Recent renewed interest in the classical Kukhtin-Ramirez adducts

Efficient Synthesis of Cyanohydrin Esters: P(NMe2)3 Mediated Direct Deoxygenation of Acyl Cyanides with Carboxylic Acids

An efficient synthesis of cyanohydrin esters via a P(NMe2)3 mediated direct deoxygenation process has been exploited, circumventing the release or transformation of the CN─ anion during the reaction. This approach possesses a broad scope and acts as a powerful supplement for the construction of diverse cyanohydrin esters. It offers advantages such as mild conditions, straightforward operations, and excellent scalability, affirming the feasibility and versatility of this approach and highlighting its potential in practical synthesis.

P(NMe2)3-Mediated Regioselective N-Alkylation of 2-Pyridones via Direct Deoxygenation of α-Keto Esters

A practical and regioselective direct N-alkylation of 2-pyridones is enabled by use of α-keto esters in the P(NMe2)3-mediated deoxygenation process. The reaction proceeds under mild conditions to produce N-alkylated 2-pyridones with high selectivity and generality, and the protocol is shown to be applicable for the scale-up synthesis, which makes it promising for practical applications.

Diastereoselective synthesis of trans-2,3-dihydroindoles via formal [4 + 1] annulation reactions of a sulfonium ylide

We have established an in situ generated sulfonium-ylide mediated annulation to construct 2,3-disubstituted-2,3-dihydroindoles. The [4 + 1] annulation approach relied on Michael addition/substitution reactions. These reactions were carried out at ambient temperature to deliver dihydroindoles with excellent yields and diastereoselectivities. Moreover, the versatility of this approach allows for the introduction of various functional groups, enabling further diversification of the dihydroindoles. Also, the cascade approach was broadened to synthesize dihydrobenzofurans.

P(III)-Mediated Formal Reductive N-H Bond Insertion Reaction of Hydrazones to α-Keto Esters

A diazo-, metal-, and base-free multi-substituted hydrazone synthesis via a formal reductive N-H bond insertion reactions of hydrazones to α-keto esters has been developed. The protocol features a broad substrate scope and good functional group tolerance, providing N-H bond insertion products in moderate to excellent yields. Moreover, P(III)-mediated N-H functionalization of pharmaceutical containing hydrazone moiety was also successfully achieved.

Relayed Heteroatom Group Transfer: A Structural Reorganization between Bisthioester and Triaminophosphine to α,α-Disulfenylamide

Simultaneous multiple displacements of organic molecules can lead to a large structural reconstruction with increased complexity that would be difficult to access otherwise. Whereas double displacement such as olefin metathesis is well-established, higher-order versions remain much more challenging, because of their intrinsic thermodynamic disadvantages. Here, we describe a newly discovered relayed heteroatom group transfer process between bisthioesters and triaminophosphines as an unusual example of a formal triple displacement. Through the oxygen/nitrogen exchange between the two simple starting materials, in addition to the 1,2-sulfur migration of a putative carbene intermediate, an organized relocation of the O/S/N groups proceeded to give a variety of α,α-disulfenylamides with excellent efficiency under ambient conditions. The experimental and computational mechanistic studies revealed the sequence of the relayed group shifts via an α,α-disulfenyl phosphonium enolate intermediate as well as the dual role of triaminophosphine as both an oxygen acceptor and a nitrogen donor.

Synthesis of oxime ethers via a formal reductive O-H bond insertion of oximes to α-keto esters

This study describes an efficient approach to access oxime ethers via P(III)-mediated O-H bond insertion reaction of oximes with α-keto esters. The strategy involves the protonation of in situ generated Kukhtin-Ramirez adducts, followed by SN2-type reaction. Important features include a good functional group tolerance, operational simplicity, and application to gram scale synthesis and the synthesis of an acaricide.

Access to Multifunctionalized Tetrasubstituted Carbon Centers Bearing up to Three Different Heteroatoms via Tandem Geminal Chlorofluorination of 1,2-Dicarbonyl Compounds

The incorporation of noncarbon heteroatoms into organic molecules typically instills characteristic and often valuable functionalities. The copresence of different heteroatoms can further broaden their utility through the synergistic cooperative effects, which may even lead to the discovery of formerly unavailable properties that are not just a simple accumulation of each function. However, despite increasing interest in the controllable installation of heteroatoms, it has been extremely challenging to construct carbon centers having three different heteroatoms in a synthetically useful manner. In this work, our group's tandem geminal chlorofluorination (Cl, F) strategy was applied to rationally designed heteroatom-bearing 1,2-dicarbonyl substrates, including α-keto thioesters (S), α-keto N-acylindoles (N), and α-keto acylsilane (Si), which resulted in the practical production of doubly or triply heterofunctionalized tetrasubstituted carbon centers with excellent site-selectivity.

Recent Advances in Palladium-Catalyzed [4 + n] Cycloaddition of Lactones, Benzoxazinanones, Allylic Carbonates, and Vinyloxetanes

Palladium-catalyzed allylation cyclization reaction has recently emerged as an efficient and powerful synthetic platform for the construction of diverse and valuable carbo- and heterocycles. Thus the development of new allylic motifs for achieving this type of transformations in high reactivity and selectivity is of great importance. Generally, these substrates have been utilized as 1,3-, 1,4-, 1,5-, 1,6-dipoles in many reactions, which are applied to prepare highly functionalized products with complete control of chemo-, regio-, diastereo-, and enantioselectivity. In this review, we focus our attention on the development of palladium-catalyzed [4 + n] cycloaddition of allylic motifs and describe a comprehensive and impressive advances in this area. Meanwhile, the related mechanism and the application of these annulation strategies in natural product total synthesis will be highlighted in detail.

Sequential In Situ-Formed Kukhtin-Ramirez Adduct and P(NMe2)3-Catalyzed O-Phosphination of α-Dicarbonyls with P(O)-H

O-Phosphination of α-dicarbonyls via sequential in situ formation of a Kukhtin-Ramirez adduct and a P(NMe2)3-catalyzed process has been exploited for the synthesis of α-phosphoryloxy carbonyls. A range of P(O)-H derivatives, including diarylphosphine oxides, arylphosphinates, and phosphinates, are competent candidates to be introduced into the α-dicarbonyls in this transformation, and various α-phosphoryloxy carbonyls are obtained. This approach possesses advantages of mild conditions, simple operations, atom economy, high efficiency, and gram-scale synthesis, which make it promising in the synthesis toolbox.

An Intramolecular Umpolung Cascade Kukhtin-Ramirez Reaction/Michael Addition-Initiated Cyclization: Stereoselective Synthesis of Tetrasubstituted Cyclopropane Fused 1-Indanones

Herein, we disclose a fascinating highly stereoselective P(NMe₂)₃ mediated intramolecular deoxygenative umpolung cascade Michael addition-initiated cyclopropanation with a diverse substrate adaptability. This methodology creates a new horizon for expedient access to valuable 6,5,3-fused scaffolds having an all-carbon quaternary stereocenter via Kukhtin-Ramirez (K-R) adduct formation, with excellent diastereoselectivity and yields under metal-free ambient conditions. A few functional group transformations have also been performed successfully. Additionally, an asymmetric catalytic attempt using (R)-(+)-H8-BINOL has delivered good enantioselectivity.

Modular Synthesis of α,α-Diaryl α-Amino Esters via Bi(V)-Mediated Arylation/SN2-Displacement of Kukhtin–Ramirez Intermediates

We report a concise and modular approach to α,α-diaryl α-amino esters from readily available α-keto esters. This mild, one-pot protocol proceeds via ketone umpolung, with in situ formation of a Kukhtin–Ramirez intermediate preceding sequential electrophilic arylation by Bi(V) and S_N2 displacement by an amine. The methodology is compatible with a wide range of anilines and primary amines - including derivatives of drugs and proteinogenic amino acids - Bi(V) arylating agents, and α-keto ester substrates.

Modular Regiodivergent Synthesis of Benzo-Fused Isocoumarins by a Cyclopropane Aromatization Strategy

Reported herein is an electrophile-modulated aromatization reaction of highly functionalized cyclopropanes to structurally diverse benzoisocoumarins featuring concurrent formation of the benzenoid and α-pyrone rings under mild conditions. An aromatization reaction of the proposed benzonorcaradiene intermediates prepared independently revealed a crucial role of the neighboring olefinic substituents in determining whether the cyclopropane ring expansion is followed by a 1,2-shift of the ester group.

α-Fluoroamine synthesis via P(III)-mediated deoxygenative geminal fluorosulfonimidation of 1,2-diketones

A deoxygenative geminal fluorosulfonimidation of 1,2-diketones was achieved for the synthesis of tetrasubstituted α-fluoroamines under mild conditions. In this study, a transition metal-free formal N-F insertion of N-fluorobenzenesulfonimide was enabled via the Kukhtin-Ramirez reaction employing a dealkylation-resistant P(III) reagent developed in our laboratory. Computational analysis was also performed to obtain a general mechanistic picture, which explained the reactivity and selectivity for this type of reaction.

Recent advances in the application of ylide-like species in [4+1]-annulation reactions: an update review

In this review, advances in [4+1]‐annulation reactions involving sulfonium, sulfoxonium and ammonium ylides, as well as diazo compounds and carbenes are summarized over the last 6 years. Newly emerged methods...

Characteristics of a novel photoinitiator aceanthrenequinone-initiated polymerization and cytocompatibility of its triggered polymer

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AATQ is a novel photosensitizer with high-photoinitiating conversion efficiency at a relatively low concentration under 455 nm-blue light.

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Cytotoxicity of AATQ to different tissue types of cells is much lower than widely used-BAPO.

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Cytocompatibility of AATQ-initiated polymer is significantly superior to PANQ, but inferior to CQ.

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AATQ offers an alternative in industrial or biomedical areas, especially in the required low concentration of photoinitiators.

A number of photoinitiators are available in chemical industry, but less of them in biomedicine or clinical therapy due to the limitation of their cytotoxicity and biocompatibility. Thus, it is urgently necessary to find non-toxic or low-toxic photoinitiators to meet clinical demands. Aceanthrenequinone (AATQ) is a novel photosensitizer with high-photoinitiating ability, but no reports contribute, to date, to its cytotoxicity and biocompatibility. Here, primary cells and various cell lines were exposed to different concentrations of AATQ with or without irradiation. AATQ had the similar photoinitiating conversion efficiency to the extensively used bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (BAPO) and higher one than 9,10-phenanthrenequinone (PANQ) with the similar extent of polymerization in depth within a certain range, but displayed much lower cytotoxicity than BAPO under non-irradiation or irradiation. The biocompatibility of BisGMA/TEGDMA polymer prepared by AATQ was superior to that of PANQ, but inferior to that of camphorquinone (CQ) although the far lower dose of AATQ is enough to initiate polymerization of monomer than that of CQ. Hence, AATQ offers a valuable alternative in applications of industrial or biomedical areas.

The formation of cage phosphoranes and their rearrangements in the reactions of substituted 2-(3-oxo-3-phenyl)ethoxybenzo[d]-1,3,2-dioxaphospholes with perfluorodiacetyl

The Kukhtin-Ramirez reaction of 2-(3-oxo-3-phenyl)ethoxy-benzo[d]-1,3,2-dioxaphospholes with perfluorodiacetyl was monitored by NMR methods. To our surprise the initial stage involved a kinetically controlled [4+4]-cycloaddition with the formation of a cage phosphorane containing a 2',5',8',9'-tetraoxa-2λ⁵-phosphaspiro[benzo[d][1,3,2]dioxaphosphole-2,1'-bicyclo[4.2.1]nonan]-3'-ene (compound 5) scaffold. Intermediate 5 then converts to spirophosphorane-4',5'-bis(trifluoromethyl)-2λ⁵-spiro[benzo[d] [1,3,2]dioxaphosphole-2-yl-2,2'-[1,3,2] dioxaphosphole (compound 4). Compound 4 further rearranges into a cage phosphorane derivative containing a [2,5]epoxybenzo[d][1,3,6,2]trioxaphosphocine] (compound 3) backbone.

Oxyphosphonium Enolate Equilibria in a (4+1)-Cycloaddition Approach toward Quaternary C3-Spirooxindole Assembly

An efficient and convergent (4+1)-cycloaddition strategy toward the construction of spirooxindole benzofurans that involves the intermediacy of an isatin-derived oxyphosphonium enolate is presented. Mechanistic investigations employing in situ NMR analysis of the reaction mixture revealed a correlation between phosphonium enolate structure and product distribution that was heavily influenced by the solvent and reaction temperature.

Molybdenum-Catalyzed Deoxygenative Cyclopropanation of 1,2-Dicarbonyl or Monocarbonyl Compounds

The transition-metal-catalyzed cyclopropanation of alkenes by the decomposition of diazo compounds is a powerful and straightforward strategy to produce cyclopropanes, but is tempered by the potentially explosive nature of diazo substrates. Herein we report the Mo-catalyzed regiospecific deoxygenative cyclopropanation of readily available and bench-stable 1,2-dicarbonyl compounds, in which one of the two carbonyl groups acts as a carbene equivalent upon deoxygenation and engages in the subsequent cyclopropanation process. The use of a commercially available Mo catalyst afforded an array of valuable cyclopropanes with exclusive regioselectivity in up to 90 % yield. The synthetic utility of this method was further demonstrated by gram-scale syntheses, late-stage functionalization, and the cyclopropanation of a simple monocarbonyl compound. Preliminary mechanistic studies suggest that phosphine (or silane) acts as both a mild reductant and a good oxygen acceptor that efficiently regenerates the catalytically active Mo catalyst through reduction of the Mo-oxo complexes.

Reactions on a 1,2-Dicarbonyl Moiety of a Fullerene Skeleton

A 1,2-dicarbonyl moiety on a cage-opened fullerene skeleton is one of suitable building blocks for the further derivatization. Herein, we discuss the chemical transformation of a 1,2-dicarbonyl compound into β-oxo-phosphorus ylide, acid anhydride, and α-methylene carbonyl derivatives. Despite possessing a sterically small methylene unit in the last one, the release of an encapsulated water molecule was significantly supressed whereas the β-oxo-phosphorus ylide bearing three bulky p-tolyl groups on the P-atom enabled the faster insertion/release dynamics, implying the flexibility of the phosphonium substituent. The replacement of the carbonyl group with phosphorus ylide and methylene units largely varied electrochemical properties of the fullerene skeleton, likely arising from the anionic charge delocalized over the entire molecule and removal of an electron-withdrawable carbonyl group, respectively.

Nucleophilic Migratory Cyclopropenation of Activated Alkynes: A Nonmetal Approach to Unbiased Cyclopropenes

An unprecedented reductive [2 + 1] annulation of α-keto esters with alkynones mediated by P(NMe₂)₃ is described. Although this nonmetal cyclopropenation is a nucleophilic process, attributed to the ester migration via a formal [2 + 2] cycloaddition reaction of Kukhtin-Ramirez adducts and alkynones followed by a fragmentation, cyclopropenes with an unbiased alkene scaffold are formed in good to excellent yields, thus providing a promising complementarity to electrophilic metal-catalyzed cyclopropenation.

Nonclassical Abramov Products Formed on Orifices of Cage-Opened C60 Derivatives

By nucleophilic addition of phosphite P(OMe)₃ to a cage-opened C₆₀ derivative, α-hydrophosphate and enol phosphate were obtained as kinetic and thermodynamic products, respectively. Different from classical Abramov products bearing a phosphorus-carbon bond, these products have a phosphorus-oxygen bond. The observed anomaly originates from the fully conjugated π system, which significantly stabilizes zwitterionic intermediates bearing a phosphorus-oxygen bond. The thus formed enol phosphate was found to exhibit an intense absorption band that extended to 730 nm, reflecting the intramolecular charge-transfer transitions. We also report domino phosphorylation reactions, which gave a cage-opened C₆₀ derivative bearing a direct P-C bond.

Organophosphorus zwitterions engaged in a conjugated macrocycle on fullerene

Organophosphorus zwitterions are one of the most important but elusive intermediates for carbon-carbon bond formation in synthetic chemistry and biology. However, a lack of isolated examples due to their lability has hampered in-depth understanding of structures and their reaction mechanisms. In this study, we crystallographically reveal the solid-state structure of a phosha-Michael adduct engaged in a cage-opened C₆₀ skeleton, which is formed as a kinetic product. This compound exhibits dark brown colour in solution with an intense absorption band that extends to 1000 nm, reflecting intramolecular charge transfer transitions. From the 1,2-dicarbonyl moiety on the conjugated orifice, β-oxo-phosphorus ylide is formed as a thermodynamic product. The reaction mechanism that has long been disputed is examined by experimental and theoretical studies, showing a pathway which includes an S_N2 reaction as a key step instead of the hitherto considered carbene pathway.

Deoxygenative α-alkylation and α-arylation of 1,2-dicarbonyls

Construction of C-C bonds at the α-carbon is a challenging but synthetically indispensable approach to α-branched carbonyl motifs that are widely represented among drugs, natural products, and synthetic intermediates. Here, we describe a simple approach to generation of boron enolates in the absence of strong bases that allows for introduction of both α-alkyl and α-aryl groups in a reaction of readily accessible 1,2-dicarbonyls and organoboranes. Obviation of unselective, strongly basic and nucleophilic reagents permits carrying out the reaction in the presence of electrophiles that intercept the intermediate boron enolates, resulting in two new α-C-C bonds in a tricomponent process.

PIII-Mediated intramolecular cyclopropanation and metal-free synthesis of cyclopropane-fused heterocycles

The carbene-like reactivity of the Kukhtin–Ramirez adduct enables the first reductive intramolecular cyclopropanation, which provides easy access to highly functionalized cyclopropane-fused heterocycles.