O-H Insertion of Hydrogenphosphate Derivatives and α-Diazo Compounds

An efficient O-H insertion of hydrogenphosphate derivatives and α-diazo compounds has been developed to construct α-phosphoryloxy scaffolds. Diverse α-phosphoryloxy skeletons could be obtained under mild and catalyst-free conditions in good yields. The control experiments suggest a protonation and nucleophilic addition process of α-diazo compounds via a diazonium ion pair for this transformation.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Theoretical Study on the Copper-Catalyzed ortho-Selective C-H Functionalization of Naphthols with α-Phenyl-α-Diazoesters

The aromatic C(sp2)-H functionalization of unprotected naphthols with α-phenyl-α-diazoesters under mild conditions catalyzed by CuCl and CuCl2 exhibits high efficiency and unique ortho-selectivity. In this study, the combination of density functional theory (DFT) calculations and experiments is employed to investigate the mechanism of C-H functionalization, which reveals the fundamental origin of the site-selectivity. It explains that CuCl-catalyzed ortho-selective C-H functionlization is due to the bimetallic carbene, which differs from the reaction catalyzed by CuCl2 via monometallic carbene. The results demonstrate the function of favourable H-bond interactions on the site- and chemo-selectivity of reaction through stabilizing the rate-determining transition states in proton (1,3)-migration.

Stereochemistry of Chiral 2-Substituted Chromanes: Twist of the Dihydropyran Ring and Specific Optical Rotation

Chiral 2-substituted chromanes are important substructures in organic synthesis and appear in numerous natural products. Herein, the correlation between specific optical rotations (SORs) and the stereochemistry at C2 of chiral 2-substituted chromanes was investigated through data mining, quantum-chemical calculations using density functional theory (DFT), and mechanistic analyses. For 2-aliphatic (including acyloxy and alkenyl) chromanes, the P-helicity of the dihydropyran ring usually corresponds to a positive SOR; however, 2-aryl chromanes with P-helicity tend to exhibit negative SORs. 2-Carboxyl (including alkoxycarbonyl and carbonyl) chromanes often display small experimental SORs, and theoretical calculations for them are prone to error because of the fluctuating conformational distribution with computational parameters. Several typical compounds were discussed, including detailed descriptions of the asymmetric synthesis, absolute configuration (AC) assignment methods, and systematic conformational analysis. We hope this work will enrich the knowledge of the stereochemistry of chiral 2-substituted chromanes.

Diastereoselective aldol-type interception of phenolic oxonium ylides for the direct assembly of 2,2-disubstituted dihydrobenzofurans

A Rh₂(OAc)₄ catalyzed intermolecular aldol-type interception of phenolic oxonium ylides with isatins has been developed, which provides an effective access to 2,2-disubstituted dihydrobenzofuran derivatives containing 3-hydroxyoxindole in high yields and with high diastereoselectivities under mild reaction conditions. The antiproliferation activity of these synthesized dihydrobenzofuran and 3-hydroxyoxindole hybrid products has been tested via the CCK8 assay in different cancer cell lines; compounds 3s and 3t exhibit good anticancer potency against human colon cancer cells (HCT116 cells, 3s: IC₅₀ = 15.99 μM; 3t: IC₅₀ = 14.48 μM) compared to other tested compounds.

Overcoming O-H Insertion to Para-Selective C-H Functionalization of Free Phenols: Rh(II)/Xantphos Catalyzed Geminal Difunctionalization of Diazo Compounds

Para-selective C-H functionalization of free phenols by metal carbenoids is rather challenging due to the generally more favorable competing O-H insertion. Herein, with the use of the combination of Rh(II) and a Xantphos ligand as the catalyst, a novel multicomponent reaction of free phenols, diazoesters, and allylic carbonates was successfully developed, affording a wide variety of phenol derivatives, bearing an all-carbon quaternary center and a synthetically useful allylic unit. This reaction is likely to occur through a tandem process of carbene-induced para-selective C-H functionalization, followed by Rh(II)/Xantphos-catalyzed allylation. The distinctive reactivity of para-selective C-H rather than O-H insertion for the carbenoid intermediate, combined with features of excellent functional group compatibility, high atom and step economy, and ease in further diversification of the products, might render this protocol highly attractive in facile functionalization of unprotected phenols.

One-Pot Transformation of Salicylaldehydes to Spiroepoxydienones via the Adler-Becker Reaction in a Continuous Flow

The Adler-Becker reaction is a useful approach for the oxidative dearomatization of salicylic alcohols to spiroepoxydienones and has been applied in the total synthesis of several natural products. Despite the advantages, the substrate and product instability under the reaction conditions can decrease the reaction efficiency, leading to lower yields. Herein, we report the Adler-Becker reaction in a continuous flow for the transformation of reduced salicylaldehydes into spiroepoxydienones in a one-pot approach. For that, a heterogeneous oxidant based on periodate is developed, leading to an efficient continuous flow process, with higher productivity and shorter reaction times, when compared with batch conditions.

Catalyst-controlled cycloisomerization/[4+3] cycloaddition sequence to construct 2,3-furan-fused dihydroazepines and 2,3-pyrrole-fused dihydrooxepines

A novel catalyst-controlled cycloisomerization/[4+3]cycloaddition sequence of readily available acyclic enyne-amides and crotonate-derived sulfur ylides is reported. This strategy enables the rapid and efficient construction of a series of bicyclic 2,3-furan-fused...

B(C6F5)3-catalyzed O-H insertion reactions of diazoalkanes with phosphinic acids

A highly efficient base-, metal-, and oxidant-free catalytic O-H insertion reaction of diazoalkanes and phosphinic acids in the presence of B(C6F5)3 has been developed. This powerful methodology provides a green approach towards the synthesis of a broad spectrum of α-phosphoryloxy carbonyl compounds with good to excellent yields (up to 99% yield). The protocol features the advantages of operational simplicity, high atom economy, practicality, easy scalability and environmental friendliness.

Chemo- and Enantioselective Insertion of Furyl Carbene into the N-H Bond of 2-Pyridones

Asymmetric carbene insertion reactions represent one of the most important protocols to construct carbon-heteroatom bonds. The use of donor-acceptor diazo compounds bearing an ester group is however a prerequisite for achieving high enantioselectivity. Herein, we report a chemo- and enantioselective formal N-H insertion of 2-pyridones that has been accomplished for the first time with enynones as the donor-donor carbene precursors. DFT calculations indicate an unprecedented enantioselective 1,4-proton transfer from O to C. The rhodium catalyst provides a chiral pocket in which the steric repulsion and the π-π interaction of the propeller ligand play a critical role in determining the selectivities.

Brønsted acid-catalyzed homogeneous O–H and S–H insertion reactions under metal- and ligand-free conditions

The economical and accessible CF₃SO₃H successfully catalyzed homogeneous O–H and S–H bond insertion reactions between hydroxyl compounds, thiols and diazo compounds under metal- and ligand-free conditions.

Highly Enantioselective O-H Bond Insertion Reaction of α-Alkyl- and α-Alkenyl-α-diazoacetates with Water

Catalytic asymmetric reactions in which water is a substrate are rare. Enantioselective transition-metal-catalyzed insertion of carbenes into the O-H bond of water can be used to incorporate water into the stereogenic center, but the reported chiral catalysts give good results only when α-aryl-α-diazoesters are used as the carbene precursors. Herein we report the first highly enantioselective O-H bond insertion reactions between water and α-alkyl- and α-alkenyl-α-diazoesters as carbene precursors, with catalysis by a combination of achiral dirhodium complexes and chiral phosphoric acids or chiral phosphoramides. Participation of the phosphoric acids or phosphoramides in the carbene transfer reaction markedly suppressed competing side reactions, such as β-H migration, carbene dimerization, and olefin isomerization, and thus ensured good yields of the desired products. Fine-tuning of the ester moiety facilitated enantiocontrol of the proton transfer reactions of the enol intermediates and resulted in excellent enantioselectivity. This protocol represents an efficient new method for preparation of multifunctionalized chiral α-alkyl and α-alkenyl hydroxyl esters, which readily undergo various transformations and can thus be used for the synthesis of bioactive compounds. Mechanistic studies revealed that the phosphoric acids and phosphoramides promoted highly enantioselective [1,2]- and [1,3]-proton transfer reactions of the enol intermediates. Maximization of molecular orbital overlap in the transition states of the proton transfer reactions was the original driving force to involve the proton shuttle catalysts in this process.

Mild and efficient synthesis of trans-3-aryl-2-nitro-2,3-dihydrobenzofurans on water

A protocol to prepare trans-3-aryl-2-nitro-2,3-dihydrobenzofurans has been developed on water. The protocol avoids the use of toxic solvents, tedious work-up procedures, and chromatographic separation.

A gold(i)-catalysed chemoselective three-component reaction between phenols, α-diazocarbonyl compounds and allenamides

A gold(i)-catalysed highly chemoselective three-component reaction of phenols, α-diazocarbonyl compounds and allenamides is presented. This transformation features mild reaction conditions, high functional group tolerance, and broad applicability.

Copper-catalysed ortho-selective C–H bond functionalization of phenols and naphthols with α-aryl-α-diazoesters

An unprecedented CuCl₂-catalysed chemo- and ortho-selective C–H bond functionalization of phenols and naphthols with diazoesters has been developed.

Synthesis of enantiomerically pure 2-aryloxy carboxylic acids and their derivatives

Recent publications on the key preparation methods of the enantiomers of 2-aryloxy carboxylic acids are summarized and comparative analysis of the methods is given. The information is arranged according to the type of the starting compound, being classified into syntheses from enantiomerically pure chiral precursors and syntheses from prochiral precursors, which imply generation of an asymmetric centre in the substrate molecule. Data on the chemical resolution of racemic mixtures of the title compounds are addressed in a separate Section. Attention is focused on the preparation of practically valuable 2-aryloxy acids. Examples of biologically active derivatives of 2-aryloxy carboxylic acids are given.

The bibliography includes 121 references.

Silver-Catalyzed Asymmetric Insertion into Phenolic O-H Bonds using Aryl Diazoacetates and Theoretical Mechanistic Studies

An enantioselective insertion reaction of silver carbenes generated from donor-acceptor-substituted diazo compounds into the O-H bond of phenols was developed. A homobinuclear silver complex with a chiral phosphorous ligand was created in situ from AgNTf₂ and (S)-XylylBINAP (in a 2:1 mole ratio). Detailed mechanistic studies using combined experimental and computational techniques revealed that one silver atom center of the catalyst forms a silver carbene and another one works as a Lewis acid for the nucleophilic addition of a phenol. Two counter-anions, two water molecules, and two silver atoms cooperatively mediate the subsequent protonation event to lower the activation energy and control enantioselectivity, affording an array of valuable α-aryl-α-aryloxy esters.

Chiral proton-transfer shuttle catalysts for carbene insertion reactions

The development of chiral proton-transfer shuttles provides a totally new enantiocontrol strategy for transition metal-catalyzed asymmetric carbene insertion reactions.

Phosphine-mediated enantioselective [4 + 1] annulations between ortho-quinone methides and Morita–Baylis–Hillman carbonates

First example of phosphine catalyzed enantioselective [4 + 1] annulations of o-QMs and MBH carbonates for chiral 2,3-dihydrobenzofurans.

Phenolate-induced intramolecular ring-opening cyclization of N-tosylaziridines: access to functionalized benzoxacycles

Phenolate-induced, diastereo- and regioselective intramolecular exo-tet ring-opening cyclization of N-tosylaziridines has been achieved for the first time.

Metal-Free Cycloetherification by in Situ Generated P-Stereogenic α-Diazanium Intermediates: A Convergent Synthesis of Enantiomerically Pure Dihydrobenzooxaphospholes

A metal-free tandem reaction, initiated by the generation of a diazonium cation and followed by cycloetherification, was developed. Acid-promoted de-tert-butylation of N-nitroso N-tert-butylamine was used to generate a diazonium cation in situ, demonstrating a new application of nitroso chemistry. This reaction was employed in the synthesis of substituted benzofuran-3(2H)-ones and dihydrobenzo[d][1,3]oxaphosphole 3-oxides.

Gold-catalyzed highly regio- and enantioselective vinylcarbene insertion into O–H bonds of 2-pyridones

The first gold-catalyzed enantioselective carbene insertion into O–H bonds has been achieved.

Enantioselective synthesis of α-alkenyl α-amino acids via N-H insertion reactions

A new highly enantioselective route to α-alkenyl α-amino acid derivatives, which are important naturally occurring compounds with attractive bioactivity and synthetic utility, was developed using a N-H insertion reaction of vinyldiazoacetates and tert-butyl carbamate cooperatively catalyzed by achiral dirhodium(ii) carboxylates and chiral spiro phosphoric acids under mild, neutral conditions. This reaction has a broad substrate scope, a fast reaction rate (turnover frequency > 6000 h^-1), a high yield (61-99%), and excellent enantioselectivity (83-98% ee). The chiral spiro phosphoric acid, which is proposed to realize the enantioselectivity of the insertion reaction by promoting the proton transfer of a ylide intermediate by acting as a chiral proton shuttle catalyst, can suppress several usual side reactions of vinyldiazoacetates and broaden the applications of these versatile carbene precursors in organic synthesis. To our knowledge, it is the first highly enantioselective carbene insertion reaction of vinyldiazoacetates with heteroatom-hydrogen bonds in which the heteroatom has lone-pair electrons.

Synthesis of Oxygen Heterocycles via Aromatic C-O Bond Formation Using Arynes

Most of the synthetic approaches to the benzo-fused heterocycles containing an oxygen atom have involved the use of phenol derivatives as a starting material. This review highlights the new synthetic approaches involving the aromatic C-O bond-forming process using arynes. The insertion of arynes into the C=O bond gives the unstable intermediates, [2 + 2] cycloaddition-type adducts, which can be easily converted into a variety of oxygen atom-containing heterocycles in a single operation. In this review, the syntheses of oxygen heterocycles, such as coumarin, chromene, xanthene, dihydrobenzofuran and benzofuran derivatives, via the insertion of arynes into the C=O bond of aldehydes or formamides are summarized.

Formal [4 + 1] cycloaddition of o-quinone methides: facile synthesis of dihydrobenzofurans

An efficient and straightforward method for the rapid synthesis of 2-substituted dihydrobenzofurans has been developed via reaction of sulfur ylides with o-quinone methides (o-QMs).

Pd-catalyzed enantioselective intramolecular α-arylation of α-substituted cyclic ketones: facile synthesis of functionalized chiral spirobicycles

Synthesis of chiral spirocyclic ketones was accomplishedviathe Pd-catalyzed intramolecular α-arylation of α-substituted cyclic ketones. The obtained spirocyclic ketone could be converted into an acid–base organocatalyst.

Synthesis of planar chiral [2.2]paracyclophane-based bisoxazoline ligands bearing no central chirality and application to Cu-catalyzed asymmetric O–H insertion reaction

C ₂-symmetric planar chiral [2.2]paracyclophane-based bisoxazoline ligands effectively control the asymmetric induction during the Cu-catalyzed O–H insertion reaction of α-diazo esters.

Efficient catalytic enantioselective Nazarov cyclizations of divinyl ketoesters

An efficient catalytic enantioselective Nazarov cyclization of divinyl ketoesters is developed using a BOX/Cu(ii) catalyst, affording various optically active cyclopent-2-enone esters.