The Wolff Rearrangement of ?-Diazo Carbonyl Compounds

Visible-Light-Driven Synthesis of Dihydroaurones from Aromatic Diazo Compounds

Dihydroaurones, which are derivatives of aurones, exhibit similar biological activity. Although there are many synthetic methods for dihydroaurones, ecofriendly methodologies that circumvent the use of precious metals still need to be explored. In this work, a catalyst-free, visible-light-driven synthesis of dihydroaurones has been developed through the cyclization of aromatic diazo compounds. The reaction proceeded smoothly under mild conditions, resulting in a series of dihydroaurones in moderate to high yields. Mechanistic investigation suggests that this process involves a radical-pair Stevens rearrangement.

Phototriggered Generation of Ynol Ethers and Their Rearrangement to Ketenes

This study developed reactions for the phototriggered generation of reactive ynol ethers using alkoxycyclopropenones. The resulting ynol ethers underwent rearrangement to ketenes, which subsequently participated in cycloaddition with alkynes and the acylation of amines. The alkoxy groups in the ynol ethers significantly influenced on the reactivity toward their rearrangement to ketenes. Reasonable transition state structures for the rearrangement reaction were found through density functional theory calculations, and the reactivity of the ynol ethers could be predicted by these calculations.

Stereoselective Synthesis of Polysubstituted Conjugated Dienes Enabled by Photo-Driven Sequential Sigmatropic Rearrangement

We here reported a highly stereoselective method for the synthesis of polysubstituted conjugated dienes from α-aryl α-diazo alkynyl ketones and pyrazole-substituted unsymmetric aminals under mild conditions, which was promoted by photo-irridation and involved with 1,6-dipolar intermediate and quadruple sigmatropic rearrangements, was successfully developed. In this transformation, the cleavage of four bonds and the recombination of five bonds were implemented in one operational step. This protocol provided a modular tool for constructing dienes from amines, pyrazoles and α-alkynyl-α-diazoketones in one-pot manner. The results of mechanistic investigation indicated that the plausible reaction path underwent the 1,6-sigmatropic rearrangement instead of the 1,5-sigmatropic rearrangement.

Ring Contraction by Rearrangement of Sterically Congested Cyclic (Amino)(aryl)carbenes

The rearrangement of sterically congested cyclic (amino)(aryl)carbenes (CAArCs) by the reaction of related iminium salts with potassium bis(trimethylsilyl)amide is reported, allowing for forming benzocyclobutanimines via a ring contraction process. Mechanistic studies by theoretical calculations indicate that the formation of conjugated ketenimines as intermediates could be considered, in which steric hindrance caused by N-alkyl motifs of CAArCs plays an important role in promoting the ring-opening by the cleavage of C-N bond.

Gold-Catalyzed Oxidation Reactions of Thioalkynes with Quinoline N-Oxides: A DFT Study

Mechanistic investigation of the gold-catalyzed oxidative reactions of thioalkynes with quinoline N-oxides was performed using density functional theory (DFT) calculations. For the oxidative rearrangement of thioalkynes with quinoline N-oxide to yield the same products, the Cβ-oxidation of thioalkynes was predicted to be competitive with Cα-oxidation, with the Cβ-oxidative process slightly more favorable. However, for the oxidative alkenylation of propargyl aryl thioethers with quinoline N-oxides, the Cβ-oxidation of thioether by quinoline N-oxide generated the product 3-hydroxy-1-alkylidene phenylthiopropan-2-one. Moreover, the ring opening of the four-membered sulfonium intermediate was achieved by the nucleophilic attack of quinoline N-oxide rather than a water molecule.

Synthesis and Characterization of Short α and β-Mixed Peptides with Excellent Anti-Lipase Activities

Obesity is a source of significant pathologies and deadly diseases, including heart disease, diabetes, and cancer. One of the most intriguing strategies in the hunt for new anti-obesity medications is the inhibition of pancreatic lipase (PL). This study presents a novel application of short α and β-mixed peptides as pancreatic lipase inhibitors. These peptides were synthesized in the solution phase and characterized using FTIR and 1H-NMR. L-proline is present in a high percentage of natural anti-lipase peptides and was used as a β-amino acid in this study to enhance anti-lipase activity and proteolytic stability. Moreover, L-α-proline was converted to β-amino acid derivatives using the Arndt-Eistert method with the advantage of stereo control at the α-carbon. The synthesized peptides with anti-lipase activity are N-Boc-β-Pro-Gly-OBz (93%), N-Boc-O-Bz-Tyr-β-Pro-β-Pro-Gly-OBz (92%), N-Boc-O-Bz-Tyr-β-Pro-COOH (91%), N-Boc-Phe-β-Pro-OCH3 (90%), and N-Boc-O-Bz-Tyr-β-Pro-OCH3 (89%). These peptides may function as lead molecules for further modification to more significant molecules, which can help control obesity.

Tetrazole Diversification of Amino Acids and Peptides via Silver-Catalyzed Intermolecular Cycloaddition with Aryldiazonium Salts

Selective structural modification of amino acids and peptides is a central strategy in organic chemistry, chemical biology but also in pharmacology and material science. In this context, the formation of tetrazole rings, known to possess significant therapeutic properties, would expand the chemical space of unnatural amino acids but has received less attention. In this study, we demonstrated that the classic unimolecular Wolff rearrangement of α-amino acid-derived diazoketones could be replaced by a faster intermolecular cycloaddition reaction with aryldiazonium salts under identical practical conditions. This strategy provides an efficient synthetic platform that could transform proteinogenic α-amino acids into a plethora of unprecedented tetrazole-decorated amino acid derivatives with preservation of the stereocenters. Density functional theory studies shed some light on the reaction mechanism and provided information regarding the origins of the chemo- and regioselectivity. Furthermore, this diazo-cycloaddition protocol was applied to construct tetrazole-modified peptidomimetics and drug-like amino acid derivatives.

Opportunities and Challenges of Switchable Materials for Pharmaceutical Use

Switchable polymeric materials, which can respond to triggering signals through changes in their properties, have become a major research focus for parenteral controlled delivery systems. They may enable externally induced drug release or delivery that is adaptive to in vivo stimuli. Despite the promise of new functionalities using switchable materials, several of these concepts may need to face challenges associated with clinical use. Accordingly, this review provides an overview of various types of switchable polymers responsive to different types of stimuli and addresses opportunities and challenges that may arise from their application in biomedicine.

Continuous Flow Generation of Acylketene Intermediates via Nitrogen Extrusion

A flow chemistry process for the generation and use of acylketene precursors through extrusion of nitrogen gas is reported. Key to the development of a suitable continuous protocol is the balance of reaction concentration against pressure in the flow reactor. The resulting process enables access to intercepted acylketene scaffolds using volatile amine nucleophiles and has been demonstrated on the gram scale. Thermal gravimetric analysis was used to guide the temperature set point of the reactor coils for a variety of acyl ketene precursors. The simultaneous generation and reaction of two reactive intermediates (both derived from nitrogen extrusion) is demonstrated.

Bis-Rhodamines Bridged with a Diazoketone Linker: Synthesis, Structure, and Photolysis

Two fluorophores bound with a short photoreactive bridge are fascinating structures and remained unexplored. To investigate the synthesis and photolysis of such dyes, we linked two rhodamine dyes via a diazoketone bridge (−COCN₂−) attached to position 5′ or 6′ of the pendant phenyl rings. For that, the mixture of 5′- or 6′-bromo derivatives of the parent dye was prepared, transformed into 1,2-diarylacetylenes, hydrated to 1,2-diarylethanones, and converted to diazoketones Ar¹COCN₂Ar². The high performance liquid chromatography (HPLC) separation gave four individual regioisomers of Ar¹COCN₂Ar². Photolysis of the model compound—C₆H₅COCN₂C₆H₅—in aqueous acetonitrile at pH 7.3 and under irradiation with 365 nm light provided diphenylacetic acid amide (Wolff rearrangement). However, under the same conditions, Ar¹COCN₂Ar² gave mainly α-diketones Ar¹COCOAr². The migration ability of the very bulky dye residues was low, and the Wolff rearrangement did not occur. We observed only moderate fluorescence increase, which may be explained by the insufficient quenching ability of diazoketone bridge (−COCN₂−) and its transformation into another (weaker) quencher, 1,2-diarylethane-1,2-dione.

Catalyst-free synthesis of oxazol-2(3H)-ones from sulfilimines and diazo compounds through a tandem rearrangement/aziridination/ring-expansion reaction

Oxazol-2(3H)-ones play a significant role in the fields of organic synthesis and drug development.

Regio- and Diastereoselective Dimerization of Diazo Carbonyls: A Cooperative Catalytic Approach to Complex Scaffolds with Four Contiguous Stereocenters

Starting from readily available o-diazoacyl-substituted arene carboxylates, scaffolds with the 5,9-epoxycyclohepta[b]pyran-2(3H)-one core were obtained by cooperative RhII , Lewis and Brønsted acid catalysis. Four new bonds, three functional groups (lactone, ketal, and alcohol) and four contiguous stereocenters are formed during this regio- and diastereoselective process in a single synthetic step. Intensive optimization and mechanistic studies, including the trapping, isolation, and elucidation of reaction intermediates, led to a plausible mechanistic scenario. The reaction is proposed to involve carbonyl ylides but also transient species of the ketocarbene equilibrium that undergo a cascade of cycloaddition and skeletal rearrangements.

Transition-Metal-Free C(sp2 )-C(sp2 ) Cross-Coupling of Diazo Quinones with Catechol Boronic Esters

A transition-metal-free C(sp² )-C(sp² ) bond formation reaction by the cross-coupling of diazo quinones with catechol boronic esters was developed. With this protocol, a variety of biaryls and alkenyl phenols were obtained in good to high yields under mild conditions. The reaction tolerates various functionalities and is applicable to the derivatization of pharmaceuticals and natural products. The synthetic utility of the method was demonstrated by the short synthesis of multi-substituted triphenylenes and three bioactive natural products, honokiol, moracin M, and stemofuran A. Mechanistic studies and density functional theory (DFT) calculations revealed that the reaction involves attack of the boronic ester by a singlet quinone carbene followed by a 1,2-rearrangement through a stepwise mechanism.

Catalyst-free formal [4+1]/[4+2] cyclization cascade sequence of isocyanides with two molecules of acylketene formed in situ from thermal-induced Wolff rearrangement of 2-diazo-1,3-diketones

An expedient and economic approach for constructing O,O,N-spiro compounds consisting of both a 1,3-oxazine and a furan ring through a catalyst-free formal [4+1]/[4+2] cycloaddition cascade sequence of isocyanides with two molecules of acylketene formed in situ through thermal-induced Wolff rearrangement of 2-diazo-1,3-diketones was developed. The reaction displayed good functional group tolerance and was compatible with different isocyanides and 2-diazo-1,3-diketones. Furthermore, preliminary asymmetric attempts of this reaction are made by utilizing optically pure isocyanides as inputs, and moderate diastereomeric induction was observed.

Stereoselective Synthesis of Diverse Lactones through a Cascade Reaction of Rhodium Carbenoids with Ketoacids

A convergent cascade approach for the stereoselective synthesis of diverse lactones is described. The Rh₂(TFA)₄-catalyzed cascade reaction proceeds via a carboxylic acid O-H insertion/aldol cyclization with high chemo-, regio-, and diastereoselectivity. The cascade reaction provides quick access to highly functionalized γ-butyro- and δ-valerolactones from readily accessible ketoacid and diazo synthons. To demonstrate the utility of this approach, a thermally induced oxy-Cope ring-expansion strategy has been incorporated in the cascade sequence to access medium-sized lactones, which can undergo a serendipitous rearrangement to form spiro-lactones through an intramolecular aldol/trans-lactonization sequence. The reaction has proven to be general, with a range of ketoacids and diazo carbonyls to provide functionalized lactones of varying ring sizes.

An excited-state Wolff rearrangement reaction of 5-diazo Meldrum's acid: an ab initio on-the-fly nonadiabatic dynamics simulation

A global switching on-the-fly trajectory surface hopping dynamics simulation at the 3SA-CASSCF(12,11)/6-31G* quantum level has been employed to probe the photo-induced Wolff rearrangement (WR) reaction of 5-diazo Meldrum's acid (DMA) within three low-lying electronic excited states. The present simulation predicted that the branching ratios for relaxing back to the ground state, isomerizing to diazirine, and reaction to ketene I via carbene I are 69% ± 0.1, 3% ± 0.4, and 28% ± 0.1, which are in excellent agreement with those obtained by the femtosecond spectroscopy experiment, 67%, 3% and 30%, respectively. In particular, the present simulation revealed that the major WR reaction to ketene I pathway is stepwise via the excited-state to carbene I (17.8% ± 0.2) and via the ground-state to carbene I (8.7% ± 0.2), and the minor pathway is concerted synchronous (1.5% ± 0.6). The photo-induced WR reaction of DMA has been quantitatively interpreted in terms of the distribution of extended seam surfaces as a function of CN dissociation bonds for two important conical intersections within three low-lying electronic excited states. Ultrafast dynamic time constants have been estimated to be about 500 fs ± 120 fs and 180 fs ± 80 fs for the stepwise and the concerted WR reaction to ketene I which are also in good agreement with those determined by the experiment. Therefore, the photo-induced excited-state WR reaction mechanism has been quantitatively revealed by the present real-time dynamics simulation.

Chiral β3-isocyanopropionates for multicomponent synthesis of peptides and depsipeptides containing a β-amino acid fragment

An efficient three-step synthesis of a novel family of enantiomerically pure isocyanides derived from β3-isocyanopropionic acids was elaborated. Easily available N-formylated α-amino acids were used as starting materials towards this aim. The 3-step sequence (Arndt-Eistert reaction-Wolff rearrangement-dehydration) resulted in target isonitriles in good yields (up to 97%). As a result a new family of isocyanides bearing a fragment of β3-amino acids with different functional groups (amides, esters and short peptides) was obtained. It was demonstrated that these new isonitriles can be used in the Ugi and Passerini reactions to prepare short peptides and depsipeptides having a β-amino acid fragment incorporated.

Light-Induced Reactions of Diazotetrahydrofuranone without Elimination of Nitrogen: Experimental and Mechanistic Study

The energies and lifetimes of the excited states (S₁, S₂, S₅, T₁) of a diazotetrahydrofuranone were determined using experimental and computational methods. It was shown that direction of the diazoketone photochemical transformations without elimination of nitrogen is determined by multiplicity and energy of the excited state, generated by UV irradiation of diazo compound: isomerization to α-ketodiazirine proceeds from the singlet S₁ state, whereas the alternative process of C-H insertion with hydrazone formation occurs through the triplet T₁ state. The most probable excited state that leads to elimination of nitrogen and Wolff rearrangement is one of the highest singlet excited states of diazotetrahydrofuranone.

Ring contraction of metallabenzooxirene to metal carbonyl complexes - a comparative study with the Wolff rearrangement of oxirene and benzooxirene

A detailed quantum mechanical study on the role of transition metal fragments towards the epoxidation reaction of metallabenzynes (1M, M = Fe, Ru, Os) to give metallabenzyne epoxide or metallabenzooxirene (2M), followed by the Wolff type 1,2-rearrangement to give metal carbonyls (4M), has been carried out at the M06/def2-TZVPP//BP86/def2-SVP level of theory. The epoxide or oxirene product (2A) of linear alkynes like acetylene is unstable and converts to a ketene (4A) by highly exothermic (-78.7 kcal mol^-1) Wolff rearrangement via an oxocarbene (3A) intermediate. The epoxide product of cyclic alkynes like benzyne (2C) is comparatively more stable than benzooxocarbene (3C), yet it undergoes rearrangement to cyclopentadienylketene (4C) through Wolff type ring contraction reactions (-65.3 kcal mol^-1). The replacement of one of the carbyne carbons in benzyne by 14 VE metal fragment M(PH₃)₂Cl₂, M = Fe, Ru and Os enhances the stability of the 18 VE metallabenzyne epoxide product (2M, -88.1 kcal mol^-1 for 2Fe, -87.2 kcal mol^-1 for 2Ru and -82.8 kcal mol^-1 for 2Os) as compared to the 16 VE metallacyclohexadienone (3M). The ring contracted 18 VE product of 2M is a carbonyl bridged metallacyclopentadienyl complex (4M, M = Fe, Ru and Os). Even though this interconversion is thermodynamically favourable, it involves a high-energy barrier (19.3, 28.1 and 27.9 kcal mol^-1 for 2Fe, 2Ru and 2Os, respectively). However, the hepta-coordinated (5M) terminal metallacyclopentadienyl carbonyl analogues of ketene (4A/4C) are not minima on the potential energy surface.

Fluoride-Mediated Desulfonylative Intramolecular Cyclization to Fused and Bridged Bicyclic Compounds: A Complex Mechanism

We previously reported the synthesis of polysubstituted chiral oxazepanes in three steps from commercially available starting materials. The unexpected reaction of one of these 1,4-oxazepanes in the presence of TBAF provided a 4-oxa-1-azabicyclo[4.1.0]heptane core. This unusual process significantly increased the complexity of the molecular scaffold by introducing a bicyclic core. Surprisingly, the generated bicyclic structure featuring three stereocenters was a mixture of enantiomers with no other diastereomers observed. These striking experimental observations deserved further investigations. A combination of experimental and computational investigations unveiled a complex diastereoselective mechanism. Mechanistic rationale is presented for this observed rearrangement.

Photo-responsive polymers: synthesis and applications

Photo-responsive polymers are able to change their structure, conformation and properties upon light irradiation.

Synthesis of Sulfonyldiazomethanes and Acetyldiazomethanes via an Alumina-Mediated Decarboxylation Strategy

Diazo compounds are widely adopted in organic synthesis due to their carbine characteristics. One of the most interesting diazo compounds is diazolsulfonyl. Here, in the current study, we found that diazolsufonyl compounds could be prepared with moderate to excellent yield through decarboxylation of diazosulfonyl acetates by neutral alumina. We hope that this mild and simple method might inspire wider application of these useful compounds.

LED lighting as a simple, inexpensive, and sustainable alternative for Wolff rearrangements

The Wolff rearrangement suffers from many drawbacks with respect to its practical execution in the laboratory. Herein, commercial LED lamps are employed as a sustainable alternative for the classic protocols typically used for Wolff rearrangements.

Nitrogen enriched mesoporous organic polymer anchored copper(ii) material: an efficient and reusable catalyst for the synthesis of esters and amides from aromatic systems

New mesoporous polymer anchored copper acetate (Cu-mPMF) has been synthesized and well characterized. The catalytic performance of this complex has been tested for the synthesis of esters and amides.

Stereospecific cis- and trans-ring fusions arising from common intermediates

Highly concise and stereospecific routes to cis and trans fusion, carrying various functionality at one of the bridgehead carbons, have been accomplished.

Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid

We investigate the gas-phase photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond photofragment ion spectroscopy. Transient-absorption studies in solution had revealed an ultrafast intramolecular Wolff rearrangement to a ketene after UV excitation, followed by reactions which also involve the solvent. Due to the absence of solvent molecules in this gas-phase study, we are able to focus purely on the photochemistry of the Wolff rearrangement and subsequent reaction steps. The observation of the time-resolved photofragment ion signals allows us to discriminate the dynamics of ketene and carbene products. By identification of the different possible molecular origins for a certain fragment ion signal, the time scale of the Wolff rearrangement and the lifetime of the ketene product are inferred. We further identified experimental signatures of a second Wolff rearrangement emanating from the carbene product, as had been conjectured indirectly for this molecule from pyrolysis studies.

Batch and flow photochemical benzannulations based on the reaction of ynamides and diazo ketones. Application to the synthesis of polycyclic aromatic and heteroaromatic compounds

Highly substituted polycyclic aromatic and heteroaromatic compounds are produced via a two-stage tandem benzannulation/cyclization strategy. The initial benzannulation step proceeds via a pericyclic cascade mechanism triggered by thermal or photochemical Wolff rearrangement of a diazo ketone. The photochemical process can be performed using a continuous flow reactor which facilitates carrying out reactions on a large scale and minimizes the time required for photolysis. Carbomethoxy ynamides as well as more ketenophilic bis-silyl ynamines and N-sulfonyl and N-phosphoryl ynamides serve as the reaction partner in the benzannulation step. In the second stage of the strategy, RCM generates benzofused nitrogen heterocycles, and various heterocyclization processes furnish highly substituted and polycyclic indoles of types that were not available by using the previous cyclobutenone-based version of the tandem strategy.

A practical method for regiocontrolled one-carbon ring contraction

A practical and efficient method for the perfluorobutanesulfonyl azide-mediated one-carbon ring contraction of cyclic enoxysilanes is described. High-yielding procedures for the elaboration of the resulting N-acyl sulfonamide products are reported.

Photoinduced ultrafast Wolff rearrangement: a non-adiabatic dynamics perspective

One reaction, two routes: full-dimensional non-adiabatic dynamics simulations shed light on the ultrafast photoinduced Wolff rearrangement in an α-diazocarbonyl compound. The trajectories show both concerted asynchronous and stepwise processes leading to the corresponding ketene.