The domestication of ortho-quinone methides

Aryne Chemistry: Generation Methods and Reactions Incorporating Multiple Arynes

Arynes hold significance for the efficient fusion of (hetero) arenes with diverse substrates, advancing the construction of complex molecular frameworks. Employing multiple equivalents of arynes is particularly effective in the rapid formation of polycyclic cores found in optoelectronic materials and bioactive compounds. However, the inherent reactivity of arynes often leads to side reactions, yielding unanticipated products and underlining the importance of a detailed investigation into the use of multiple arynes to fine-tune their reactivity. This review centers on methodologies and syntheses in organic reactions involving multiple arynes, categorizing based on mechanisms like cycloadditions, σ-bond insertions, nucleophilic additions, and ene reactions, and discusses aryne polymerization. The categorization based on these mechanisms includes two primary approaches: the first entails multiple aryne engagement within a single step while the second approach involves using a single equivalent of aryne sequentially across multiple steps, with both requiring strict reactivity control to ensure precise aryne participation in each respective step. Additionally, the review provides an in-depth analysis of the selection of aryne precursors, organized chronologically and by activation strategy, offering a comprehensive background that supports the main theme of multiple aryne utilization. The expectation remains that this comprehensive review will be invaluable in designing advanced syntheses engaging multiple arynes.

Targeted Discovery of a Natural ortho-Quinone Methide Precursor and Green Generation of Its Oligomers

ortho-Quinone methides (o-QMs) are a class of highly reactive intermediates that serve as important nonisolable building blocks (NBBs) in organic synthesis and small-molecule library construction. Because of their instability and nonisolability, most reported o-QMs are generated through in situ chemical synthesis, and only a few natural o-QMs have been reported due to the lack of directed discovery strategies. Herein, a new natural o-QM precursor (trichophenol A, 2) was identified from the fungal strain of Trichoderma sp. AT0167 through genome mining, which was generated by trilA (nonreducing polyketide synthase) and trilB (2-oxoglutarate dependent dioxygenase). Combinatorial biosynthesis via two other known NRPKS genes with trilA and trilB was performed, leading to the generation of five new trichophenol o-QM oligomers (trichophenols D-H, 5-9). The strategy combining genome mining with combinatorial biosynthesis not only targetedly uncovered a new natural o-QM precursor but also produced various new molecules through oligomerization of the new o-QM and its designated o-QM acceptors without chemical synthesis and isolation of intermediates, which was named NBB genome mining-combinatorial biosynthesis strategy for o-QM molecule library construction. This study provides a new strategy for the targeted discovery of natural o-QMs and small-molecule library construction with natural o-QMs.

Trifluoroethanol promoted formal nucleophilic substitution of indol-2-yl diaryl methanol for the synthesis of tetraarylmethanes

The synthesis of tetraarylmethanes has long been a challenge in the field of synthetic chemistry. In this study, a series of tetraarylmethanes were successfully synthesized through the formal nucleophilic substitution reaction of indol-2-yl diaryl methanol catalyzed by Brønsted acid. The key success of this study lies in suppressing the influence of water molecules by forming hydrogen bonds with the TFE solvent. This process leads to the formation of active 2-indole imine methide (2-IIM) intermediates, ensuring the successful synthesis of tetraarylmethanes. Furthermore, some of the products also exhibited potential anticancer activity.

Chiral Brønsted Acid-Catalyzed Asymmetric Reaction via Vinylidene Ortho-Quinone Methides

Vinylidene ortho-quinone methides (VQMs) have been proven to be versatile and crucial intermediates in the catalytic asymmetric reaction in last decade, and thus have drawn considerable concentrations on account of the practical application in the construction of enantiomerically pure functional organic molecules. However, in comparison to the well established chiral Brønsted base-catalyzed asymmetric reaction via VQMs, chiral Brønsted acid-catalyzed reaction is rarely studied and there is no systematic summary to date. In this review, we summarize the recent advances in the chiral Brønsted acid-catalyzed asymmetric reaction via VQMs according to three types of reactions: a) intermolecular asymmetric nucleophilic addition to VQMs; b) intermolecular asymmetric cycloaddition of VQMs; c) intramolecular asymmetric cyclization of VQMs. Finally, we put forward the remained challenges and opportunities for potential breakthroughs in this area.

Different Reaction Mechanisms Triggered by the Meta Effect: Photoinduced Generation of Quinone Methides from Hydroxybiphenyl Derivatives

A series of sterically congested quinone methides (QMs) exhibit photoinduced antiproliferative activity against some human cancer cell lines. To elucidate the structure-reactivity relationship and details of mechanisms of the photogeneration of sterically congested QMs, we chose phenylphenol derivatives 1-3 as QM precursors and investigated their photodehydration processes in aqueous solutions using ultrafast spectroscopy and theoretical computations. We found that meta derivatives 1 and 2 undergo water-mediated excited-state proton transfer (ESPT) from the phenol OH, followed by expulsion of the OH- to form QMs. By comparison, para derivative 3 proceeds via water-mediated ESPT from H2O to benzyl alcohol coupled with dehydration as the first step, delivering a cation intermediate, which further deprotonates to yield QM. Such results would help chemists understand more about the meta effects in photochemistry and about ESPT and would help synthetic chemists design sterically congested QM precursors with extraordinary reactivities and expand applications of QMs in biological and medical systems.

Kinetic resolution of 1-(1-alkynyl)cyclopropyl ketones via gold-catalyzed divergent (4 + 4) cycloadditions: stereoselective access to furan fused eight-membered heterocycles

Chiral eight-membered heterocycles comprise a diverse array of natural products and bioactive compounds, yet accessing them poses significant challenges. Here we report a gold-catalyzed stereoselective (4 + 4) cycloaddition as a reliable and divergent strategy, enabling readily accessible precursors (anthranils and ortho-quinone methides) to be intercepted by in situ generated gold-furyl 1,4-dipoles, delivering previously inaccessible chiral furan/pyrrole-containing eight-membered heterocycles with good results (56 examples, all >20 : 1 dr, up to 99% ee). Moreover, we achieve a remarkably efficient kinetic resolution (KR) process (s factor up to 747). The scale-up synthesis and diversified transformations of cycloadducts highlight the synthetic potential of this protocol. Computational calculations provide an in-depth understanding of the stereoselective cycloaddition process.

Brønsted acid catalyzed [4 + 2] cycloaddition for the synthesis of bisbenzannulated spiroketals with antifungal activities

The intermolecular [4 + 2] cycloaddition of o-hydroxy benzyl alcohols with isochroman ketals was realized by CF3CO2H catalysis. A broad range of bisbenzannulated [6,6]-spiroketals were formed under the metal-free mild conditions in moderate to excellent yields (45-98%) with mostly excellent diastereoselectivities (up to >20 : 1 dr). Furthermore, the enantioselective version was also preliminarily investigated and the bisbenzannulated [6,6]-spiroketal was obtained with 61% ee in the presence of Sc(OTf)3/Feng's chiral N,N'-dioxide ligand. Some of the bisbenzannulated [6,6]-spiroketal products showed good in vitro antifungal activities against Sclerotinia sclerotiorum and Rhizoctonia solani.

Bioorthogonal Quinone Methide Decaging Enables Live-Cell Quantification of Tumor-Specific Immune Interactions

Effective antitumor immunity hinges on the specific engagement between tumor and cytotoxic immune cells, especially cytotoxic T cells. Although investigating these intercellular interactions is crucial for characterizing immune responses and guiding immunotherapeutic applications, direct and quantitative detection of tumor-T cell interactions within a live-cell context remains challenging. We herein report a photocatalytic live-cell interaction labeling strategy (CAT-Cell) relying on the bioorthogonal decaging of quinone methide moieties for sensitive and selective investigation and quantification of tumor-T cell interactions. By developing quinone methide-derived probes optimized for capturing cell-cell interactions (CCIs), we demonstrated the capacity of CAT-Cell for detecting CCIs directed by various types of receptor-ligand pairs (e.g., CD40-CD40L, TCR-pMHC) and further quantified the strengths of tumor-T cell interactions that are crucial for evaluating the antitumor immune responses. We further applied CAT-Cell for ex vivo quantification of tumor-specific T cell interactions on splenocyte and solid tumor samples from mouse models. Finally, the broad compatibility and utility of CAT-Cell were demonstrated by integrating it with the antigen-specific targeting system as well as for tumor-natural killer cell interaction detection. By leveraging the bioorthogonal photocatalytic decaging chemistry on quinone methide, CAT-Cell provides a sensitive, tunable, universal, and noninvasive toolbox for unraveling and quantifying the crucial but delicate tumor-immune interactions under live-cell settings.

Synthesis and Photochemical Properties of Fluorescent Metabolites Generated from Fluorinated Benzoylmenadiones in Living Cells

This work describes the reactivity and properties of fluorinated derivatives (F-PD and F-PDO) of plasmodione (PD) and its metabolite, the plasmodione oxide (PDO). Introduction of a fluorine atom on the 2-methyl group markedly alters the redox properties of the 1,4-naphthoquinone electrophore, making the compound highly oxidizing and particularly photoreactive. A fruitful set of analytical methods (electrochemistry, absorption and emission spectrophotometry, and HRMS-ESI) have been used to highlight the products resulting from UV photoirradiation in the absence or presence of selected nucleophiles. With F-PDO and in the absence of nucleophile, photoreduction generates a highly reactive ortho-quinone methide (o-QM) capable of leading to the formation of a homodimer. In the presence of thiol nucleophiles such as β-mercaptoethanol, which was used as a model, o-QMs are continuously regenerated in sequential photoredox reactions generating mono- or disulfanylation products as well as various unreported sulfanyl products. Besides, these photoreduced adducts derived from F-PDO are characterized by a bright yellowish emission due to an excited-state intramolecular proton transfer (ESIPT) process between the dihydronapthoquinone and benzoyl units. In order to evidence the possibility of an intramolecular coupling of the o-QM intermediate, a synthetic route to the corresponding anthrones is described. Tautomerization of the targeted anthrones occurs and affords highly fluorescent stable hydroxyl-anthraquinones. Although probable to explain the intense visible fluorescence emission also observed in tobacco BY-2 cells used as a cellular model, these coupling products have never been observed during the photochemical reactions performed in this study. Our data suggest that the observed ESIPT-induced fluorescence most likely corresponds to the generation of alkylated products through reduction species, as demonstrated with the β-mercaptoethanol model. In conclusion, F-PDO thus acts as a novel (pro)-fluorescent probe for monitoring redox processes and protein alkylation in living cells.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

gem-Difluoro-Masked o-Quinone Methides Generated by Photocatalytic Radical (3+3) Annulation and Their (4+1) Cycloaddition with Sulfur Ylides

A visible light-promoted radical (3+3) annulation of vinyldiazo compounds and bromodifluoromethyl alkynyl ketones for the construction of gem-difluoro-masked o-quinone methides (o-QMs) is described. The reactivity of this new type of o-QM precursor is demonstrated by its (4+1) cycloaddition with sulfur ylides, affording monofluorinated aromatic benzofurans by the elimination of HBr without external oxidants.

Dipeptides Containing Pyrene and Modified Photochemically Reactive Tyrosine: Noncovalent and Covalent Binding to Polynucleotides

Dipeptides 1 and 2 were synthesized from unnatural amino acids containing pyrene as a fluorescent label and polynucleotide binding unit, and modified tyrosine as a photochemically reactive unit. Photophysical properties of the peptides were investigated by steady-state and time-resolved fluorescence. Both peptides are fluorescent (Φf = 0.3-0.4) and do not show a tendency to form pyrene excimers in the concentration range < 10-5 M, which is important for their application in the fluorescent labeling of polynucleotides. Furthermore, both peptides are photochemically reactive and undergo deamination delivering quinone methides (QMs) (ΦR = 0.01-0.02), as indicated from the preparative photomethanolysis study of the corresponding N-Boc protected derivatives 7 and 8. Both peptides form stable complexes with polynucleotides (log Ka > 6) by noncovalent interactions and similar affinities, binding to minor grooves, preferably to the AT reach regions. Peptide 2 with a longer spacer between the fluorophore and the photo-activable unit undergoes a more efficient deamination reaction, based on the comparison with the N-Boc protected derivatives. Upon light excitation of the complex 2·oligoAT10, the photo-generation of QM initiates the alkylation, which results in the fluorescent labeling of the oligonucleotide. This study demonstrated, as a proof of principle, that small molecules can combine dual forms of fluorescent labeling of polynucleotides, whereby initial addition of the dye rapidly forms a reversible high-affinity noncovalent complex with ds-DNA/RNA, which can be, upon irradiation by light, converted to the irreversible (covalent) form. Such a dual labeling ability of a dye could have many applications in biomedicinal sciences.

Antiproliferative activity of meso-substituted BODIPY photocages: Effect of electrophiles vs singlet oxygen

A series of BODIPY compounds with a methylphenol substituent at the meso-position and halogen atoms on the BODIPY core, or OCH3 or OAc substituents at the phenolic moiety was synthesized. Their spectral and photophysical properties and the photochemical reactivity upon irradiation in CH3OH were investigated. The molecules with the phenolic substituent at the meso-position undergo more efficient photo-methanolysis at the boron atom, while the introduction of the OCH3 group at the phenolic moiety changes the reaction selectivity towards the cleavage at the meso-position. The introduction of the halogen atoms into the BODIPY increases the photo-cleavage reaction efficiency, as well as the ability of the molecules to sensitize oxygen and form reactive oxygen species (ROS). The efficiency of the ROS formation was measured in comparison with that of tetraphenylporphyrin. The antiproliferative effect of BODIPY molecules was investigated against three human cancer cell lines MCF-7 (breast carcinoma), H460 (lung carcinoma), HCT116 (colon carcinoma), and two non-cancer cell lines, HEK293T (embryonic kindey) and HaCaT (keratinocytes), with the cells kept in the dark or irradiated with visible light. For most of the compounds a modest or no antiproliferative activity was observed for cells in the dark. However, when cells were irradiated, a dramatic increase in cytotoxicity was observed (more than 100-fold), with IC50 values in the submicromolar concentration range. The enhancement of the cytotoxic effect was explained by the formation of ROS, which was studied for cells in vitro. However, for some BODIPY compounds, the effects due to the formation of electrophilic species (carbocations and quinone methides, which react with biomolecules) cannot be disregarded. Confocal fluorescence microscopy images of H460 cells and HEK293T show that the compounds enter the cells and are retained in the cytoplasm and membranes of the various organelles. When the cells treated with the compounds are irradiated, photo-processes lead to cell death by apoptosis. The study performed is important because it provides bases for the development of novel photo-therapeutics capable of exerting photo-cytotoxic effects in both oxygenated and hypoxic cells.

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.

Phosphine-catalysed transformations of ortho- and para-quinone methides

Organocatalytic methodologies for the derivatization of o-QM, p-QM and the analogous aza-QM have been recently developed and involve different catalytic systems such as phosphoric acids, thioureas, squaramides, NHC carbenes or chiral ammonium salts. Besides, phosphines, commonly used as ligands in metal-catalysed reactions, can be also used as organocatalysts. In this case, they are mainly involved as nucleophilic catalysts in reactions such as the Rauhut-Currier (RC) reaction. In this review, an analysis of the recent developments in racemic and enantioselective phosphine-catalysed transformations of o-QM, p-QM and aza-o-QM has been carried out.

Chiral Lewis Base-Catalysed Asymmetric Syntheses of Benzo-fused ϵ-Lactones

We herein report a two-step protocol for the asymmetric synthesis of novel chiral benzofused ϵ-lactones starting from O-protected hydroxymethyl-para-quinone methides and activated aryl esters. By using chiral isothiourea Lewis base catalysts a broad variety of differently substituted products could be obtained in yields of around 50 % over both steps with high levels of enantioselectivities, albeit low diastereoselectivities only.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Enzymatic Fluoromethylation Enabled by the S-Adenosylmethionine Analog Te-Adenosyl-L-(fluoromethyl)homotellurocysteine

Fluoromethyl, difluoromethyl, and trifluoromethyl groups are present in numerous pharmaceuticals and agrochemicals, where they play critical roles in the efficacy and metabolic stability of these molecules. Strategies for late-stage incorporation of fluorine-containing atoms in molecules have become an important area of organic and medicinal chemistry as well as synthetic biology. Herein, we describe the synthesis and use of Te-adenosyl-L-(fluoromethyl)homotellurocysteine (FMeTeSAM), a novel and biologically relevant fluoromethylating agent. FMeTeSAM is structurally and chemically related to the universal cellular methyl donor S-adenosyl-L-methionine (SAM) and supports the robust transfer of fluoromethyl groups to oxygen, nitrogen, sulfur, and some carbon nucleophiles. FMeTeSAM is also used to fluoromethylate precursors to oxaline and daunorubicin, two complex natural products that exhibit antitumor properties.

Lewis Acid-Catalyzed 1,4-Addition and Annulation of 4-Hydroxy-coumarins with o-Hydroxyphenyl Propargyl Amines: Entry to Regio-Selective Synthesis of Furano[3,2-c]coumarins and Pyrano[3,2-c]coumarins

A facile and regioselective Lewis acid-catalyzed cascade annulation of o-hydroxyphenyl propargyl amines with 4-hydroxycoumarin to afford furano[3,2-c]coumarin and pyrano[3,2-c]coumarin derivatives is reported. The reaction presumably proceeds by the conjugate addition of 4-hydroxycoumarin to the in situ-generated alkynyl o-quinone methide and is followed by intramolecular 5-exo-dig and 6-endo-dig annulation to form furano[3,2-c]coumarins and pyrano[3,2-c]coumarins, respectively. The prepared o-hydroxyl substituted pyrano[3,2-c]coumarins could be readily transformed into the corresponding coumarin-derived dioxabicycles by acid-mediated cyclization.

Catalyst-free thiophosphorylation of in situ formed ortho-quinone methides

A metal-, chloride reagent and base-free thiophosphorylation reaction of in situ formed ortho-quinone methide (o-QM) to synthesize functionalized thiophosphates has been developed. The reaction is an atom-economical process, producing water as the sole byproduct. (EtO)₂P(O)SH functions as both a Brønsted acid and nucleophilic thiolate to produce the o-QM intermediate and the thiophosphate product, respectively. The aza o-QMs were also successfully thiophosphorylated in the presence of catalytic TsOH to form sulfonamido thiophosphates.

Brønsted Acid Catalyzed Friedel-Crafts Alkylation of Naphthols with In Situ Generated Naphthol-Derived ortho-Quinone Methides: Synthesis of Chiral and Achiral Xanthene Derivatives

We disclose herein an enantioselective protocol for the Brønsted acid catalyzed addition of naphthols to in situ generated naphthol-derived ortho-quinone methides (o-QMs) followed by intramolecular cyclization, which delivers substituted chiral xanthene derivatives, in a one-pot reaction sequence under mild conditions. This process serves to convert naphthol-derived ortho-hydroxyl benzylic alcohols into reactive naphthol-derived o-QMs using a chiral phosphoric acid (CPA) catalyst. Moreover, it is helpful in controlling the enantioselectivity of the carbon-carbon bond-forming event via hydrogen-bonding followed by intramolecular cyclization. Additionally, for the first time, we observe a Brønsted acid catalyzed C(sp²)-C(sp³) bond cleavage of naphthol-derived ortho-hydroxyl benzylic alcohols for the synthesis of achiral xanthene (sigma plane containing) derivatives in good to excellent yields.

Nucleophilic Addition of 4,5-Dihydrooxazole Derivatives to Base Generated o-Quinone Methides: A Four-Component Reaction

A novel method for joining four components together in a single pot leading to an assortment of N-amino-benzylated phenols is described. The method involves the addition of different Grignard reagents to various o-OBoc salicylaldehydes in the presence of assorted 4,5-dihydrooxazoles, followed by aqueous workup. Seventeen examples are presented with varied (-R, -R' -R″, -R‴, -R⁗, and C_n) substituents.

Recent Advances in Quinone Methide Chemistry for Protein-Proximity Capturing

Here we summarize the most recent findings in the chemical-, photo-, or enzyme-triggered generation of nitrogen and oxygen anions leading to the formation of quinone methide intermediates (QMIs). This short review is divided into two categories: generation of nitrogen and oxygen anions. Based on quinone methide intermediates (QMIs), proximate capture of a wide range of proteins has been widely determined and studied. Generally, the triggers include, photoirradiation using 365/254 nm UV light, small molecules (ROS/TBAF/s-tetrazine), metal catalysis (iridium catalysis), and enzymes (NQO1/β-galactosidase). New directions including far-red light, heat, force, microwave, and more practical approaches are explored and illustrated.

1 Introduction

2 Generation of the Nitrogen Anion

3 Generation of the Oxygen Anion

4 Conclusion

Reactivity of quinone methides with carbenes generated from α-diazocarbonyl compounds and related compounds

Over the years, quinone methides have broadly been applied in synthesis and biological systems for synthesizing heterocyclic compounds and biologically active molecules. In this feature article, we have discussed the novel and uncovered reactivity of o-quinone methides, p-quinone methides, aza-o-quinone methides, and indolyl-2-methides with carbenes generated from α-diazocarbonyl compounds and related compounds. Two in situ-generated transient intermediates undergo cycloannulation reactions, metathesis-type reactions, 1,6-conjugate addition reactions, cyclopropanation reactions, and many other transformations to access nitrogen- and oxygen-containing heterocyclic compounds and beyond. The reactivity of quinone methides and carbenes is observed in various metal catalysts, Brønsted-acids, Lewis acids, phase transfer catalysts, additives, and visible-light-induced transformations.

A Novel Method to Construct 2-Aminobenzofurans via [4 + 1] Cycloaddition Reaction of In Situ Generated Ortho-Quinone Methides with Isocyanides

A new approach for the synthesis of 2-aminobenzofurans has been described via Sc(OTf)₃ mediated formal cycloaddition of isocyanides with the in situ generated ortho-quinone methides (o-QMs) from o-hydroxybenzhydryl alcohol. Notably, as a class of readily available and highly active intermediates, o-QMs were first used in the construction of benzofurans. This [4 + 1] cycloaddition reaction provides a straightforward and efficient methodology for the construction of 2-aminobenzofurans scaffold in good yield (up to 93% yield) under mild conditions.

An expeditious FeCl3-catalyzed cascade 1,4-conjugate addition/annulation/1,5-H shift sequence for modular access of all-pyrano-moiety-substituted chromenes

ortho-Alkynyl quinone methides are well-known four-atom synthons for direct [4 + n] cycloaddition in constructing useful oxa-heterocyclic compounds owing to their high reactivity as well as the thermodynamically favored aromatization nature of this process. Herein we report an operationally simple and eco-friendly protocol for the modular and regioselective access of (E)-4-(vinyl or aryl or alkynyl)iminochromenes from propargylamines and S-methylated β-ketothioamides in the presence of FeCl3, and particularly under undried acetonitrile and air atmosphere conditions. This method exhibits a broad substrate scope and displays nice functional group compatibility, thus providing an efficient access of 3,4-disubstituted iminochromenes.

Insights on Chemical Crosslinking Strategies for Proteins

Crosslinking of proteins has gained immense significance in the fabrication of biomaterials for various health care applications. Various novel chemical-based strategies are being continuously developed for intra-/inter-molecular crosslinking of proteins to create a network/matrix with desired mechanical/functional properties without imparting toxicity to the host system. Many materials that are used in biomedical and food packaging industries are prepared by chemical means of crosslinking the proteins, besides the physical or enzymatic means of crosslinking. Such chemical methods utilize the chemical compounds or crosslinkers available from natural sources or synthetically generated with the ability to form covalent/non-covalent bonds with proteins. Such linkages are possible with chemicals like carbodiimides/epoxides, while photo-induced novel chemical crosslinkers are also available. In this review, we have discussed different protein crosslinking strategies under chemical methods, along with the corresponding crosslinking reactions/conditions, material properties and significant applications.

Theoretical calculations of formation and reactivity of o-quinomethide derivatives of resorcin[4]arene with reference to empirical data

This paper describes theoretical reaction pathways of alkoxybenzyl derivatives of resorcin[4]arene leading to the formation of o-quinomethide derivatives of resorcin[4]arene (o-QMR[4]A). For each case, the activation energies for the formation of one o-QMR[4]A unit and the activation energies for the backward reaction were calculated. Based on the calculated reaction pathways, the reaction mechanism of o-QMR[4]A formation was proposed. Using the example of o-QMR[4]A generated from a methoxy derivative of resorcin[4]arene, the activation energies with selected nucleophiles were calculated and the reaction mechanisms discussed. Reaction path calculations were performed using the nudged elastic band method and semiempirical extended tight-binding method (GFN2-xTB). Using hydroxybenzyl derivatives of resorcin[4]arene as an example, a comparison of calculated activation energies by selected density-functional theory methods with GFN2-xTB and B97-3c geometries was performed. B97-3c and wB97XD methods were used to calculate the energies of the reactants (R), transition states (TS) and products (P) of the analysed reactions. Theoretical reaction mechanisms were discussed with respect to the orbital-weighted Fukui dual descriptor (Δfw ) and experimental data.

Diastereoselective Synthesis of Chromeno[3,2-d]isoxazoles via Brønsted Acid Catalyzed Tandem 1,6-Addition/Double Annulations of o-Hydroxyl Propargylic Alcohols

A Brønsted acid catalyzed tandem process to access densely functionalized chromeno[3,2-d]isoxazoles with good to excellent yields and diastereoselectivities was disclosed. The procedure is proposed to involve a 1,6-conjugate addition/electrophilic addition/double annulations process of alkynyl o-quinone methides (o-AQMs) in situ generated from o-hydroxyl propargylic alcohols with nitrones. Mild conditions, good functional group compatibility, easy scale-up of the reaction, and further product transformation demonstrated its potential application.

Palladium-Catalyzed Intermolecular Asymmetric Dearomative Annulation of Phenols with Vinyl Cyclopropanes

Herein, we report the Pd(0)-catalyzed intermolecular asymmetric dearomative [3 + 2] annulation of phenols with vinyl cyclopropanes via in situ generated ortho-quinone methide intermediates. A series of highly functionalized spiro-[5,6] bicycles which bear three contiguous stereogenic centers including one all-carbon quaternary were obtained with excellent stereoselectivities. Density functional theory (DFT) calculations indicate that the reactions were controlled by thermodynamics.

Selective radical cascade (4+2) annulation with olefins towards the synthesis of chroman derivatives via organo-photoredox catalysis

Due to the importance of chroman frameworks in medicinal chemistry, the development of novel synthetic methods for these structures is gaining increasing interest of chemists. Reported here is a new (4 + 2) radical annulation approach for the construction of these functional six-membered frameworks via photocatalysis. Featuring mild reaction conditions, the protocol allows readily available N-hydroxyphthalimide esters and electron-deficient olefins to be converted into a wide range of valuable chromans in a highly selective manner. Moreover, the present strategy can be used in the late-stage functionalization of natural product derivatives and biologically active compounds, which demonstrated the potential application. This method is complementary to the traditional Diels–Alder [4 + 2] cycloaddition reaction of ortho-quinone methides and electron-rich dienophiles, since electron-deficient dienophiles were smoothly transformed into the desired chromans.

We have developed a (4 + 2) radical annulation approach for the synthesis of diverse chromans. This method is complementary to the traditional Diels–Alder [4 + 2] annulation of ortho-quinone methides and electron-rich dienophiles.

Reactivity vs. selectivity of quinone methides: synthesis of pharmaceutically important molecules, toxicity and biological applications

Quinone methides (QMs) are considered to be highly reactive intermediates because of their aromatization both in chemical and biological systems. Being highly accessible, quinone methides (QMs) have been widely exploited and their concurrent use has been manifested for the synthesis of tertiary and quaternary carbon centers of bioactives, drugs and drug-like molecules. In this feature article, the synthetic routes, structure-reactivity relationships and synthetic applications of quinone methides are discussed. Formation of the intermediates during bioactivation of different chemical entities and possible chemical manifestations leading to their toxicity in biological systems are also covered.

DMSO-assisted environmentally benign synthesis of benzo[c]-chromeno[4,3,2-gh]phenanthridines by remote oxidative hetero cross-coupling cyclization and aromatization reaction

An unprecedented metal-free and catalyst-free synthesis of benzo[c]chromeno[4,3,2-gh]phenanthridine derivatives, a class of 1,6-diheterophenalenoid heterocycle, is reported for the first time. The oxidative cross-coupling reaction for the remote cyclization is achieved through the in situ generated o-quinone methide intermediate followed by an electrocyclic ring closure reaction. The aromatization of the cyclohexane ring is achieved by sequential H shift, hydroxylation, and elimination reaction. DMSO-assisted concomitant cyclization and aromatization reactions are also disclosed for the first time.

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

Harnessing ortho-Quinone Methides in Natural Product Biosynthesis and Biocatalysis

The implementation of ortho-quinone methide (o-QM) intermediates in complex molecule assembly represents a remarkably efficient strategy designed by Nature and utilized by synthetic chemists. o-QMs have been taken advantage of in biomimetic syntheses for decades, yet relatively few examples of o-QM-generating enzymes in natural product biosynthetic pathways have been reported. The biosynthetic enzymes that have been discovered thus far exhibit tremendous potential for biocatalytic applications, enabling the selective production of desirable compounds that are otherwise intractable or inherently difficult to achieve by traditional synthetic methods. Characterization of this biosynthetic machinery has the potential to shine a light on new enzymes capable of similar chemistry on diverse substrates, thus expanding our knowledge of Nature's catalytic repertoire. The presently known o-QM-generating enzymes include flavin-dependent oxidases, hetero-Diels-Alderases, S-adenosyl-l-methionine-dependent pericyclases, and α-ketoglutarate-dependent nonheme iron enzymes. In this review, we discuss their diverse enzymatic mechanisms and potential as biocatalysts in constructing natural product molecules such as cannabinoids.

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

The ultrafast photochemical reaction of quinone methide (QM) formation from adamantylphenol was monitored in real time using femtosecond transient absorption spectroscopy and fluorescence upconversion in solution at room temperature. Experiments were complemented by theoretical studies simulating the reaction pathway and elucidating its mechanism. Excitation with sub-20 fs UV pulses and broadband probing revealed ultrafast formation of the long-lived QM intermediate directly in the ground state, occurring with a time constant of around 100 fs. UV-vis transient absorption data covering temporal dynamics from femtoseconds to hundreds of milliseconds revealed persistence of the absorption band assigned to QM and partially overlapped with other contributions tentatively assigned to triplet excited states of the adamantyl derivative and the phenoxyl radical that are clearly distinguished by their evolution on different time scales. Our data, together with the computations, provide evidence of a non-adiabatic photodehydration reaction, which leads to the formation of QM in the ground state via a conical intersection, circumventing the generation of a transient QM excited state.

Multicomponent Reactions (MCRs) with o-Quinone Methides

This article presents a comprehensive overview of multicomponent reactions (MCRs) that proceed via ortho-quinone methide intermediates (o-QM) generated in the reaction medium. Examples of applications involving these highly reactive intermediates in organic synthesis and biological processes (e. g., biosynthetic pathways, prodrug cleavage and electrophilic capture of biological nucleophiles) are also described. QMs are often generated by eliminative processes of phenol derivatives or by photochemical reactions, including reversible generation in photochromic substances. This class of compounds can undergo various reaction types, including nucleophilic attack at the methide carbon, with subsequent rearomatization, and react with electron-rich dienophiles in inverse-electron demand hetero-Diels-Alder reactions. Its versatile reactivity has been explored in the context of cascade reactions for the construction of several classes of substances, including complex natural products.

Trapping Reactions of Benzynes Initiated by Intramolecular Nucleophilic Addition of a Carbonyl Oxygen to the Electrophilic Aryne

We describe here reactions in which a carbonyl oxygen atom initiates cascade reactions by nucleophilic attack on a covalently attached benzyne. The benzynes are produced by thermal cyclization of triynes via hexadehydro-Diels-Alder reaction. The initially produced oxocarbenium/aryl carbanionic zwitterion is protonated in situ by an external protic nucleophile (NuH) of appropriate acidity. The resulting ion pair (oxocarbenium⁺/Nu^-) collapses through several different mechanistic manifolds, adding to the diversity of structural classes that can be generated.

A new cycloaddition profile for ortho-quinone methides: photoredox-catalyzed [6+4] cycloadditions for synthesis of benzo[b]cyclopenta[e]oxepines

Visible-light-induced [6+4] cycloaddition reactions of ortho-quinone methides have been developed. The reaction of ortho-quinone methides with pentafulvenes in the presence of a thioxanthylium photoredox catalyst afforded benzo[b]cyclopenta[e]oxepines. The present reaction represents a promising tool for the synthesis of natural products and bioactive compounds that contain a benzoxepine structure.