Synthesis of Secondary Unsaturated Lactams via an Aza-Heck Reaction

Revisiting the 1,3-azadiene-succinic anhydride annulation reaction for the stereocontrolled synthesis of allylic 2-oxopyrrolidines bearing up to four contiguous stereocenters

Polysubstituted 2-oxopyrrolidines bearing at least two contiguous stereocenters constitute the core of several pharmaceuticals, including clausenamide (antidementia). Here, we describe a flexible annulation strategy, which unites succinic anhydride and 1,3-azadienes to produce allylic 2-oxopyrrolidines bearing contiguous stereocenters. The approach is chemoselective, efficient, modular, scalable, and diastereoselective. The scalable nature of the reactions offers the opportunity for post-diversification, leading to incorporation of motifs with either known pharmaceutical value or that permit subsequent conversion to medicinally relevant entities.

Enantioselective Synthesis of Isoindolinone by Palladium-Catalyzed Aminoalkynylation of O-Phenyl Hydroxamic Ethers with Alkynes

A highly efficient palladium-catalyzed asymmetric tandem aza-Heck/Sonogashira coupling reaction of O-phenyl hydroxamic ethers with terminal alkynes is described. This protocol enables versatile access to challenging chiral isoindolinone derivatives bearing a quaternary stereogenic center. The palladium-catalyzed aminoalkynylation reaction shows broad functional group tolerance and allows the straightforward preparation of isoindolinones with high efficiency and excellent enantioselectivity under mild conditions. DFT calculations were performed to disclose the reaction mechanism and the origins of the enantioselectivity.

Palladium-Catalyzed E-Selective Oxidative Amination of Aromatic Amine with 3-Butenoic Acid

A palladium-catalyzed oxidative amination of inactive olefins with an aromatic amine was developed using a copper acetate oxidant to yield corresponding secondary and tertiary enamines in moderate to good yields. This new procedure outlines an efficient approach for the construction of enamine skeletons.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Diastereospecific arylation and cascade deconstructive amidation/thioesterification of readily available lactam-fused bromolactones

An intrinsic goal when designing synthetic methodology is to identify approaches whereby readily accessible precursors are converted into an array of products, which efficiently tap into new 3D-chemical space. In these studies, readily available bicyclic lactam-bromolactones have been interrogated in several fragment growth protocols by utilizing the halogen and lactone motifs as versatile linchpins for strategic construction of C-C, C-N, C-O, and C-S bonds. Diastereospecific C(sp3)-C(sp2) Kumada coupling of sterically imposing [5,5]-bicyclic lactam-bromolactones with several aryl Grignard reagents, under palladium catalysis, furnishes diarylmethane-tethered lactam-lactones in synthetically attractive yields, stereoinvertive fashion, and with a tolerance for many functional groups. When [5,6]-bicyclic lactam-bromolactones, which are prone to β-hydride elimination are employed, efficient arylation is observed only under Co(acac)3-catalyzed conditions. Importantly, these [5,6]-bicyclic lactam-bromolactones undergo retentive arylation, independent of the transition metal catalyst. A base-mediated cascade deconstructive amidation of the [5,6]-bicyclic lactam-bromolactones with primary aliphatic amines proceeds efficiently to afford epoxide-tethered lactam carboxamides, which bear four contiguous stereocenters. Furthermore, an unusual route to homoallylic thioesters has been uncovered through deconstructive contra-thermodynamic thioesterification of the lactam-fused bromolactone precursors.

Unexpected Cascade Sequence Forming the C(sp3)-N/C(sp2)-C(sp2) Bond: Direct Access to γ-Lactam-Fused Pyridones with Anticancer Activity

An unexpected Ugi cascade reaction was developed for the facile construction of γ-lactam-fused pyridone derivatives with high tolerance of substrates. A C(sp3)-N bond and a C(sp2)-C(sp2) bond were formed together, accompanied by a chromone ring-opening in Ugi adducts, under the basic conditions without any metal catalyst for the whole process. Screening data of several difficult-to-inhibit cancer cell lines demonstrated that 7l displayed a high cytotoxicity against HCT116 cells (IC50 = 5.59 ± 0.78 μM). Taken together, our findings revealed new insights into the molecular mechanisms underlying compound 7l and provided potential usage of this scaffold for cancer therapeutics.

Palladium-Catalyzed Alkenyl C-H Activation/Diamination toward Tetrahydrocarbazole and Analogs Using Hydroxylamines as Single-Nitrogen Sources

A palladium-catalyzed alkenyl C-H activation/diamination reaction of cycloalkenyl bromoarenes with hydroxylamines is described. A wide range of tetrahydrocarbazoles and analogs has been prepared using fine-tuning bifunctional secondary hydroxylamines as the single-nitrogen sources. Mechanistic investigations suggest that the selective alkenyl C-H activation/diamination cascade process should build the N-heterocycles.

Cyclopropane-Fused N-Heterocycles via Aza-Heck-Triggered C(sp3)-H Functionalization Cascades

Unique examples of aza-Heck-based C(sp³)–H functionalization cascades are described. Under Pd(0)-catalyzed conditions, the aza-Heck-type cyclization of N-(pentafluorobenzoyloxy)carbamates generates alkyl–Pd(II) intermediates that effect C(sp³)–H palladation en route to cyclopropanes. Key factors that control the site selectivity of the cyclopropanation process have been elucidated such that selective access to a wide range of ring- or spiro-fused systems can be achieved.

Radical Aza-Heck Cyclization of Imidates via Energy Transfer, Electron Transfer, and Cobalt Catalysis

A radical aza-Heck cyclization has been developed to afford functionally rich products with four contiguous C-heteroatom bonds. This multi-catalytic strategy provides rapid syntheses of dense, medicinally relevant motifs by enabling the conversion of alcohol-derived imidates to heteroatom-rich fragments containing vinyl oxazolines/oxazoles, allyl amines, β-amino alcohols/halides, and combinations thereof. Mechanistic insights of this process show how three distinct photocatalytic cycles cooperate to enable: (1) imidate radical generation by energy transfer, (2) dehydrogenation by Co catalysis, and (3) catalyst turnover by electron transfer.

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C-N bond-forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Alkyne Aminopalladation/Heck and Suzuki Cascades: An Approach to Tetrasubstituted Enamines

Alkyne aminopalladation reactions starting from tosylamides are reported. The emerging vinylic Pd species are converted either in an intramolecular Heck reaction with olefinic units or in an intermolecular Suzuki reaction by using boronic acids exhibiting broad functional group tolerance. Tetra(hetero)substituted tosylated enamines are obtained in a simple one-pot process.

Complex Polyheterocycles and the Stereochemical Reassignment of Pileamartine A via Aza-Heck Triggered Aryl C-H Functionalization Cascades

Structurally complex benzo- and spiro-fused N-polyheterocycles can be accessed via intramolecular Pd(0)-catalyzed alkene 1,2-aminoarylation reactions. The method uses N-(pentafluorobenzoyloxy)carbamates as the initiating motif, and this allows aza-Heck-type alkene amino-palladation in advance of C–H palladation of the aromatic component. The chemistry is showcased in the first total synthesis of the complex alkaloid (+)-pileamartine A, which has resulted in the reassignment of its absolute stereochemistry.

Total Synthesis of (±)-Impatien A via Aza-Heck Cyclization

The first total synthesis of the natural product impatien A is described. This concise synthesis features an aza-Heck cyclization to construct the complex spirocyclic ring system and provides a rare example of the use of aza-Heck cyclizations in complex molecule synthesis. To enable this key cyclization of an electrophilic nitrogen atom with a tetrasubstituted alkene, we utilized high-throughput experimentation to identify a new ligand and ultimately deliver impatien A in seven steps from known compounds.

A Bridge to Alkaloid Synthesis

The construction of a structurally rigid architecture with chiral complexity, necessary to enhance the interaction with binding sites of drug targets, has been adapted as an intriguing approach in drug development. In the past few years, we have been interested in the synthesis of biologically significant and bridged alkaloids via novel synthetic methods and strategies based on recognition of the privileged pattern. Therefore, nitroso-ene and aza-Wacker cyclizations were elevated for the first time to construct bridged alkaloids, such as hosieine A, kopsone, melinonine-E and strychnoxanthine. Mechanistic investigations, including computational calculations for nitroso-ene reaction and deuterated experiments for aza-Wacker reaction, enable us to gain more insights into the chemical reactivity and selectivity of specific functional groups in developing viable synthetic methods.

Tandem aza-Heck Suzuki and carbonylation reactions of O-phenyl hydroxamic ethers: complex lactams via carboamination

The palladium-catalysed tandem aza-Heck-Suzuki and aza-Heck-carbonylation reactions of O-phenyl hydroxamic ethers are reported. These formal alkene carboamination reactions provide highly versatile access to wide range complex, stereogenic secondary lactams and exhibit outstanding functional group tolerance and high diastereoselectivity.

Asymmetric synthesis of 3-benzyl and allyl isoindolinones by Pd-catalyzed dicarbofunctionalization of 1,1-disubstituted enamides

The Pd-catalyzed enantioselective Heck/Suzuki reaction of 1,1-disubstituted enamides with aryl/vinyl boronic acids has been developed to access 3-benzyl/allyl substituted isoindolinones bearing a tetrasubstituted stereogenic carbon center.

Photosensitized Intramolecular [2+2] Cycloaddition of 1H-Pyrrolo[2,3-b]pyridines Enabled by the Assistance of Lewis Acids

The [2+2] photocycloaddition of alkenyl-tethered 1H-pyrrolo[2,3-b]pyridine derivatives sensitized with 3',4'-dimethoxyacetophenone under irradiation by a high-pressure mercury lamp through Pyrex glass was dramatically accelerated by the addition of Lewis acids, preferably Mg(OTf)₂, to give the products stereoselectively in high yields. The reaction without a Lewis acid gave only small amounts of the [2+2] cycloaddition products. Conformational fixation of the substrates by coordination with a Lewis acid was presumed to facilitate the cycloaddition.

Concise Synthesis and Antimicrobial Evaluation of the Guanidinium Alkaloid Batzelladine D: Development of a Stereodivergent Strategy

Herein, we describe a stereodivergent route to (±)-batzelladine D (2), (+)-batzelladine D (2), (-)-batzelladine D (2), and a series of stereochemical analogues and explore their antimicrobial activity for the first time. The concise synthetic approach enables access to the natural products in a sequence of 8-12 steps from readily available building blocks. Highlights of the synthetic strategy include gram-scale preparation of a late stage intermediate, pinpoint stereocontrol around the tricyclic skeleton, and a modular strategy that enables analogue generation. A key bicyclic β-lactam intermediate not only serves as the key controlling element for pyrrolidine stereochemistry but also serves as a preactivated coupling partner to install the ester side chain. The stereocontrolled synthesis allowed for the investigation of the antimicrobial activity of batzelladine D, demonstrating promising activity that is more potent for non-natural stereoisomers.

Hydroxylamines As Bifunctional Single-Nitrogen Sources for the Rapid Assembly of Diverse Tricyclic Indole Scaffolds

Conventional approaches on using hydroxylamine derivatives as single nitrogen sources, for the construction of n-membered (n > 3) N-heterocycles, rely upon two chemical operations by involving sequential nucleophilic and electrophilic C-N bond formations. Here, we report a highly efficient cascade of alkyne insertion/C-H activation/amination for the rapid preparation of a myriad of tricyclic indoles, in a single-step transformation, by using bifunctional secondary hydroxylamines. It is noteworthy that judicious selection of applicable amino agents, for enabling the prior oxidative addition of aryl iodide to initial Pd(0) species and subsequent two C-N bonds formation, was the key to the success of this reaction. Control experiments indicated that a five-membered palladacyclic intermediate played a crucial role in promoting the final aminative ring closure.

Synthesis of Lactams via Ir-Catalyzed C-H Amidation Involving Ir-Nitrene Intermediates

x-membered lactams were synthesized via either an amidation of sp³ C-H bonds or an electrophilic substitution of arenes via Ir-nitrene intermediates. With the employment of a readily available iridium catalyst in dichloromethane or hexafluoro-2-propanol, a wide range of lactams were synthesized in good to excellent yields with high selectivity.

A Facile Direct Route to N-(Un)substituted Lactams by Cycloamination of Oxocarboxylic Acids without External Hydrogen

Lactams are privileged in bioactive natural products and pharmaceutical agents and widely featured in functional materials. This study presents a novel versatile approach to the direct synthesis of lactams from oxocarboxylic acids without catalyst or external hydrogen. The method involves the in situ release of formic acid from formamides induced by water to facilitate efficient cycloamination. Water also suppresses the formation of byproducts. This unconventional pathway is elucidated by a combination of model experiments and density functional theory calculations, whereby cyclic imines (5-methyl-3,4-dihydro-2-pyrrolone and its tautomeric structures) are found to be favorable intermediates toward lactam formation, in contrast to the conventional approach encompassing cascade reductive amination and cyclization. This sustainable and simple protocol is broadly applicable for the efficient production of various N-unsubstituted and N-substituted lactams.

Synthesis of Indolines and Derivatives by Aza-Heck Cyclization

For the first time, an aza-Heck cyclization that allows the preparation of indoline scaffolds is described. Using N-hydroxy anilines as electrophiles, which can be easily accessed from the corresponding nitroarenes, this method provides indolines bearing pendant functionality and complex ring topologies. Synthesis of challenging indolines, such as those bearing fully substituted carbon atoms at C2, is also possible using this method.

Cu-catalyzed oxygenation of alkene-tethered amides with O2 via unactivated C[double bond, length as m-dash]C bond cleavage: a direct approach to cyclic imides

An efficient aerobic unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage process was achieved, in which the succinimide or glutarimide derivatives could be prepared directly from alkenyl amides.

The transformations of unactivated alkenes through C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond double cleavage are always attractive but very challenging. We report herein a chemoselective approach to valuable cyclic imides by a novel Cu-catalyzed geminal amino-oxygenation of unactivated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds. O₂ was successfully employed as the oxidant as well as the O-source and was incorporated into alkenyl amides via C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond cleavage for the efficient preparation of succinimide or glutarimide derivatives. Moreover, the present strategy under simple conditions can be used in the late-stage modification of biologically active compounds and the synthesis of pharmaceuticals, which demonstrated the potential application.

Stereoselective Conjugate Addition of the Lithium Anion of N-Allyl Imine to Unsaturated Esters: Application to the Enantiospecific Total Synthesis of (-)-Epibatidine

A regio- and diastereoselective conjugate addition of the lithium anion of N-allyl imine (prepared from allylamine and benzophenone) to α,β-unsaturated esters in good yields is reported. The reaction was general and provided the γ-amino esters resulting from the regioselective C-C bond formation between the α-carbon to the nitrogen in the imine and the β-carbon of the unsaturated ester. Synthetic utility of the formed products was illustrated in the nonracemic total synthesis of the bioactive alkaloid (-)-epibatidine.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

Formal Aza-Wacker Cyclization by Tandem Electrochemical Oxidation and Copper Catalysis

In oxidative electrochemical organic synthesis, radical intermediates are often oxidized to cations on the way to final product formation. Herein, we describe an approach to transform electrochemically generated organic radical intermediates into neutral products by reaction with a metal catalyst. This approach combines electrochemical oxidation with Cu catalysis to effect formal aza-Wacker cyclization of internal alkenes. The Cu catalyst is essential for transforming secondary and primary alkyl radical intermediates into alkenes. A wide range of 5-membered N-heterocycles including oxazolidinone, imidazolidinone, thiazolidinone, pyrrolidinone, and isoindolinone can be prepared under mild conditions.

Construction of N-Heterocyclic Systems Containing a Fully Substituted Allylic Carbon by Palladium/Phosphine Catalysis

The unique cyclization of benzamide derivatives that contain an alkyne by a Pd(0)/dialkyl(biaryl)phosphine catalytic system is reported. The reaction efficiently provides a variety of six-membered N-heterocyclic compounds that contain a fully substituted carbon center without the need for a stoichiometric additive. Mechanistic studies suggest that this unprecedented cyclization starts with the cleavage of a propargylic C-O bond, and a 1,3-diene has been identified as a relevant intermediate.

Intramolecular Palladium-Catalyzed Oxidative Amination of Furans: Synthesis of Functionalized Indoles

Unconventional modification of palladium-catalyzed oxidative amination where a furan ring serves as a masked olefin is described. The designed chemical process provides 2-(2-acylvinyl)indole derivatives with up to a 93% yield and excellent E-selectivity. A highly reactive α,β-unsaturated carbonyl moiety of the obtained compounds allows for accessing various heteroaromatic scaffolds through simple synthetic procedures.

Synthesis of N-H Bearing Imidazolidinones and Dihydroimidazolones Using Aza-Heck Cyclizations

The synthesis of unsaturated, unprotected imidazolidinones via an aza-Heck reaction is described. This palladium-catalyzed process allows for the cyclization of N-phenoxy ureas onto pendant alkenes. The reaction has broad functional group tolerance, can be applied to complex ring topologies, and can be used to directly prepare mono- and bis-unprotected imidazolidinones. By addition of Bu4 NI, dihydroimidazolones can be accessed from the same starting materials. Improved conditions for preparing unsaturated, unprotected lactams are also reported.

Approach to Chiral 1-Substituted Isoquinolone and 3-Substituted Isoindolin-1-one by Addition-Cyclization Process

An approach to access 1-substituted isoquinolones has been developed through the addition-cyclization of imines with Grignard reagents in the presence of 2,2'-dipyridyl. A number of substituted aromatic magnesium reagents were amenable to this process, and the desired products were obtained with excellent yields and outstanding diastereoselectivities ( dr > 99:1). The utility of this convenient approach is demonstrated by the formal synthesis of ( S)-cryptostyline II. Moreover, N-methylmorpholine (NMM) was found to be an effective additive for the formation of 3-substituted isoindolin-1-ones using one-pot addition-cyclization-deprotection of imine with Grignard reagents.

Pyrrolidines and Piperidines by Ligand-Enabled Aza-Heck Cyclizations and Cascades of N-(Pentafluorobenzoyloxy)carbamates

Ligand-enabled aza-Heck cyclizations and cascades of N-(pentafluorobenzoyloxy)carbamates are described. These studies encompass the first examples of efficient non-biased 6-exo aza-Heck cyclizations. The methodology provides direct and flexible access to carbamate protected pyrrolidines and piperidines.