Stereoselective Synthesis of Hexahydroimidazo[1,2-a]quinolines via SN2-Type Ring-Opening Hydroarylation-Hydroamination Cascade Cyclization of Activated Aziridines with N-Propargylanilines

Recent breakthroughs in ring-opening annulation reactions of aziridines

Aziridines, characterized by their highly constrained three-membered nitrogen-containing heterocyclic ring system, serve as compelling synthetic intermediates for synthesizing numerous naturally occurring alkaloids and pharmaceuticals. The distinct ring strain arising from the geometric constraints of these sp3-rich trigonal rings imparts high reactivity, thereby offering a wealth of intriguing synthetic opportunities. Recent advances in the chemistry and reactivity of aziridines have unveiled significant progress in preparing more complex heterocycles. This review consolidates and examines recent publications on the ring-opening annulation reactions of aziridines, highlighting the latest breakthroughs, emerging trends, and future directions in this dynamic field.

A Synthetic Route to Tetrahydro-1H-azepino[4,3,2-cd]indoles via Ring-Opening Cyclization of Activated Azetidines with 4-Bromoindole: Toward a Vasopressin V2 Receptor Antagonist

A simple one-pot, two-step strategy for the synthesis of tetrahydro-1H-azepino[4,3,2-cd]indoles via Lewis acid-catalyzed SN2-type ring opening of activated azetidines with 4-bromoindole, followed by a Pd-catalyzed intramolecular C-N cyclization reaction, with good to excellent yields is described. Utilizing this protocol, the vasopressin V2 receptor antagonist precursor has been synthesized easily. Enantioenriched tetrahydro-1H-azepino[4,3,2-cd]indoles were obtained by starting from enantiopure azetidine.

Copper-catalyzed reaction of aziridine for the synthesis of substituted imidazolidine and imidazolidinone

Herein we report a copper-catalyzed synthesis of imidazolidine by employing the reaction of aziridine with imine. The reaction smoothly provided a diverse range of 2-substituted imidazolidines with high compatibility with various functional groups. Moreover, during our investigation, we discovered that isocyanate also reacted with aziridine to yield substituted imidazolidinones efficiently. The versatility of these reactions was further demonstrated by their application in the synthesis of hybrid molecules derived from two pharmaceutical compounds. This approach opens new possibilities for the discovery of novel classes of bioactive molecules.

Ring-Opening Cyclization (ROC) of Aziridines with Propargyl Alcohols: Synthesis of 3,4-Dihydro-2H-1,4-oxazines

Activated aziridines react with propargyl alcohols in the presence of Zn(OTf)₂ as the Lewis acid catalyst following an S_N2-type ring-opening mechanism to furnish the corresponding amino ether derivatives. Those amino ethers further undergo intramolecular hydroamination via 6-exo-dig cyclization in the presence of Zn(OTf)₂ as the catalyst and tetrabutylammonium triflate salt as an additive under one-pot two-step reaction conditions. However, for nonracemic examples, ring-opening and cyclization steps were conducted under two-pot conditions. The reaction works well without any additional solvents. The final 3,4-dihydro-2H-1,4-oxazine products were obtained with 13 to 84% yield and 78 to 98% enantiomeric excess (for nonracemic examples).

Ag2O/squaramide cocatalyzed asymmetric interrupted Barton-Zard reaction of 8-nitroimidazo[1,2-a]pyridines

Imidazo[1,2-a]pyridines are present in numerous biologically active compounds as the core structural motif. Herein, we report an asymmetric interrupted Barton-Zard reaction of electron-deficient imidazo[1,2-a]pyridines with α-substituted isocyanoacetates. The reaction enables the dearomatization of 8-nitroimidazo[1,2-a]pyridines and hence offers straightforward access to an array of optically active highly functionalized imidazo[1,2-a]pyridine derivatives that possess three contiguous stereogenic centers in good yields (up to 98%) with high stereoselectivities (>19:1 dr, >99% ee). It is worth noting that the catalytic system consisting of a chiral squaramide and silver oxide displays remarkable reactivity and stereoselectivity, and a gram-scale reaction is compatible with the catalyst loading of 0.5 mol%. In addition, the synthetic potential of this method was showcased by versatile transformations of the product.

Construction of Bridged Aza- and Oxa-[n.2.1] Skeletons via an Intramolecular Formal [3+2] Cycloaddition of Aziridines and Epoxides with Electron-Deficient Alkenes

An intramolecular formal [3+2] cycloaddition of activated aziridines and epoxides with electron-deficient alkene has been developed for the general and efficient construction of bridged aza- and oxa-[n.2.1] (n = 3 or 4) skeletons. This strategy can be efficiently promoted by lithium iodide. To demonstrate its potential, the intramolecular formal [3+2] cycloaddition was used to access the important intermediate of homoepiboxidine.

An Access to Benzo[a]fluorenes, Benzo[b]fluorenes, and Indenes Triggered by Simple Lewis Acid

This report illustrates BF₃·OEt₂ promoted intramolecular cascade cycloaromatization of 1,7-ynones toward synthesizing structurally diverse benzofluorene scaffolds. Remarkably, the present protocol promotes the formation of two consecutive C-C bonds intramolecularly and undergoes aromatization under mild reaction conditions to afford the tetracyclic benzo[a]fluorene frameworks. Besides, the formation of indenes was observed when 1-bromo-2-iodoarenes are relatively more electron-rich when compared with the one originating from the terminal arylacetylenes, under controlled conditions, wherein triple bond polarity has been just reversed due to the change of electronic effects exerted by the strong +M group of 1-bromo-2-iodoarenes, which is in conjugation to the connected triple bond. The same concept to generate indenes has also been extended by using aliphatic alkyne tethered ynones. Further, it was noticed that 1,7-ynones bearing the more electron-rich 1-bromo-2-iodoarenes than the arene ring arriving from the terminal arylacetylenes lead to benzo[b]fluorenes, under thermodynamic conditions, instead of delivering the benzo[a]fluorenes. In addition, this method features metal-free conditions, easily accessible starting materials, operational simplicity, gram-scale synthesis, and a wide range of substrate scopes.

Lewis Acid Mediated Domino Intramolecular Cyclization: Synthesis of Dihydrobenzo[a]fluorenes

An efficient and facile method for the regioselective synthesis of novel dihydrobenzo[a]fluorenes from readily accessible alkynols is presented. The current strategy triggers the formation of a dual C-C bond intramolecularly via Lewis acid catalysis under mild reaction conditions. Notably, secondary as well as tertiary alcohols bearing an alkyne moiety have been smoothly transformed into the corresponding products. As a result, novel tetracyclic dihydrobenzo[a]fluorenes have been accomplished using this approach.

Photoinduced triiodide-mediated [3 + 2] cycloaddition of N-tosyl aziridines and alkenes

A photoinduced triiodide-mediated [3 + 2] cycloaddition of N-Ts aziridines and alkenes is described herein. This operationally simple protocol enables regioselective access to a wide range of substituted pyrrolidines under mild-free conditions.