δ-Acetoxy Allenoate as a 5C-Synthon in Domino-Annulation with Sulfamidate Imines: Ready Access to Coumarins

Acetoxy allenoates as emerging synthons in annulation/cycloaddition reactions

In contrast to the normal allenoates (2,3-butadienoates) that undergo phosphine/amine addition generating dipolar-type intermediates, acetoxy allenoates that contain a facile-leaving -OAc group create electrophilic diene-phosphonium/ammonium intermediates rendering the reactivity pattern vastly different in the two cases. This review highlights the fascinating chemistry of acetoxy allenoates and related substrates. The diene-phosphonium/ammonium intermediate thus formed from acetoxy allenoate [substituted 5-acetoxypenta-2,3-dienoate or 2-(acetoxymethyl)buta-2,3-dienoate] participates in a plethora of cycloaddition/annulation reactions with bisnucleophiles generating multifunctional hetero- or homo-cycles in very few steps. Thus the electron-withdrawing effect of the phosphonium as well as carboxylate groups, making the diene-phosphonium/ammonium intermediates quite electrophilic, has been pivotal to the growth and significant interest during the last 10-15 years in this emerging class of substrates as revealed in this article. A comparison of this chemistry with that of (acetoxy)(acyl)allenes and allenyl acetates is also made briefly.

Base-Promoted One-Pot Strategy for Expeditious Assembly of Highly Functionalized Benzocoumarin Derivatives

A versatile and atom-economic three-component reaction between 3-cyano-4-styrylcoumarins, 1,3-indandione, and aliphatic alcohols is described for synthesizing diversely multifunctionalized benzocoumarins. This strategy allows facile access to various benzocoumarins bearing an amine and a substituted benzoyl scaffold under simple heating conditions. Readily available precursors, operational simplicity, acceptable functional group tolerance, and excellent yields are some highlighted advantages of this transformation.

Phosphine vs DBU-Catalyzed Annulation Reactions of β'-Acetoxy Allenoates with Acyl-Tethered Benzothiazole Bisnucleophiles: (4 + 3) or (4 + 1) vs (3 + 3) Annulation

Dearomative annulation reaction of acyl-tethered benzothiazole bisnucleophiles with β'-acetoxy allenoates by switching the Lewis base is developed. The DBU-catalyzed reaction gives benzothiazole-fused 1,4-dihydropyridine carboxylates by (3 + 3) annulation chemoselectively. By contrast, the PR3-catalyzed reaction gives benzothiazole-fused azepines by (4 + 3) annulation and cyclopentene carboxylates by (4 + 1) annulation; the ratio of the latter two products depends on the solvent. A possible rationale for the difference in the reactivity, based on the 1,4/1,5-addition of the 2-acyl-tethered benzothiazole to the key phosphonium intermediate, is provided.

Synthesis of Isoxazol-5(2H)-one Derivatives via (tBuO)2Mg Promoted [3 + 2] Annulations of δ-Acetoxy Allenoates with Hydroxylamine

Herein, an efficient (tBuO)2Mg promoted [3 + 2] annulation of δ-acetoxy allenoates with N-benzylhydroxylamine has been developed. This method provides a concise and facile protocol to synthesize vinylated isoxazol-5(2H)-one derivatives stereospecifically in a broad substrate scope with high efficiency (31 examples, up to 87% yield).

Syntheses, reactivity, and biological applications of coumarins

This comprehensive review, covering 2021-2023, explores the multifaceted chemical and pharmacological potential of coumarins, emphasizing their significance as versatile natural derivatives in medicinal chemistry. The synthesis and functionalization of coumarins have advanced with innovative strategies. This enabled the incorporation of diverse functional fragments or the construction of supplementary cyclic architectures, thereby the biological and physico-chemical properties of the compounds obtained were enhanced. The unique chemical structure of coumarine facilitates binding to various targets through hydrophobic interactions pi-stacking, hydrogen bonding, and dipole-dipole interactions. Therefore, this important scaffold exhibits promising applications in uncountable fields of medicinal chemistry (e.g., neurodegenerative diseases, cancer, inflammation).