Pyridine vs DABCO vs TBAB in Annulations of δ-Acetoxy Allenoates with Thioamides Leading to Dihydrothiophene, Thiopyran, and Thiazole Scaffolds

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.

Direct Synthesis of α-Ketothioamide Derivatives through a One-Pot Reaction of Sulfur Ylides, Nitrosobenzenes, and Thioacetic Acid

A one-pot approach has been developed for the synthesis of α-ketothioamide derivatives from sulfur ylides, nitrosobenzenes, and thioacetic acid. This protocol is carried out under mild reaction conditions in generally moderate to excellent yields without any precious catalysts, affording the derivatives with structural diversity. Additionally, a possible mechanism for this chemical transformation is proposed.

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).

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

A DMAP-catalyzed sequential benzannulation and lactonization strategy in which δ-acetoxy allenoate functions as a 5C-synthon in its reaction with cyclic sulfamidate imines is reported. This platform delivers π-extended coumarin frameworks under metal-free conditions via allylic elimination followed by Mannich coupling, proton shifts, C-N bond cleavage, and lactonization as key steps. The driving force for this domino reaction is the formation of the diene-ammonium intermediate and O-S bond cleavage. ESI-HRMS has been useful in gaining insights into the reaction pathway.

DBU-Catalyzed Ring Expansion or Ene-amine Formation Involving δ-Acetoxy Allenoates and N-Sulfonyl Hydrazides

DBU-catalyzed spiro-annulation and concomitant ring expansion/domino reaction of δ-acetoxy allenoates with cycl-2-ene-N-sulfonyl hydrazides afford ring-expanded (5 → 6, 6 → 7, and 7 → 8) products. By contrast, cycl-3-ene/ane-N-sulfonyl hydrazones under similar conditions deliver pyrazole cores with the same allenoate that involves allylic elimination in which δ-acetoxy allenoate serves as 3C-synthon. The key spirocyclic intermediates, as well as dienyl-amine intermediates, are isolated and characterized. An extension to (R)-(-)-carvone-derived sulfonyl hydrazide also led to ring expansion and gave pyrazoloazepine.