indolines in Pharmaceutical Intermediates

A transition-metal/quinone complex, [Ru(phd)₃]²⁺ (phd = 1,10-phenanthroline-5,6-dione), is shown to be effective for aerobic dehydrogenation of 3° indolines to the corresponding indoles. The results show how low potential quinones may be tailored to provide a catalytic alternative to stoichiometric DDQ, due to their ability to mediate efficient substrate dehydrogenation while also being compatible with facile reoxidation by O₂. The utility of the method is demonstrated in the synthesis of key intermediates to pharmaceutically important molecules.

Copper-catalyzed highly diastereoselective cross-dehydrogenative coupling between 8-hydroxyisochromanes and 1,3-dicarbonyl compounds

A novel copper-catalyzed highly diastereoselective cross-dehydrogenative coupling reaction for the access of tricyclic chromanes from 8-hydroxyisochromanes and 1,3-dicarbonyl compounds.

Bimolecular oxidative C–H alkynylation of α-substituted isochromans

The first bimolecular oxidative C–H functionalization of secondary benzylic ethers for tertiary ether synthesis has been established in high efficiency.

Stereoselective synthesis of sugar-fused (or 1,2-annulated) isochromans and isochromanones by using oxa-Pictet-Spengler reaction

A highly efficient stereoselective synthesis of sugar fused 1,2-annulated isochromans is reported by utilizing the oxa-Pictet-Spengler cyclization reaction. The process is found to be very general as both glucal and galactal derived C2-hydroxy-α-C-aryl glycosides lead to the target molecules in good yields. The utility of this strategy was extended to the synthesis of sugar fused isochromanones, which are bergenin type molecules.

Direct Arylation of Benzyl Ethers with Organozinc Reagents

A novel C(sp³)-H bond arylation of benzyl ethers with Knochel-type arylzinc reagents has been developed. This transition-metal-catalyst-free reaction proceeds well under mild conditions in a simple and effective manner and enables the synthesis of a wide range of potentially biologically active benzyl ethers by using highly functionalized organozinc reagents as a carbon nucleophile.

Evolution of a Strategy for the Enantioselective Total Synthesis of (+)-Psiguadial B

(+)-Psiguadial B is a diformyl phloroglucinol meroterpenoid that exhibits antiproliferative activity against the HepG2 human hepatoma cancer cell line. This full account details the evolution of a strategy that culminated in the first enantioselective total synthesis of (+)-psiguadial B. A key feature of the synthesis is the construction of the trans-cyclobutane motif by a Wolff rearrangement with in situ catalytic, asymmetric trapping of the ketene. An investigation of the substrate scope of this method to prepare enantioenriched 8-aminoquinolinamides is disclosed. Three routes toward (+)-psiguadial B were evaluated that featured the following key steps: (1) an ortho-quinone methide hetero-Diels-Alder cycloaddition to prepare the chroman framework, (2) a Prins cyclization to form the bridging bicyclo[4.3.1]decane system, and (3) a modified Norrish-Yang cyclization to generate the chroman. Ultimately, the successful strategy employed a ring-closing metathesis to form the seven-membered ring and an intramolecular O-arylation reaction to complete the polycyclic framework of the natural product.

Cross-dehydrogenative coupling and oxidative-amination reactions of ethers and alcohols with aromatics and heteroaromatics

This is a perspective on cross-dehydrogenative coupling reactions of aromatic and heteroaromatic systems with ethers and alcohols via metal or organic catalysis, and through uncatalyzed means, leading to C–C and C–N bond formation.

Cross-dehydrogenative coupling (CDC) is a process in which, typically, a C–C bond is formed at the expense of two C–H bonds, either catalyzed by metals or other organic compounds, or via uncatalyzed processes. In this perspective, we present various modes of C–H bond-activation at sp³ centers adjacent to ether oxygen atoms, followed by C–C bond formation with aromatic systems as well as with heteroaromatic systems. C–N bond-formation with NH-containing heteroaromatics, leading to hemiaminal ethers, is also an event that can occur analogously to C–C bond formation, but at the expense of C–H and N–H bonds. A large variety of hemiaminal ether-forming reactions have recently appeared in the literature and this perspective also includes this complementary chemistry. In addition, the participation of C–H bonds in alcohols in such processes is also described. Facile access to a wide range of compounds can be attained through these processes, rendering such reactions useful for synthetic applications via C_sp³ bond activations.

A Total Synthesis of Bifidenone

The first total synthesis of bifidenone, a novel natural tubulin polymerization inhibitor, has been achieved in 12 steps starting from commercially available 1,4-dioxaspiro[4.5]decan-8-one. The synthesis includes a newly developed method to generate the dihydrobenzodioxolone core by palladium-catalyzed aerobic dehydrogenation. The three stereocenters were installed with an AD-mix-β dihydroxylation step followed by a late-stage palladium-catalyzed decarboxylation-allylation procedure. The absolute stereochemistry of 3 was determined via 13a by single-crystal X-ray analysis.

Chemoselective Palladium-Catalyzed Deprotonative Arylation/[1,2]-Wittig Rearrangement of Pyridylmethyl Ethers

Control of chemoselectivity is one of the most challenging problems facing chemists and is particularly important in the synthesis of bioactive compounds and medications. Herein, the first highly chemoselective tandem C(sp3)-H arylation/[1,2]-Wittig rearrangement of pyridylmethyl ethers is presented. The efficient and operationally simple protocols enable generation of either arylation products or tandem arylation/[1,2]-Wittig rearrangement products with remarkable selectivity and good to excellent yields (60-99%). Choice of base, solvent, and reaction temperature play a pivotal role in tuning the reactivity of intermediates and controlling the relative rates of competing processes. The novel arylation step is catalyzed by a Pd(OAc)2/NIXANTPHOS-based system via a deprotonative cross-coupling process. The method provides rapid access to skeletally diverse aryl(pyridyl)methanol core structures, which are central components of several medications.

Diastereoselective synthesis of substituted hexahydrobenzo[de]isochromanes and evaluation of their antileishmanial activity

Hexahydrobenzo[de]isochromanes and hexahydropyrano[3,4,5-ij]isoquinolines can be efficiently synthesized via Friedel Crafts and oxa Pictet-Spengler reaction of acrylyl enol ethers mediated by triflic acid in good yields. The reaction is highly stereoselective. Two of the hexahydrobenzo[de]isochromanes are found to have moderate antileishmanial activity.

Quinone-Catalyzed Selective Oxidation of Organic Molecules

Quinones are common stoichiometric reagents in organic chemistry. Para-quinones with high reduction potentials, such as DDQ and chloranil, are widely used and typically promote hydride abstraction. In recent years, many catalytic applications of these methods have been achieved by using transition metals, electrochemistry, or O2 to regenerate the oxidized quinone in situ. Complementary studies have led to the development of a different class of quinones that resemble the ortho-quinone cofactors in copper amine oxidases and mediate the efficient and selective aerobic and/or electrochemical dehydrogenation of amines. The latter reactions typically proceed by electrophilic transamination and/or addition-elimination reaction mechanisms, rather than hydride abstraction pathways. The collective observations show that the quinone structure has a significant influence on the reaction mechanism and has important implications for the development of new quinone reagents and quinone-catalyzed transformations.

Gold-Catalyzed Benzylic Azidation of Phthalans and Isochromans and Subsequent FeCl3-Catalyzed Nucleophilic Substitutions

The benzylic positions of the phthalan and isochroman derivatives (1) as benzene-fused cyclic ethers effectively underwent gold-catalyzed direct azidation using trimethylsilylazide (TMSN3) to give the corresponding 1-azidated products (2) possessing the N,O-acetal partial structure. The azido group of the N,O-acetal behaved as a leaving group in the presence of catalytic iron(III) chloride, and 1-aryl or allyl phthalan and isochroman derivatives were obtained by nucleophilic arylation or allylation, respectively. Meanwhile, a double nucleophilic substitution toward the 1-azidated products (2) occurred at the 1-position using indole derivatives as a nucleophile accompanied by elimination of the azido group and subsequent ring opening of the cyclic ether nucleus produced the bisindolylarylmethane derivatives.

Catalytic amounts of CBr4 mediated dehydrogenative coupling of isochromans with aromatic ketones

In the presence of catalytic amounts of CBr₄ (a metal-free mediator), an unexpected oxidative dehydrogenative coupling of isochromans with ketones occurred to construct new C^sp3–C^sp3 bonds. The reactions were performed under simple solvent-free aerobic conditions.

Selective formation of C–N and CN bonds via C(sp3)–H activation of isochroman in the presence of DTBP

An organocatalytic approach for the synthesis of isochroman derivatives via direct C(sp³)–H bond and N–H bond coupling is described. The C–N (amine or amide) and CN (imidate) products can be selectively achieved by controlling the amount of oxidants.