Access to Cycloalkeno[c]-Fused Pyridines via Pd-Catalyzed C(sp2)-H Activation and Cyclization of N-Acetyl Hydrazones of Acylcycloalkenes with Vinyl Azides

Access to [2.2]Paracyclophane-Fused N-Heterocycles via Palladium-Catalyzed C-H Activation and Cyclization of [2.2]Paracyclophaneacylhydrazones with Vinyl Azides

A novel protocol for the synthesis of [2.2]paracyclophane-fused N-heterocycles via palladium-catalyzed C-H activation and cyclization of [2.2]paracyclophaneacylhydrazones with vinyl azides has been developed. High regioselectivity control was realized by selective activation/functionalization of the C-H bond of highly rigid [2.2]paracyclophanes. A broad range of [2.2]paracyclophane-fused isoquinolinophane derivatives were prepared in sole regioselectivity with good functional group tolerance, which would be difficult to synthesize by other approaches in a one-step manner.

Transaminase-Triggered Cascades for the Synthesis and Dynamic Kinetic Resolution of Chiral N-Heterocycles

Biocatalysis is now a well-established branch of catalysis and the growing toolbox of natural, evolved and designer enzymes is enabling chemistry previously deemed inaccessible. However, most enzyme methodologies have been developed for functional group interconversions, such as the conversion of a ketone into an amine or alcohol, and do not result in the generation of significant 3D molecular complexity. The application of enzyme-triggered reaction cascade methodologies has the potential to transform simple substrates into complex sp3-rich molecules in one step. Herein, we describe a single-step biocatalytic route to high-value, complex indolizidine, and quinolizidine alkaloids, which relies on a transaminase-triggered double intramolecular aza-Michael reaction. This approach allows access to architecturally complex, natural-product-like N-heterocycles and reveals intriguing examples of diastereoselectivity in these enzyme-triggered reactions. Significantly, we demonstrate an elegant example of a biocatalytic cascade where the transaminase plays a dual role in generating complex N-heterocycles and where a retro-double intramolecular aza-Michael reaction mediates a dynamic kinetic resolution and enables the isolation of sp3-rich indolizidine diastereoisomers containing five stereocenters, as single isomers.

Gold(I)-Catalyzed Rautenstrauch/Hetero-Diels-Alder/Retro-aza-Michael Cascade Reaction for the Synthesis of α-Hydrazineyl-2-cyclopentenones

A one-pot synthesis of ring-fused, α-hydrazineyl-2-cyclopentenone derivatives is achieved by a gold(I)-catalyzed Rautenstrauch/hetero Diels-Alder/ring opening tandem reaction of suitable propargyl esters. By mixing the latter with a dialkylazodicarboxylate in the presence of a gold(I) catalyst, the 1,2-acyloxy migration/cyclization process (Rautenstrauch reaction) leads to cyclopentadienyl ester intermediates which are trapped by the heterodienophile present in situ. This provides strained intermediates which spontaneously undergo highly regioselective ring opening by a retro aza-Michael reaction promoted by the gold(I) catalyst, eventually yielding the target compounds. Six- and seven-membered ring-fused cyclopentenones bearing a pendant α-hydrazineyl moiety can be obtained in moderate to excellent yield (50-98%) by this approach, with a minimal erosion of the initial optical purity when using enantioenriched substrates.

Visible/solar-light-driven thiyl-radical-triggered synthesis of multi-substituted pyridines

A light-triggered synthesis of thio-functionalized pyridines is demonstrated using γ-ketodinitriles, thiols, and eosin Y as the photocatalyst. The reaction proceeds via the selective attack on one of the cyano groups by an in situ generated thiyl radical. The reaction also proceeds with nearly equal efficiency using direct sunlight. Large-scale synthesis and a few useful synthetic transformations of the substituted pyridines are also performed.

Iron-Catalysed [3+3] Annulation of Oxime Acetates and Enaminones towards the Synthesis of Multi-Substituted Pyridines

A direct access to unsymmetrical and symmetrical multi-substituted pyridines has been accomplished via iron-catalysed [3+3] annulation of oxime acetates with enaminones. This protocol is featured by easily available starting materials, no requirement of expensive additives and ligands, operational simplicity, and good tolerance with diverse functional groups.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Brønsted Acid-Catalyzed Synthesis of 1,2,5-Trisubstituted Imidazoles via a Multicomponent Reaction of Vinyl Azides with Aromatic Aldehydes and Aromatic Amines

A facile and efficient approach to the synthesis of 1,2,5-trisubstituted imidazoles is developed via a multicomponent reaction under metal-free catalysis. Under Brønsted acid catalysis, the desired products can be obtained from readily available vinyl azides, aromatic aldehydes, and aromatic amines without generating any toxic waste. The convenient operations and high functional group compatibility indicate that this approach offers an attractive alternative method for the synthesis of imidazole derivatives.

Synthesis of gem-Disulfonyl Enamines via an Iminyl-Radical-Mediated Formal 1,3-HAT/Radical Coupling Cascade

We herein report the first example of an iminyl-radical-mediated formal 1,3-HAT/radical coupling cascade of vinyl azides leading to the synthesis of tetrasubstituted gem-disulfonyl enamines. It is possible to employ a variety of vinyl azides and sulfinate salt coupling elements without sacrificing effectiveness and scalability. The combination of experimental studies and DFT calculations showed that this reaction proceeds via a radical addition/formal 1,3-HAT/radical coupling mechanism.

S-triggered Schmidt-type rearrangement of vinyl azides to access N-aryl-(trifluoromethylsulfinyl)acetamides

A novel S-induced Schmidt-type rearrangement of vinyl azides with CF3SO2Na is developed for synthesis of N-arylated 2-(trifluoromethylsulfinyl)acetamieds, which is mediated by triphosgene (BTC) under mild reaction conditions.

Pd(II)-Catalyzed Synthesis of Alicyclic[b]-Fused Pyridines via C(sp2)-H Activation of α,β-Unsaturated N-Acetyl Hydrazones with Vinyl Azides

A new synthetic protocol for alicyclic[b]-fused pyridines with complete regioselectivity from α,β-unsaturated N-acetyl hydrazones and vinyl azides via Pd(II)-catalyzed C-H activation/cyclization/aromatization strategy has been described. A series of five- to eight-membered alicyclic[b]-fused pyridines were prepared in a one-step manner with wide substrate scope and good functional group tolerance.

cis-3-Azido-2-methoxyindolines as safe and stable precursors to overcome the instability of fleeting 3-azidoindoles

Use of 3-azidoindoles in organic synthesis remains a difficult task owing to their instabilities. Herein, we report a general and concise approach for tackling this problem by using 3-azidoindole surrogates. The surrogates are bench-stable, presumably due to the observed intramolecular O-N_β bonding. The resultant fleeting intermediates undergo capturing in situ to afford 3-substitued indoles through formal ipso-substitution of the azide group by nucleophiles. In these investigations, we found that the fleeting 3-azidoindoles show a C3-electrophilic character for the first time.

Synthesis of Poly-Substituted Pyridines via Noble-Metal-Free Cycloaddition of Ketones and Imines

An eco-friendly and noble-metal-free formal [4+2] cycloaddition reaction was developed for the efficient synthesis of biologically interesting poly-substituted pyridines from easily available ketones and imines, whereby two sequential C-C bonds are formed. The given approach features a unique synthetic strategy of imines and ketones with wide substrate scope, good functional group tolerance, mild conditions and operational simplicity, which represents a more direct pathway to synthesize poly-substituted pyridines than traditional methods.

Regioselective Synthesis of 2,4-Diaryl-6-trifluoromethylated Pyridines through Copper-Catalyzed Cyclization of CF3-Ynones and Vinyl Azides

A novel copper-catalyzed cyclization of readily available vinyl azides with CF₃-ynones is steadily achieved under mild conditions to furnish the versatile 2,4-diaryl-6-trifluoromethylated pyridine products, which are of great interest in medicinal chemistry. The generation of the vinyl iminophosphorane intermediates from vinyl azides through the Staudinger-Meyer reaction ensures the subsequent 1,4-addition process with CF₃-ynones in this transformation.

Metal-free [3 + 2 + 1] annulation of allylic alcohols, ketones, and ammonium acetate: radical-involving synthesis of 2,3-diarylpyridine derivatives

A three-component [3 + 2 + 1] annulation strategy for the synthesis of biologically and pharmaceutically active 2,3-diarylpyridine derivatives by using a series of allylic alcohols, ketones, and ammonium acetate as substrates has been developed. The method proceeds efficiently under metal-free conditions, and the desired heterocycles could be obtained in a site-specific selectivity manner with good functional group tolerance.

Ruthenium-catalysed C–H/C–N bond activation: facile access to isoindolinones

A facile ruthenium-catalysed C–H/C–N bond activation and the subsequent annulation of readily available benzoic acids with in situ generated formaldimines are developed for the efficient synthesis of a wide range of biologically important isoindolinones.

Recent advances in transition metal-catalyzed olefinic C–H functionalization

Recent advances during 2015–2020 in the field of metal-catalyzed olefinic C–H functionalization are organized according to the metal center of the catalyst, with an emphasis on the similarities and differences among different catalysts.