Phosphine-Catalyzed γ'-Carbon 1,6-Conjugate Addition of α-Succinimide Substituted Allenoates with Para-Quinone Methides: Synthesis of 4-Diarylmethylated 3,4-Disubstituted Maleimides

In this paper, an interesting γ'-carbon 1,6-conjugate addition for phosphine-catalyzed α-succinimide substituted allenoates has been disclosed. A wide array of substrates was found to participate in the reaction, resulting in the production of diverse 4-diarylmethylated 3,4-disubstituted maleimides with satisfactory to outstanding yields. Furthermore, a plausible mechanism for the reaction was proposed by the investigators.

Organocatalytic Asymmetric Conjugate Addition of Fluorooxindoles to Quinone Methides

Fluorooxindoles undergo asymmetric Michael addition to para-quinone methides under phase-transfer conditions with 10 mol% of a readily available cinchona alkaloid ammonium catalyst. This reaction affords sterically encumbered, multifunctional fluorinated organic compounds displaying two adjacent chirality centers with high yields, ee's and dr's.

Cu(I)-Catalyzed Three-Component Annulation for the Synthesis of 3-Acyl Imidazo[1, 5-a]Pyridines from 2-Pyridinyl-Substituted p-Quinone Methides, Terminal Alkynes, and TsN3 Using O2 as the Oxygen Source

Currently, most conventional methods to achieve imidazo[1,5-a]pyridines have limitations for the synthesis of 3-acyl imidazo[1,5-a]pyridines. Herein, a novel and efficient Cu(I)-catalyzed three-component annulation method for the synthesis of valuable 3-acyl imidazo[1,5-a]pyridines by the reaction of 2-pyridinyl-substituted p-QMs, terminal alkynes, and TsN3 in the presence of O2 under mild conditions have successfully been developed. The investigation indicated that molecular oxygen (O2) and TsN3, respectively, serving as oxygen and nitrogen sources, were essential for the successful completion of the reaction system.

Total Syntheses of Hosieines A-C

The collective total syntheses of (±)-hosieines A-C with a cage-like tetracyclic framework have been realized, which includes the first syntheses of hosieines B-C. The key strategy of the synthesis employs a one-pot domino reaction that involves Cu-catalyzed [3+2] cycloaddition, 1,6-enone formation, and 1,6-aza-Michael addition forming the 5/6/6-aza-tricyclic skeleton. Other salient synthetic tactics comprise a challenging double bond migration and a 1,4-aza-Michael addition reaction to afford the tetracyclic framework.

Synthesis of Fluorine-Containing Multisubstituted Oxa-Spiro[4,5]cyclohexadienones via a Fluorinated Alcohol-Catalyzed One-Pot Sequential Cascade Strategy

In recent years, the application of fluorinated alcohols as solvents, cosolvents, or additives has become important in modern organic synthesis. However, their potential as efficient catalysts in organic synthesis has not been well-explored. In this article, we report on the development of a one-pot sequential cascade reaction of p-quinone methides with difluoroenoxysilanes using hexafluoroisopropanol as catalyst. This reaction allows for the preparation of fluorinated multisubstituted oxa-spiro[4,5]cyclohexadienones. By using 50 mol % 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP), the reaction proceeds smoothly to yield 1,6-conjugated products, which are then subjected to oxidative dearomatization/hemiacetalization using PhI(OAc)2. The overall process affords moderate to high yields and excellent diastereoselectivities.

An Open-Air Palladium-Catalyzed Stereoselective O-Glycosylation of Glycals via in-situ Generation of gem-Disubstituted Methanols from p-Quinone Methides

We devised a palladium-catalyzed α-stereoselective glycosylation that incorporates oxygen via in-situ generation of gem-disubstituted methanols from p-quinone methides to access 2,3-unsaturated gem-diarylmethyl O-glycosides under open-air atmosphere at room temperature. Advantages of this environmentally friendly strategy include the absence of additives and ligands, using water as the green source of oxygen, mildest, operationally simple, exhibiting a wide functional group tolerance, and compatibility with a variety of glycal progenitors in appreciable yields. A mechanistic study has been verified via H2 18 O labeling, which validates that water (moisture) is a sole source of oxygen.

Non-enzymatic synthesis of C-methylated fluostatins: discovery and reaction mechanism

Two C-methylated fluostatins (FSTs) B3 (1) and B4 (2) were synthesized from flavin-mediated nonenzymatic epoxide ring-opening reactions of FST C. The structures of 1 and 2 were elucidated by HRESIMS, NMR, and ECD spectroscopic analyses. A subsequent 13C labeling study demonstrated that the C-methyl groups of 1 and 2 were derived from DMSO and enabled the mechanistic proposal of a nonenzymatic C-methylation.

Olefin skeletal rearrangement enabling access to multiarylated N-sulfonyl amidines

A base-promoted olefin skeletal rearrangement strategy from para-quinone methides (p-QMs) and N-fluoroarenesulfonamides is reported, enabling direct nitrogen insertion of olefins to produce a series of multiarylated (Z)-N-sulfonyl amidines with complete stereoselectivity and generally good yields. Using p-QMs without o-hydroxy substituents gave triarylated N-sulfonyl amidines, whereas tetraarylated N,N'-disulfonyl amidines were synthesized with the existence of o-hydroxy groups.

Synthetic Studies toward 5,6,7,3',4'-Monomethoxytetrahydroxyflavones: Synthesis of Pedalitin

During the synthetic studies toward 5,6,7,3',4'-monomethoxytetrahydroxyflavones, a concise pedalitin synthesis procedure was achieved. As previously reported, 6-hydroxy-2,3,4-trimethoxyacetophenone was prepared by Friedel-Crafts acylation of 1,4-dihydroxy-2,6-dimethoxybenzene with boron trifluoride diethyl etherate in acetic acid. When aldol condensation of 6-hydroxy-2,3,4-trimethoxyacetophenone 2b with vanillin was performed in basic conditions, it produced 2'-hydroxychalcone 3b, and, surprisingly, along with 3-hydroxyflavone 4 in a considerable amount. We propose that this oxidative cyclization is presumably due to the contribution of a quinone methide, likely to be subjected to aerobic oxidation. The chalcone was then subjected to oxidative cyclization with iodine in dimethyl sulfoxide to afford flavone 5 in good yield. To our delight, serial demethylation of the three methoxy groups at the 5-, 6-, and 3'-positions of 5 proceeded smoothly to produce pedalitin 1, under hydrogen bromide solution (30% in acetic acid). The crystal structures of 3-hydroxyflavone 4 and pedalitin tetraacetate 6 were unambiguously determined by X-ray crystallography.

Synthesis and Photophysical Properties of 3-Substituted-1H-Indazoles: A Pd-Catalyzed Double C-N Bond Formation Strategy via 1,6-Conjugate Addition

A Pd-catalyzed cascade process for the direct synthesis of 3-substituted-1H-indazole employing p-quinone methide (p-QM) and arylhydrazine through Pd-catalyzed double C-N bond formation via 1,6-conjugate addition is reported. This reaction strategy affords efficient and practical access to synthetically important diverse 3-substituted-1H-indazoles in good yields. The photophysical properties of the synthesized 3-substituted-1H-indazoles are investigated, and some of them showed very good fluorescence properties with quantum yields up to 85%. Also, the synthesized 3-substituted-1H-indazole exhibits an acid-sensitive fluorescence turn-off activity.

Catalyst- and Additive-free, Regioselective 1,6-Hydroarylation of para-Quinone Methides with Anilines in HFIP

A simple and efficient method for the synthesis of diarylmethyl-functionalized anilines through the hexafluoroisopropanol (HFIP)-mediated regioselective 1,6-hydroarylation reaction of para-quinone methides (p-QMs) with anilines under catalyst- and additive-free conditions is reported. Various kinds of p-QMs and amines (e. g. primary, secondary and tertiary amines) are well tolerated in this transformation without the pre-protection of amino group, and the corresponding products could be generated with good to excellent yields and satisfactory regioselectivity under the optimized reaction conditions. In addition to adaptable amine compounds, indoles and their derivatives are also compatible with this reaction system. This transformation can be easily extended to a gram scale-synthesis level to synthesize the target product. Furthermore, it is worth noting that some complex small aniline molecules with biological activity can be selectively modified using this method. The possible reaction mechanism is proposed through the step-by-step control experiments and DFT calculations, showing that the key process for achieving the regioselective 1,6-hydroarylation of p-QMs is the hydrogen bonding effect of HFIP to substrates.

Ruthenium-Catalyzed 1,6-Hydroalkylation of para-Quinone Methides with Ketones via the in Situ Activation of C(sp3)-H Bonds

A simple and efficient method for the ruthenium-catalyzed 1,6-hydroalkylation of para-quinone methides (p-QMs) with ketones via the in situ activation of C(sp3)-H bonds has been disclosed. Without the need for preactivation of the substrates and oxidant, a broad range of p-QMs and ketones are well tolerated, producing the expected 1,6-hydroalkylation products with moderate to good yields. Step-by-step control experiments and DFT calculation were conducted systematically to gain insights for the plausible reaction mechanism. This finding may have potential application in the selective diarylmethylation of ketones at the α-C position in organic synthesis.

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.

One-pot synthesis of thioethers from indoles and p-quinone methides using thiourea as a sulfur source

Thiourea is an inexpensive and user friendly sulfur reagent that acts as a sulfur source. A simple and efficient protocol has been developed to access thioethers by reacting indoles with p-quinone methides using thiourea as the sulfur source. In our experiments, the reaction apparently proceeded through an S-(3-indolyl)isothiuronium iodide intermediate and subsequent generation of indolethiol that attacked the 1,6 position of p-quinone methides to give desired thioethers in good to excellent yields.

Magnesium-Mediated Umpolung Carboxylation of p-Quinone Methides with CO2

Magnesium-mediated reductive carboxylation of p-QMs with CO2 via an Umpolung strategy has been developed, which can be used for the preparation of various aryl acetic acids. This protocol featured high atom economy, mild conditions, and operational simplicity. The creation of this Umpolung carboxylation of p-QMs will unprecedentedly extend the application of p-QMs to nucleophilic reagents.

Site-selective ring opening of bicyclo[n.1.0]alkanols: an Fe(II)-catalyzed 1,6-conjugate addition to p-quinone methides

Herein, we report an efficient synthetic strategy for an Fe(ii)-catalyzed site-selective ring opening of bicyclo[n.1.0]alkanols and their concomitant 1,6-conjugate addition to p-quinone methides. Access to tertiary carbon centers with appendaged carbocycles of distinct sizes and functional groups are achieved, under a substrate-controlled bond scission of the fused cyclopropanols. Synthetic derivatizations further enhance the utility of the protocol.

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.

BF3 ⋅Et2 O-Catalyzed Selective C-4 Alkylation of Isoquinolin-1(2H)-ones Employing p-Quinone Methides

The direct C-4 alkylation of isoquinolin-1(2H)-one moiety is a challenging transformation in organic synthesis. Here we present a practical and efficient synthesis of C-4 alkylated isoquinolin-1(2H)-ones through conjugate addition of isoquinolin-1(2H)-ones to p-quinone methides for the first time. The process is facilitated by Lewis acid catalysis and this operationally straightforward, mild, metal-free and one-pot transformation provides a wide range of C-4 alkylated isoquinolin-1(2H)-ones at ambient temperature in good to excellent yields.

Regio- and diastereoselective synthesis of diverse spirocyclic indenes by cyclization with indene-dienes as two carbon building blocks

We report a base-promoted cyclization with indene-dienes as two carbon building blocks toward diverse spirocyclic indene scaffolds including hexacyclic spiroindenes bearing benzo pyran motifs and pentacyclic spiroindenes containing oxindole units in high yields with excellent diastereoselectivities.

Dibenzoylperoxide as a Versatile Oxidant for Oxidative Azidations of Phenols Using Quaternary Ammonium Iodides or Bromides

Dibenzoylperoxide emerged as a versatile oxidant for quaternary ammonium iodide or bromide‐catalyzed reactions of phenol derivatives with NaN3. While the use of iodides allowed for efficient benzylic azidations under these oxidative conditions, the use of bromides allowed for dearomative azidations instead. Both approaches have been successfully applied to different phenol derivatives and a first proof‐of‐concept for an enantioselective variant using chiral quat. ammonium bromides has been obtained as well.

Stereoselective Reductive Coupling Reactions Utilizing [1,2]-Phospha-Brook Rearrangement: A Powerful Umpolung Approach

[1,2]-Phospha-Brook rearrangement entails the generation of α-oxygenated carbanions via the umpolung process. Recently, these anionic species have been widely utilized for several C-C bond forming strategies, providing various useful frameworks that are difficult to access through conventional approaches. However, the application of this powerful methodology in the development of chiral strategies is still at the nascent stage due to challenges involved in controlling chemoselectivity and enantioselectivity. This synopsis provides a detailed summary of diastereo- and/or enantioselective chemical transformations using [1,2]-phospha-Brook rearrangement.

3-Nitroindoles Serving as N-Centered Nucleophiles for Aza-1,6-Michael Addition to para-Quinone Methides

An unprecedented N-alkylation of 3-nitroindoles with para-quinone methides was developed for the first time. Using potassium carbonate as the base, a wide range of structurally diverse N-diarylmethylindole derivatives were obtained with moderated to good yields via the protection group migration/aza-1,6-Michael addition sequences. The reaction process was also demonstrated by control experiments. Different from the previous advances where 3-nitrodoles served as electrophiles trapping by various nucleophiles, the reaction herein is featured that 3-nitrodoles is defined with latent N-centered nucleophiles to react with ortho-hydrophenyl p-QMs for construction of various N-diarylmethylindoles.

Catalytic vs. uncatalyzed [2 + 2] photocycloadditions of quinones with alkynes

Photoreactions of quinones with alkynes under catalytic and non-catalytic conditions were studied. In contrast to recent reports, simple irradiation with blue light is sufficient to trigger [2 + 2] photocycloadditions, which afford either fused cyclobutenes or reactive para-quinone methides (p-QMs) depending on the quinone structure. Revision of the chemo- and regioselectivity of the uncatalyzed photoreactions provided useful insight into their overlooked relatedness to the recently developed catalytic protocols. Experimental evidence indicates that the reactivity of the photochemically generated p-QMs is sufficient to perform uncatalyzed reactions with nucleophiles, which can help to explain the existing transformations and develop new cascade transformations involving quinones and alkynes.

Palladium-Catalyzed Annulative Coupling of Spirovinylcyclopropyl Oxindoles with p-Quinone Methides

Pd-catalyzed annulative coupling of spirovinylcyclopropyl oxindoles with p-quinone methides has been accomplished via cascade carbon-carbon bond formation to afford bis-spirooxindole scaffolds. The mild reaction conditions, diastereoselectivity, functional group diversity, post-synthetic transformations, and mechanistic studies using DFT calculations are the important practical features.

Metal-Free Approach for Oxa-spirocyclohexadienones through [3 + 2]/[4 + 2] ipso-Cyclization of para-Quinone Methides with Halo Alcohols

A facile one-pot metal-free, base-mediated formal [3 + 2] and [4 + 2] dearomative ipso-cycloaddition of para-quinone methides (p-QMs) with halo alcohols has been designed for the efficient construction of 2-oxa-spirocyclohexadienones in excellent yield under mild reaction conditions. The commercial availability of the bases, reagents, and convenient reaction procedure makes it an attractive method for ipso-cyclization.

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.

Design and development of intramolecular doubly vinylogous Michael addition to access 3-aryl substituted 2-alkenyl-benzofurans and -indoles

Herein, we have disclosed a rare example of an intramolecular doubly vinylogous Michael addition (DVMA). The reaction design exploits the innate reactivity of ortho-heteroatom substituted para-quinone methide (p-QM) derivatives. The sequential reaction of p-QMs and activated allyl halides proceeds through heteroatom-allylation, DVMA and oxidation to furnish a diverse range of 2-alkenyl benzofuran and 2-alkenyl indole derivatives in high yields.

Organophotoredox 1,6-Addition of 3,4-Dihydroquinoxalin-2-ones to para-Quinone Methides Using Visible Light

An organophotoredox 1,6-radical addition of 3,4-dihidroquinoxalin-2-ones to para-quinone methides catalyzed by Fukuzumi's photocatalyst is described under the irradiation of a HP Single LED (455 nm). The corresponding 1,1-diaryl compounds bearing a dihydroquinoxalin-2-one moiety (20 examples) are obtained with good to excellent yields under mild reaction conditions. Several experiments have been carried out in order to propose a reaction mechanism.

(Z)-Selective Synthesis of Bromofluoroalkenes via the TMSCF2Br-Mediated Tandem Reaction with para-Quinone Methides

We report herein a tandem reaction of para-quinone methides with TMSCF₂Br to construct bromofluoroalkenes in a Z-selective manner. While TMSCF₂Br has been documented as the precursor of difluoro carbene, it exhibits another possibility in this transformation, a formal bromofluoro carbene surrogate. The alkenyl bromide unit of the products could directly engage in a variety of transformations.

Synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of p-quinone methides with organosulfur reagents

A rapid and simple method was developed for the synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of para-quinone methides (p-QMs) with organosulfur reagents. A series of diarylmethyl thioethers were synthesized at 13-85% yields by this method. After that, the antibacterial activities of synthesized diarylmethyl thioethers and their derivatives were evaluated. The MIC range (μg mL^-1) against Staphylococcus aureus ATCC 25923 and clinically isolated methicillin-resistant S. aureus was 8-128 and 64-128, respectively.

Synthesis of π-Extended Benzofulvenes: A Pd-Catalyzed Cascade Annulation Process Comprising C-C Bond Formation Followed by 1,6-Conjugate Addition

A Pd-catalyzed cascade annulation process comprising C-C bond formation followed by 1,6-conjugate addition for π-extended benzofulvenes is reported. This process is compatible with a wide range of functionalities for both the p-quinone methides and internal alkynes, leading to diverse π-extended benzofulvenes. Additionally, this strategy is also applicable for aryne annulation with p-quinone methides.

Addition of sulfonylphthalides to para-quinone methides: Selective 1,6-additions and oxidative annulations

KO^tBu mediated addition of sulfonylphthalides to p-quinone methides led to the selective synthesis of isochroman-1,4-diones and addition products. Interestingly, isochroman-1,4-diones were obtained via an unprecedented oxidative annulation pathway. The present work highlights a wide variety of substrates, good yields, shorter reaction time and ambient reaction conditions. Furthermore, a few addition products were transformed into functionalized heterocyclic molecules. Additionally, the scale-up experiment suggests the practical feasibility of preparing isochroman-1,4-diones in higher-scale reactions.

Asymmetric Synthesis of Triphenylmethanes via Organocatalytic Regio- and Enantioselective Friedel-Crafts Alkylation of Aniline Derivatives

Herein, we described a highly regio- and enantioselective Friedel-Crafts alkylation of aniline derivatives with in situ generated ortho-quinone methides enabled by chiral phosphoric acid, furnishing a wide range of enantioenriched triarylmethanes bearing three similar benzene rings in high yields (up to 98%) with excellent stereoselectivities (up to 98% ee). Furthermore, the large-scale reactions and diversified transformations of product demonstrate the practicality of the protocol. Density functional theory calculations elucidate the origin of the enantioselectivity.

Copper (I) Chloride-Catalyzed Photoredox Synthesis of Multifunctionalized Compounds at Room Temperature and Their Antifungal Activities

A simple visible-light-induced CuCl-catalyzed synthesis was developed for highly functionalized carbon-centered compounds (α-alk/aryloxy-α-diaryl/alkylaryl-acetaldehydes/ketones) at room temperature using benzoquinone, alkyl/aryl alcohol, and alkyl/aryl terminal/internal alkynes. Late-stage functionalized compounds show good antifungal activities, especially against Candida krusei fungal strain, in in vitro experiments (the Broth microdilution method). Moreover, toxicity tests (zebrafish egg model experiments) indicated that these compounds had negligible cytotoxicity. The green chemistry metrics (E-factor value is 7.3) and eco-scale (eco-scale value is 58.8) evaluations show that the method is simple, mild, highly efficient, eco-friendly, and environmentally feasible.

Bismuth(III)-Catalyzed 1,8-Addition/Cyclization/Rearrangement of Propargylic para-Quinone Methides with 2-Vinylphenol: Synthesis of Indeno[2,1-c]chromenes

The unique reactivity of in situ generated propargylic para-quinone methides as a new type of five-carbon synthon has been discovered by a novel bismuth(III)-catalyzed tandem annulation reaction. This 1,8-addition/cyclization/rearrangement cyclization cascade reaction is characterized by unusual structural reconstruction of 2-vinylphenol, involving cleavage of the C1'═C2' bond and formation of four new bonds. This method provides a convenient and mild approach to generate synthetically important functionalized indeno[2,1-c]chromenes. The mechanism of the reaction is proposed from several control experiments.

Solvent-Mediated Tunable Regiodivergent C6- and N1-Alkylations of 2,3-Disubstituted Indoles with p-Quinone Methides

Indium-catalyzed, solvent-enabled regioselective C6- or N1-alkylations of 2,3-disubstituted indoles with para-quinone methides are developed under mild conditions. Notably, highly selective and switchable alkylations were selectively achieved by adjusting the reaction conditions. Moreover, scalability and further transformations of the alkylation products are demonstrated, and this operationally simple methodology is amenable to the late-stage C6-functionalization of the indomethacin drug. The reaction pathways were explained with the support of experimental and density functional theory studies.

Diastereoselective synthesis of polycyclic indolines via dearomative [4 + 2] cycloaddition of 3-nitroindoles with ortho-aminophenyl p-quinone methides

A formal [4 + 2] cycloaddition of 3-nitroindoles with ortho-aminophenyl p-quinone methides via a dearomatization process was developed. This method provides a facile approach for preparing tetrahydro-5H-indolo[2,3-b]quinolones with good results. With the bifunctional Cinchona alkaloid-squaramide as the catalyst, the asymmetric version of the reaction successfully afforded the corresponding chiral products with moderate to good enantioselectivities. This work represents the first dearomative cycloaddition of electron-deficient heteroarenes triggered by aza-Michael addition from p-QMs.

Transition-Metal-Free Multicomponent Approach to the Regioselective Synthesis of Highly Substituted N-Alkylpyrazoles

An efficient transition-metal-free multicomponent approach to the regioselective synthesis of highly substituted N-alkylpyrazoles through 1,6-addition of pyrazole (in situ generated from α,β-unsaturated aldehyde and hydrazide) to para-Quinone Methides has been developed. The N-alkylpyrazole containing triarylmethanes having several heteroaryl rings (quinoline, pyridine, thiophene) at the central methine carbon atom was developed. This chemical process may be used for large-scale synthesis and provides a novel way to produce triarylmethanes with diverse functional groups.

Atom-Economic Synthesis of Unsymmetrical gem-Diarylmethylthio/Seleno Glycosides via Base Mediated C(O)-S/Se Bond Cleavage and Acyl Transfer Approach of Glycosylthio/Selenoacetates

Herein, we invented the Cs₂CO₃-mediated atom economic method that streamlines the scission of the C(O)-S/Se bond involving the in situ generation of an anomeric thiolate/selenolate anion, which reacted with p-QMs to yield novel unsymmetrical gem-diarylmethylthio/seleno glycosides while retaining the anomeric stereochemistry. Notably, the key features of this protocol involve unprecedented long-range acyl transfer (from S/Se to O), thus affording acylation of the final product which is not yet reported by classical methods. This straightforward protocol offers a mild, short reaction time, synthetically simple approach, and compatibility with 8 types of sugar along with phenylthio/benzylseleno esters.

Enantioselective 1,6-Addition of β-Ketoester Enolates to In Situ Generated para-Quinone Methides Enabled by Cooperative Palladium and Brønsted Acid Catalysis

We report herein an asymmetric cooperative process for the enantioselective 1,6-addition of β-ketoesters to in situ generated para-quinone methides with chiral Pd-aqua complexes as mixed Brønsted acid-base catalysts. Excellent yields, outstanding enantiocontrol, and good diastereoselectivity across a broad substrate range are highlights of this transformation. The utility of this reaction is further demonstrated by a facile scale up and subsequent complexity-increasing modifications.

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.

Boosting the Enantioselectivity of Conjugate Borylation of α,β-Disubstituted Cyclobutenones with Monooxides of Chiral C2 -Symmetric Bis(phosphine) Ligands

Chiral bis(phosphine) monooxides (BPMOs) derived from C₂ -symmetric bis(phosphines) have been found to induce superior levels of enantioselection in copper-catalyzed conjugate borylation of α,β-disubstituted cyclobutenones. More precisely, enantiomeric excesses as well as chemical yields are exceedingly high with (R,R)-Bozphos as the chiral ligand while these values are low with parent (R,R)-Me-Duphos. A similar yet less pronounced effect was seen in the corresponding 1,6-addition to para-quinone methides.

Organocatalytic Asymmetric Synthesis of Tetrahydroquinolines from ortho-Aminophenyl para-Quinone Methides

The asymmetric catalytic [4 + 2] cycloannulation of ortho-aminophenyl p-QMs with different types of alkenes for the construction of tetrahydroquinolines containing three contiguous stereogenic centers was developed. This is the first example of catalytic asymmetric cycloannulation of ortho-aminophenyl p-QMs. This reaction exhibits excellent functional group tolerance. Excellent yields, exclusive diastereoselectivities, and high enantioselectivities were obtained in this efficient organocatalytic reaction.

Photocatalyst-free hydroacylations of electron-poor alkenes and enones under visible-light irradiation

Herein we present a photocatalyst- and additive-free radical hydroacylation of electron-poor double bonds under mild reaction conditions. Using 4-acyl-Hantzsch ester radical reservoirs, various Michael acceptors, enones and para-quinone methide substrates could be used. The protocol enabled further derivatizations and it could also be extended to a few unactivated alkenes. Moreover, the nature of the radical process was also investigated.

Catalytic Access to 4-(sec-Alkyl)Anilines via 1,6-Conjugate Addition of Grignard Reagents to in Situ Generated aza-p-Quinone Methides

The synthesis of aniline derivatives, common building blocks in many pharmaceuticals, agrochemicals, dyes or polymers, has been limited to reactions based on benzene-toluene-xylene derivatives (BTX) due to their ample availability. Despite the large number of existing methodologies, the synthesis of chiral 4-(sec-alkyl)anilines has not been accomplished so far. In this work, a tandem strategy based on the generation of a reactive aza-p-quinone methide (aza-p-QM) intermediate followed by Cu(I)-catalyzed addition of Grignard reagents has been developed.

Iron(III)-catalyzed tandem annulation of indolyl-substituted p-quinone methides with ynamides for the synthesis of cyclopenta[b]indoles

The unique reactivity of indolyl-substituted p-QMs as a new type of two-carbon synthon has been explored for the first time in a novel iron(III)-catalyzed tandem annulation. This (2+2) annulation/retro-4π electrocyclization/imino-Nazarov cyclization cascade reaction is characterized by an unusual structural reconstruction of indolyl-substituted p-QMs, leading to an expeditious assembly of synthetically important functionalized cyclopenta[b]indoles.