Type of Tetrahydronaphthalene-Fused 1,5-Dipoles and Their Application in Polycyclic Compounds Synthesis

Palladium-catalyzed dipolar cycloaddition reactions represent an efficient strategy for the construction of cyclic compounds, with the development of novel dipolar precursors being a key focus. In this study, a new type of dipolar precursor was synthesized through the assembly of the vinylethylene carbonate unit and the tetrahydronaphthalene skeleton. This dipolar precursor can undergo [3 + 2], [5 + 4], and [5 + 2] cycloaddition reactions, leading to the construction of tetrahydronaphthalene-fused oxazolidin-2-ones, 1,5-oxazonines, and tetrahydrooxepines. In general, all of these reactions exhibited good reaction efficiency and functional group tolerance.

BF3·OEt2 Catalyzed Cascade [4 + 2] Benzannulation of Vinyloxiranes with Coumarins to Construct Benzocoumarin Derivatives

A BF3·OEt2-catalyzed cascade cyclization reaction of vinyloxirane with coumarin is described, affording the benzocoumarin derivatives with moderate to excellent yields (72-92%). The reaction demonstrates exceptional substrate tolerance and has been extensively explored for its potential in drug development, including scale-up experiments, functional group transformations, and screening of the products for anticancer activity. Moreover, the reaction mechanism has been rigorously validated through intermediate trapping and control experiments. Additionally, this reaction represents the uncommon nonmetal catalyzed intermolecular cyclization of vinyloxiranes.

Enantio- and Diastereoselective Copper-Catalyzed Borylative Coupling of Styrenes and Azadienes

Here we disclose a CuB-catalyzed reaction between aurone-derived α,β-unsaturated imines and styrenes to produce 2-substituted benzofuran derivatives bearing both the γ-boryl functionality and α,β-unsymmetric stereogenic centers. The reaction represents the first transition-metal-catalyzed unsymmetric 1,4-Michael additions of azadienes, which would enrich the arsenal of CuB catalysis in organic synthesis. In addition, the synthetically versatile boron-alkylated products can be elaborated by chemical transformations to useful optically active benzofuran heterocycles.

Pd-Catalyzed Three-Component Coupling of Cyclopropenones via Sequential C-C Bond Activation and Allylation

A novel Pd-catalyzed three-component domino reaction for the stereoselective synthesis of highly functionalized allyl cinnamates has been developed. In this protocol, a sequential process of C-C bond activation and intermolecular allylic substitution was well-organized. The key for this transformation is the in situ generated hydrolysis product of cyclopropenone, which triggered a new reaction with vinylethylene carbonates. The reaction mechanism was investigated, demonstrating the high stereoselectivity and excellent atomic economy in this process.

Copper-Catalyzed Enantioselective 1,2-Allylation of Azadienes with Allylboronates

Catalytic asymmetric 1,2-allylation of aurone-derived azadienes is very difficult to achieve due to the driving force for aromatization and the greater steric hindrance of 1,2-addition compared with 1,4-addition. By taking advantage of the ability of nitrogen ligated metal complexes, we successfully demonstrated the first example of copper-catalyzed 1,2-allylation of azadienes with allylboronates for the highly enantioselective synthesis of homoallylic amines. Meanwhile, the enantioenriched 1,4-addition products could also be obtained through a subsequent 3,3-sigmatropic rearrangement of the 1,2-addition products. Extensive DFT calculations were carried out to elucidate the origins of high regioselectivity (1,2- vs 1,4-) and enantioselectivity.

Palladium-Catalyzed [5 + 4] Cycloaddition of 4-Vinyl-4-Butyrolactones with N-Tosyl Azadienes: Construction of Nine-Membered Ring

In this paper, we reported the palladium-catalyzed formal [5 + 4] cycloaddition reactions between 4-vinyl-4-butyrolactones (VBLs) and azadienes. Under mild reaction conditions, a wide range of benzofuran-fused 9-membered heterocyclic compounds had been provided in moderate to excellent yields with exclusive regioselectivities and excellent diastereoselectivities. The practical applicability of the synthesis was demonstrated through scale-up reaction and further transformation.

Enantioselective Construction of Eight-Membered N-Heterocycles from Simple 1,3-Dienes via Pd(0) Lewis Base Catalysis

We report herein an unprecedented enantioselective (4+4) cycloaddition of simple 1,3-dienes with azadienes for the construction of fused eight-membered N-heterocycles. In this transformation, the π-Lewis basic Pd(0) catalyst achieves activation of 1,3-dienes to induce nucleophilic addition to azadienes followed by ring cyclization via a selective terminal allylic substitution. Furthermore, highly efficient and diastereoselective derivatizations of the eight-membered rings provide a facile access to diverse enantiopure fused tetra- to hexacyclic compounds with potential application in medicinal chemistry.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Synthesis of 10-Membered Azecines through Pd-Catalyzed Formal [6+4] Cycloaddition and Their Transannular Reaction to Polycyclic Compounds

Azecine fragments are frequently presented in natural products and bioactive compounds. However, minor efforts have been devoted to these 10-membered N-heterocycles, and their synthesis is still challenging. Reported herein is the first catalytic formal [6+4] cycloaddition for the synthesis of 10-membered azecines. Under palladium catalysis, the reaction of δ-vinylvalerolactones and benzofuran-derived azadienes proceeds smoothly to afford benzofuran-fused azecines with high diastereoselectivity in moderate to good yields. A unique transannular reaction of these 10-membered azecines for the construction of polycyclic compounds is also demonstrated.

Studies on the [4 + 2] cycloaddition and allylic substitution of indole-fused zwitterionic π-allylpalladium

The zwitterionic π-allylpalladium species, also known as dipoles, are important synthons widely used in various reactions including cycloaddition and allylic substitution. This study reported the development of a new indole-fused zwitterionic π-allylpalladium precursor compound and its application in [4 + 2] cycloaddition and allylic substitution reactions. As a result, the synthesis of pyrrolo[3,2,1-ij]quinazolin-3-one and 7-vinyl indole compounds was achieved with moderate to good yields. Notably, the allylic substitution reaction exhibited excellent regio- and stereoselectivity.

Asymmetric cascades of the π-allyl complex: a journey from transition-metal catalysis to metallaphotocatalysis

The enantioselective catalytic cascade involving Tsuji-Trost allylation has provided a viable strategy for the construction of multiple asymmetric C-C and C-X centres and numerous methods have been developed around it for the synthesis of various vital scaffolds. The synthetic utility of this strategy was enhanced by replacing the customary allyl acetates with ethylene diacetates/dicarbonates, vinyl epoxides, vinyl oxetanes, vinyl ethylene carbonates, vinyl cyclopropanes, enynes, and dienes using transition-metal catalysis. One more milestone was achieved when metallaphotocatalysis provided the necessary platform for these cascades by using a cheaper metal. This review will provide a summary of these enantioselective catalytic cascades from 2015.

Highly Regio- and Diastereoselective Phosphine-Catalyzed [2 + 4] Annulation of Benzofuran-Derived Azadienes with Allyl Carbonates: Access to Spiro[benzofuran-cyclohexanes]

A novel highly regio- and diastereoselective phosphine-catalyzed [2 + 4] annulation of benzofuran-derived azadienes (BDAs) with acidic hydrogen-tethered allyl carbonates has been developed ingeniously. A range of functionalized spiro[benzofuran-cyclohexane] derivatives with two consecutive stereocenters were smoothly obtained in moderate to excellent yields under mild reaction conditions from readily available materials. Moreover, this method is a practical and scalable strategy that creates the core structural motif of the fungistatic drug, griseofulvin.

Pd-catalyzed decarboxylative 1,4-addition reactions of benzofuran-based azadienes with allyl phenyl carbonates

Under the catalysis of Pd(OAc)2/dppf/Na2CO3, the decarboxylative 1,4-addition reaction of benzofuran-based azadienes with allyl phenyl carbonates took place easily and delivered the desired products in reasonable chemical yields. The chemical structure of the target compounds was clearly identified by single crystal X-ray structural analysis.

Palladium-catalysed enantio- and regioselective (3 + 2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes towards spirocyclic cyclopentanes

An enantio- and diastereoselective Pd-catalysed (3 + 2) cycloaddition of bis(trifluoroethyl) 2-vinyl-cyclopropane-1,1-dicarboxylate (VCP) with cyclic sulfamidate imine-derived 1-azadienes (SDAs) has been developed. These reactions provide highly functionalized spiroheterocycles having three contiguous stereocentres, including a tetrasubstituted carbon bearing an oxygen functionality. The two geminal trifluoroethyl ester moieties can be manipulated in a facially selective manner to afford more diversely decorated spirocycles with four contiguous stereocentres. In addition, diastereoselective reduction of the imine moiety can also afford a fourth stereocentre and exposes the important 1,2-amino alcohol functionality.

Sterically Hindered and Deconjugative α-Regioselective Asymmetric Mannich Reaction of Meinwald Rearrangement-Intermediate

Compared to γ-addition, the α-addition of α-branched β,γ-unsaturated aldehydes faces larger steric hindrance and disrupts the π-π conjugation, which might be why very few examples are reported. In this article, a highly diastereo- and enantioselective α-regioselective Mannich reaction of isatin-derived ketimines with α-, β- or γ-branched β,γ-unsaturated aldehydes, generated in situ from Meinwald rearrangement of vinyl epoxides, is realized by using chiral N,N'-dioxide/Sc^III catalysts. A series of chiral α-quaternary allyl aldehydes and homoallylic alcohols with vicinal multisubstituted stereocenters are constructed in excellent yields, good d.r. and excellent ee values. Experimental studies and DFT (density functional theory) calculations reveal that the large steric hindrance of the ligand and the Boc (tButyloxy carbonyl) protecting group of imines are critical factors for the α-regioselectivity.

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

Palladium-Catalyzed Asymmetric Cascade Intramolecular Cyclization/Intermolecular Michael Addition Reaction of Allenyl Benzoxazinones with 1-Azadienes

We herein designed and synthesized allenyl benzoxazinones of a novel type, which were then involved in a Pd-catalyzed asymmetric cascade intramolecular cyclization/intermolecular Michael addition reaction with 1-azadienes. A broad range of chiral C2-functionalized quinoline derivatives were afforded in moderate to good yields (up to 93%) with high enantioselectivities (up to 93% ee) in this reaction.

Atroposelective Construction of Nine-Membered Carbonate-Bridged Biaryls

We herein demonstrated an efficient method for the atroposelective construction of nine-membered carbonate-bridged biaryls through vinylidene ortho-quinone methide (VQM) intermediates. Diverse products with desirable pharmacological features were synthesized in satisfying yields and good to excellent enantioselectivities. In subsequent bioassays, several agents showed considerable antiproliferative activity via the mitochondrial-related apoptosis mechanism. Further transformations produced more structural diversity and may inspire new ideas for developing functional molecules.

Pd-Catalyzed [4 + 2] cycloaddition of methylene cyclic carbamates with dihydropyrazolone-derived alkenes: synthesis of spiropyrazolones

In this paper, a palladium-catalyzed [4 + 2] cycloaddition of 5-methylene-1,3-oxazinan-2-ones with 4-arylidene-2,4-dihydro-3H-pyrazol-3-ones has been developed to produce spiropyrazolones in high yields with excellent diastereoselectivities in nearly all cases. The cycloaddition reaction was scaled-up without significant loss of yield, and its synthetic utility has been demonstrated by further transformations of the products. The reaction type of N-Ts cyclic carbamates under palladium catalysis was extended to include [4 + 2] cycloaddition for the first time.

Phosphine-Catalyzed (4 + 2) Annulation of Allenoates with Benzofuran-Derived Azadienes and Subsequent Thio-Michael Addition

A phosphine-catalyzed (4 + 2) annulation of tetrahydrobenzofuranone-derived allenoates and benzofuran-derived azadienes (BDAs) has been achieved to construct the decahydro-2H-naphtho[1,8-bc]furan derivatives, which were subsequently treated with 4-methylbenzenethiol and trimethylamine to produce thio-Michael addition products in high to excellent yields with good diastereoselectivities.

Reductive Opening of Oxetanes Catalyzed by Frustrated Lewis Pairs: Unexpected Aryl Migration via Neighboring Group Participation

B(C₆F₅)₃ was found to catalyze an unusual double reduction of oxetanes by hydrosilane with aryl migration via neighboring group participation. Control experiments suggested that the phenonium ion serves as the key intermediate. Minor modification of this protocol also led to simple hydrosilylative opening of oxetanes.

Atroposelective Synthesis of 1,1'-Bipyrroles Bearing a Chiral N-N Axis: Chiral Phosphoric Acid Catalysis with Lewis Acid Induced Enantiodivergence

We present herein a highly efficient atroposelective synthesis of axially chiral 1,1'-bipyrroles bearing an N-N linkage from simple hydrazine and 1,4-diones. Further product derivatizations led to axially chiral bifunctional compounds with high potential in asymmetric catalysis. For this chrial phosphoric acid (CPA)-catalyzed double Paal-Knorr reaction, an intriguing Fe(OTf)₃ -induced enantiodivergence was also observed.

Asymmetric Synthesis of Pyrrolidines via Oxetane Desymmetrization

Asymmetric synthesis of chiral pyrrolidines bearing an all-carbon quaternary stereocenter in the 3-position remains challenging. Herein we report two efficient protocols by means of oxetane desymmetrization, featuring the use of a readily available tert-butylsulfinamide chiral auxiliary and a catalytic system with chiral phosphoric acid as the source of chirality, respectively.

Substrate-Switched Chemodivergent Pyrazole and Pyrazoline Synthesis: [3 + 2] Cycloaddition/Ring-Opening Rearrangement Reaction of Azadienes with Nitrile Imines

By virtue of a fundamentally new reaction model of benzofuran-derived azadienes (BDAs), an unprecedented synthesis of biologically important pyrazoles has been achieved through a tandem [3 + 2] cycloaddition/ring-opening rearrangement reaction of BDAs with nitrile imines. The nature and type of substrates are found to act as a chemical switch to trigger two distinct reaction pathways. A minor modification to the substrates allows the access to spiro-pyrazolines.

Construction of nine-membered N,N,O-heterocycles via Pd-catalyzed [6+3] dipolar cycloaddition

A new approach for the synthesis of 9-membered N,N,O-heterocycles by Pd-catalyzed [6+3] dipolar cycloaddition of N-iminoisoquinolinium ylides and 2-vinyl oxetanes has been developed.

Palladium-Catalyzed Rearrangement Reaction to Access 1-Phenanthrol Derivatives

This manuscript describes an unusual Pd-catalyzed rearrangement reaction. It provides efficient access to 1-phenanthrol derivatives using allyloxy-tethered aryl iodides. This rearrangement process involves the cleavage of C-I bond, C-O bond...

Asymmetric higher-order [10+n] cycloadditions of palladium-containing 10π-cycloaddends

We uncovered an asymmetric higher-order [10 + 2] cycloaddition reaction between diverse activated alkenes and a new type of π-allylpalladium complex-containing dipole-type 10π-cycloaddend, which was generated in situ from 2-methylene-1-indanols via a dehydrative insertion and deprotonation strategy under double activation of Pd(0) and phosphoric acid. A similar strategy was applied to an asymmetric higher-order [10 + 8] cycloaddition reaction or [10 + 4] cycloaddition reaction by using a heptafulvene derivative or a cyclic enone, respectively, as the acceptor. A variety of polycyclic frameworks imbedding an indene core were generally furnished in moderate to excellent yields with high levels of enantioselectivity by employing a newly designed chiral phosphoramidite ligand.

A type of π-allylpalladium complex-containing 10π-cycloaddend generated from 2-methylene-1-indanols under double activation of Pd(0) and phosphoric acid undergoes asymmetric higher-order [10 + 2] cycloadditions with diverse activated alkenes.

Catalytic Enantioselective Synthesis of 2,3-Dihydrobenzo[b]oxepines via Asymmetric Oxetane Opening by Internal Carbon Nucleophiles

An intramolecular C-C formation process based on catalytic asymmetric oxetane opening by carbon nucleophiles has been developed, which provides rapid access to a range of valuable enantioenriched 2,3-dihydrobenzo[b]oxepines. With the combination of Sc(OTf)₃ and a Box ligand, good chemical efficiency and enantioselectivity were achieved under mild conditions. The products are also useful precursors to other valuable structures, such as the bicyclo[3.2.2]nonane derivatives.

Enantio- and Diastereodivergent Synthesis of Spirocycles through Dual-Metal-Catalyzed [3+2] Annulation of 2-Vinyloxiranes with Nucleophilic Dipoles

The development of efficient and straightforward methods for obtaining all optically active isomers of structurally rigid spirocycles from readily available starting materials is of great value in drug discovery and chiral ligand development. However, the stereodivergent synthesis of spirocycles bearing multiple stereocenters remains an unsolved challenge owing to steric hindrance and ring strain. Herein, we report an enantio- and diastereodivergent synthesis of rigid spirocycles through dual-metal-catalyzed [3+2] annulation of oxy π-allyl metallic dipoles with less commonly employed nucleophilic dipoles (imino esters). A series of spiro compounds bearing a pyrroline and an olefin were easily synthesized in an enantio- and diastereodivergent manner (up to 19:1 dr, >99 % ee), which showed great promise as a new type of N-olefin ligand. Preliminary mechanistic studies were also carried out to understand the process of this bimetallic catalysis.

Stereoselective [4 + 3] annulation of azadienes and ethyl 4-bromo-3-oxobutanoate: construction of benzindeno-fused azepine derivatives

The benzindenoazepine ring system is an attractive scaffold for biologically active compounds. This work reported a NaH-promoted cycloaddition between azadienes and ethyl 4-bromo-3-oxobutanoate, which delivered a series of benzindenoazepines with good yields and stereoselectivities. Such benzindenoazepine derivatives were not easily obtained by using a traditional approach. The application of this cycloaddition strategy has been extended to azadienes bearing a benzofuran or benzothiophene moiety. The utility of this method was showcased by gram-scale experiments and synthetic transformations of the product.

Catalyst-controlled switchable [4 + 1], [4 + 3] and [3 + 2] domino reactions of azadienes and MBH carbonates: diverse synthesis of benzothiophene fused derivatives

Catalyst-controlled switchable domino reactions between azadienes bearing a benzothiophene moiety and isatin-derived MBH carbonates were developed. The [4 + 1] annulation was triggered in the presence of DABCO, giving a variety of benzothiophene fused pyrrole derivatives, while the [4 + 3] annulation occurred when changing the catalyst to DMAP. Furthermore, the [3 + 2] annulation mode was observed with the use of catalytic Ph₂PMe. Additionally, the synthetic utility of these domino reactions was demonstrated by gram-scale experiments and simple transformations of the products. To the best of our knowledge, catalyst-controlled synthesis of benzothiophene fused or spiro derivatives has rarely been reported.

Access to Indole-Annulated Medium-Sized Lactams through Protonation/Deuteration-Induced Ring-Opening of Spiroindolines

A protocol has been developed to access indole-annulated eight- and nine-membered lactams through protonation-induced ring-opening of spiroindolines, which are dearomative Heck products of tetrahydro-β-carbolines or hexahydroazepino[3,4-b]indoles. Brønsted acids and nucleophiles were explored and compared in the transformation. A combination of deuterated hydrochloride and deuterated methanol enables deuterative ring-opening of spiroindolines to afford medium-sized lactam diastereoisomers with a deuterium content ratio around 1:1.

Diversity-oriented synthesis of benzofuro[3,2-b]pyridine derivatives from aurone-derived α,β-unsaturated imines and activated terminal alkynes

An efficient annulation reaction of aurone-derived α,β-unsaturated imines and activated terminal alkynes mediated by triethylamine is described, which enables the facile synthesis of 1,4-dihydrobenzofuro[3,2-b]pyridines in high yields. When the nucleophile of triethylamine was replaced with triphenylphosphine, another class of 1,4-dihydrobenzofuro[3,2-b]pyridines tethered with an additional acrylate motif were obtained instead. These two types of 1,4-dihydrobenzofuro[3,2-b]pyridines could be aromatized in the presence of DBU to afford benzofuro[3,2-b]pyridines, which could also be accessed via a one-pot procedure.

Designing and Accurately Developing a [6 + 2] Dipolar Cycloaddition for the Synthesis of Benzodiazocines

1,6-Dipolar cycloadditions represent a valuable strategy for the rapid construction of medium-sized rings. Herein, we describe the concept for the design of 1,6-dipoles that bypasses the regioselectivity. Through the introduction of an amino group into Morita-Baylis-Hillman (MBH) carbonates, unprecedented [6 + 2] dipolar cycloadditions were accurately developed with Cs₂CO₃, efficiently delivering a series of benzodiazocines in mild conditions. Computational studies bring a deeper understanding of this reaction.

Asymmetric Catalytic Vinylogous Addition Reactions Initiated by Meinwald Rearrangement of Vinyl Epoxides

The first catalytic asymmetric multiple vinylogous addition reactions initiated by Meinwald rearrangement of vinyl epoxides were realized by employing chiral N,N'-dioxide/Sc^III complex catalysts. The vinyl epoxides, as masked β,γ-unsaturated aldehydes, via direct vinylogous additions with isatins, 2-alkenoylpyridines or methyleneindolinones, provided a facile and efficient way for the synthesis of chiral 3-hydroxy-3-substituted oxindoles, α,β-unsaturated aldehydes and spiro-cyclohexene indolinones, respectively with high efficiency and stereoselectivity. The control experiments and kinetic studies revealed that the Lewis acid acted as dual-tasking catalyst, controlling the initial rearrangement to match subsequent enantioselective vinylogous addition reactions. A catalytic cycle with a possible transition model was proposed to illustrate the reaction mechanism.

Ligand-Controlled Regiodivergent Asymmetric [5 + 2] and [3 + 2] Annulations of Vinyl Indoloxazolidones Catalyzed by Palladium

Here, we present palladium-catalyzed regiodivergent asymmetric annulations of vinyl indoloxazolidones, which can act as 1,5-carbodipoles or 1,3-carbodipoles by tuning the chiral ligands and conditions, in the assemblies with sulfamate-derived cyclic imines and even activated alkenes. A diversity of polycyclic products are generally constructed with high regio- and enantioselectivity.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Rhodium-Catalyzed Denitrogenative Diazole-Triazole Coupling toward Aza-Bridged Structures and Imidazole-Based Chelating Ligands

1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by Rh₂(Piv)₄-catalyzed denitrogenative coupling of pyrazoles with 1-sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes via intramolecular aza-Diels-Alder cycloaddition. This domino reaction, combined with the subsequent thermal or acid-catalyzed rearrangement of the cycloadducts, provides direct and flexible access to N-sulfonylated (Z)-2-(2-aminovinyl)imidazoles.

A Palladium-Catalyzed Oxa-(4+4)-Cycloaddition Strategy Towards Oxazocine Scaffolds

A Pd-catalyzed oxa-(4+4)-cycloaddition between 1-azadienes and (2-hydroxymethyl)allyl carbonates is described. Aurone-derived azadienes furnished polycyclic 1,5-oxazocines in good yields. Interestingly, linear azadienes have also been involved and yielded monocyclic heterocycles with complete regioselectivity. DFT calculations were carried out to gain insight on this observation.