An intramolecular [2 + 2] cycloaddition of ketenimines via palladium-catalyzed rearrangements of N-allyl-ynamides

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Radical Chain Isomerization of N-Sulfonyl Ynamides to Ketenimines and Its Application to Furan Dearomatization

A radical chain isomerization of N-sulfonyl ynamides to isolable ketenimines is developed, featuring mild reaction conditions, a high efficiency, ∼100% atom economy, a broad substrate scope, and column chromatography-free workup in most cases. Meanwhile, an unprecedented dearomatization of furans is achieved by the radical chain isomerization-triggered aza-Claisen rearrangement, providing highly chemo-, regio-, stereo-, and diastereoselective access to functionalized quaternary nitriles.

Media-Driven Pd-Catalyzed Reaction Cascades with 1,3-Diynamides Leading Selectively to Either Indoles or Quinolines

Divergent Pd-catalyzed reaction cascades with various 1,3-diynamides yielding either 2-amino-3-alkynylindoles or 2-amino-4-alkenylquinolines were established. Omitting or adding TBAF (tetrabutylammonium fluoride) to the reaction of N,N-(2-iodophenyl)(4-toluenesulfonyl)-1,3-diynamides with secondary or primary amines in the presence of KOH in THF and catalytic amounts of Pd(PPh₃ )₄ completely changed the outcome of the reaction. In the absence of TBAF, 2-amino-3-alkynylindoles were the sole products, while the presence of TBAF switched the product formation to 2-amino-4-alkenylquinolines. Deuterium labeling proceeded selectively at the C3 and C11 positions of the 2-amino-4-alkenylquinoline products and this suggests the unprecedented formation of [4]cumulenimines from 1,3-diynamides as reactive key intermediates.

Synthesis of N-(Isoquinolin-1-yl)sulfonamides via Ag2O-Catalyzed Tandem Reaction of ortho-Alkynylbenzaldoximes with Benchtop Stabilized Ketenimines

In this project, a moderately efficient approach to multisubstituted N-(isoquinolin-1-yl)sulfonamide derivatives was illustrated, utilizing ortho-alkynylbenzaldoximes and zwitterionic ketenimine salts in a tandem reaction catalyzed by silver oxide. The oxophilicity of Ag₂O, along with its nature as Lewis acid, pave the way for a smooth [3 + 2] cycloaddition between isoquinoline N-oxides and ketenimine species, which is a key step in this reaction. DFT calculation suggests that 1,3-dipolar cycloaddition of nitrone and ketenimine proceeds through a selective stepwise mechanism.

Copper-catalyzed alkynylation/annulation cascades of N-allyl ynamides: regioselective access to medium-sized N-heterocycles

A synthetic strategy based on sequential application of aza-Claisen rearrangement, C–H functionalization, C–N coupling and cyclization as key steps has been developed for the synthesis of 6-, 7-, 8-, and 9-membered rings N-heterocycles.

Synthesis of polysubstituted azetidines via cascade trifluoromethylation/cyclization of N-allyl ynamides

A cascade trifluoromethylation/cyclization of N-allyl sulfonylynamides is developed, providing a direct access to azetidine-fused tricyclic compounds at room temperature.

Asymmetric synthesis with ynamides: unique reaction control, chemical diversity and applications

Ynamides are among the most powerful building blocks in organic synthesis and have become invaluable starting materials for the construction of multifunctional compounds and challenging architectures that would be difficult to prepare otherwise. The rapidly growing popularity originates from the unique reactivity and ease of manipulation of the polarized ynamide triple bond, the advance of practical methods for making them, and the simplicity of storage and handling. These attractive features and the demonstration of numerous synthetic applications have spurred the development of intriguing asymmetric reaction strategies during the last decade. An impressive variety of chemo-, regio- and stereoselective carbon-carbon and carbon-heteroatom bond forming reactions with ynamides have been developed and now significantly enrich the toolbox of synthetic chemists. This review provides a comprehensive overview of asymmetric ynamide chemistry since 2010 with a focus on the general scope, current limitations, stereochemical reaction control and mechanistic aspects.

Annulation Cascades of N-Allyl- N-((2-bromoaryl)ethynyl)amides Involving C-H Functionalization

An annulation cascades of N-allyl- N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C-H functionalization for producing 2,3-functionalized indoles has been first developed by means of Cu catalysis. The method is enabled by the formation of the ketenimine intermediates to deliver 2,3-disubstituted indoles through a sequence of aza-Claisen rearrangement, C-H functionalization, Ullmann C-N coupling, and cyclization.

Catalytic Double [2 + 2] Cycloaddition Relay Enabled C-C Triple Bond Cleavage of Yne-Allenones

An unusual catalytic double [2 + 2] cycloaddition relay reaction of yne-allenones with unactivated alkenes and alkynes has been achieved, which enabled C-C triple-bond cleavage to access more than 60 examples of functionalized phenanthren-9-ols with generally good yields. This reaction provides a regioselective and practical method for the construction of carbocyclic ring systems with a high degree of functional group compatibility. Aside from surveying the scope of this transformation, mechanistic details of this process are provided by conducting systematic theoretical calculations.

A copper-catalyzed three component reaction of aryl acetylene, sulfonyl azide and enaminone to form iminolactone via 6π electrocyclization

We developed a copper-catalyzed three component reaction of aryl acetylene, enaminone and sulfonyl azide to construct iminolactone via copper-catalyzed alkyne–azide cycloaddition (CuAAC), Michael addition of metalated ketenimine followed by elimination and 6π electrocyclization.

Total Syntheses of (±)-Rhodonoids A and B and C12-epi-Rhodonoid B

Total syntheses of (±)-rhodonoids A and B and C12-epi-rhodonoid B are described here. A unified strategy employed in these syntheses is an intramolecular oxa-[3 + 3] annulation for accessing the chromene unit. A Fe(OTf)₃-promoted diastereoselective cationic [2 + 2] cycloaddition and a photochemical [2 + 2] cycloaddition were featured to construct the cyclobutane core of (±)-rhodonoids A and B and C12-epi-rhodonoid B, respectively. Fe(OTf)₃ also leads to an interesting bridged tetracycle, which was unambiguously confirmed by single crystal X-ray analysis.

Divergent C-H Insertion-Cyclization Cascades of N-Allyl Ynamides

Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp(3) )H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N-allyl ynamides to form fused nitrogen-heterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold-catalyzed synthesis of densely functionalized C(sp(3) )-rich polycycles and a copper-catalyzed synthesis of fused pyridine derivatives. The respective gold-keteniminium and ketenimine activation pathways have been explored through a structure-reactivity study, and isotopic labeling identified turnover-limiting CH bond-cleavage in both processes.

Ynamides in ring forming transformations

The ynamide functional group activates carbon-carbontriple bonds through an attached nitrogen atom that bears an electron-withdrawing group. As a result, the alkyne has both electrophilic and nucleophilic properties. Through the selection of the electron-withdrawing group attached to nitrogen, chemists can modulate the electronic properties and reactivity of ynamides, making these groups versatile synthetic building blocks. The reactions of ynamides also lead directly to nitrogen-containing products, which provides access to important structural motifs found in natural products and molecules of medicinal interest. Therefore, researchers have invested increasing time and research in the chemistry of ynamides in recent years. This Account surveys and assesses new organic transforma-tions involving ynamides developed in our laboratory and in others around the world. We showcase the synthetic power of ynamides for rapid assembly of complex molecular structures. Among the recent reports of ynamide transformations, ring-forming reactions provide a powerful tool for generating molecular complexity quickly. In addition to their synthetic utility, such reactions are mechanistically interesting. Therefore, we focus primarily on the cyclization chemistry of ynamides. This Account highlights ynamide reactions that are useful in the rapid synthesis of cyclic and polycyclic structural manifolds. We discuss the mechanisms active in the ring formations and describe representative examples that demonstrate the scope of these reactions and provide mechanistic insights. In this discussion, we feature examples of ynamide reactions involving radical cyclizations, ring-closing metathesis, transition metal and non-transition metal mediated cyclizations, cycloaddition reactions, and rearrangements. The transformations presented rapidly introduce structural complexity and include nitrogen within or in close proximity to a newly formed ring (or rings). Thus, ynamides have emerged as powerful synthons for nitrogen-containing heterocycles and nitrogen-substituted rings, and we hope this Account will promote continued interest in the chemistry of ynamides.

SYNTHESIS OF DE NOVO CHIRAL γ-AMINO-YNAMIDES USING LITHIATED YNAMIDES. OBSERVATION OF A UNIQUE 5-ENDO-DIG CYCLIZATION WITH AN INVERSION OF S-CENTER

We describe herein details of our efforts in developing a highly stereoselective synthesis of de novo chiral γ-amino-ynamides through additions of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines. While additions of ynamides could be highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. On the other hand, additions of oxazolidinone-substituted, oxazinanone-substituted and tetrahydropyrimidinone-substituted ynamides behaved quite differently and functioned better with BF3-OEt2. The chirality of the oxazolidinone ring exerts no impact on the selectivity. This work also features a unique 5-endo-dig cyclization of oxazolidinone-substituted γ-amino-ynamides that could be promoted with acid, leading to isothiazoles and 2,3-dihydro-isothiazole S-oxides.

A Convenient Synthesis of γ-Amino-Ynamides via Additions of Lithiated Ynamides to Aryl Imines. Observation of an Aza-Meyer-Schuster Rearrangement

Efforts in developing an expeditious and convenient method for synthesizing γ-amino-ynamides via nucleophilic addition of lithiated ynamides to aryl imines are described. This work also features an aza-variant of a Meyer-Schuster rearrangement of γ-amino-ynamides and the synthetic utility of γ-amino-ynamides in an intramolecular ketenimine-[2 + 2] cycloaddition.

Synthesis of cyclopentenimines from N-allyl ynamides via a tandem aza-Claisen rearrangement-carbocyclization sequence

We describe here details of our investigations into Pd-catalyzed and thermal aza-Claisen-carbocyclizations of N-allyl ynamides to prepare a variety of α,β-unsaturated cyclopentenimines. The nature of the ynamide electron-withdrawing group and β-substituent plays critical roles in the success of this tandem cascade. With N-sulfonyl ynamides, the use of palladium catalysis is required, as facile 1,3-sulfonyl shifts dominate under thermal conditions. However, since no analogous 1,3-phosphoryl shift is operational, N-phosphoryl ynamides could be used to prepare similar cyclopentenimines under thermal conditions through zwitter ionic intermediates that undergo N-promoted H-shifts. Alternatively, by employing ynamides bearing tethered carbon nucleophiles, the zwitter ionic intermediates could be intercepted, giving rise rapidly to more complex fused bi- and tricyclic scaffolds.

A highly stereoselective addition of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines

A highly diastereoselective addition of lithiated ynamides to Ellman-Davis chiral imines is described. While additions of N-sulfonyl ynamides are highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. In addition, oxazolidinone-substituted ynamides behaved differently and functioned better with BF3-OEt2, and the chirality of the oxazolidinone ring exerts no impact on the selectivity.

Rhodium-catalyzed [2 + 2] cycloaddition of ynamides with nitroalkenes

In the presence of a diene-ligated rhodium complex, ynamides and nitroalkenes undergo catalytic [2 + 2] cycloadditions to provide cyclobutenamides. The presence of sodium tetraphenylborate was found to be crucial for the reactions to proceed efficiently.