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.

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

The same δ-acetoxy allenoates and thioamides, under DABCO, pyridine, or tetra-n-butyl ammonium bromide (TBAB) catalysis, undergo distinctly different annulations giving chemoselective routes to dihydrothiophene, thiopyran, or thiazole motifs. Thus, using pyridine in [3 + 2] annulation, dihydrothiophenes are obtained as essentially single diastereomers. By contrast, under DABCO catalysis, allenoates deliver thiopyran motifs in good to high yields through 6-exo-dig cyclization. In the thiazole forming [3 + 2] annulation, tetra-n-butyl ammonium bromide (TBAB) facilitates addition-elimination and 5-exo-trig cyclization in which ß- and γ-carbons of allenoates participate to deliver thiazole cores exclusively with a Z-isomeric exocyclic double bond. A possible rationale for these observations is delved into.

Divergent Reactivity of δ- and ß'-Acetoxy Allenoates with 2-Sulfonamidoindoles via Phosphine Catalysis: Entry to Dihydro-α-carboline, α-Carboline, and Spiro-cyclopentene Motifs.

The reactivity of 2-sulfonamidoindoles with acetoxy allenoates under phosphine catalysis depends on the disposition of the acetoxy (OAc) group on the allenoate. In the temperature-controlled [3 + 3] annulations, δ-acetoxy allenoates afforded dihydrocarboline and carboline scaffolds with carbon-nitrogen nucleophilic 2-sulfonamidoindoles, in which allenoate serves as a ß-, γ-, and δ-carbon donor. At room temperature (25 °C), dihydro-α-carboline motifs were obtained exclusively through Michael addition, 1,4-proton shift, isomerization, 1,2-proton transfer, phosphine elimination, and aza-Michael addition. The higher temperature (80 °C) cascade protocol using Ph3P-Cs2CO3 combination involves addition-elimination, aza-Claisen rearrangement, tosyl migration, and aromatization as key steps to give α-carbolines containing tosyl functionality at the γ-carbon. In contrast, with ß'-acetoxy allenoate, 2-sulfonamidoindole acts only as a carbo-nucleophile in (p-tolyl)3P-directed [4 + 1] spiro-annulation, leading to five-membered spiro-carbocyclic motifs essentially as single diastereomers (dr >20:1) via chemoselective carbo-annulation.

Contrasting Carboannulation Involving δ-Acetoxy Allenoate as a Four-Carbon Synthon Using DABCO and DMAP: Access to Spiro-carbocyclic and m-Teraryl Scaffolds.

Spiro-annulation involving δ-acetoxy allenoate and alkyl benzoisothiazole dioxide (N-sulfonyl ketimine) triggered by DABCO/MeCO2H combination leads to an essentially single diastereomer via chemo- and regiospecific [4 + 2]-carboannulation and a new hydroxyl group is introduced. In contrast, DMAP-catalyzed benzannulation using the same reactants affords unsymmetrical m-teraryls via Mannich coupling, sequential proton transfers, and C-N bond cleavage. Here, δ-acetoxy allenoate serves as a 4C-synthon and the carboannulation is completely base dependent and mutually exclusive.

A Synthesis of "Dual Warhead" ß-Aryl Ethenesulfonyl Fluorides and One-Pot Reaction to ß-Sultams.

Herein, we report an operationally simple, ligand- and additive-free oxidative boron-Heck coupling that is compatible with the ethenesulfonyl fluoride functional group. The protocol proceeds at room temperature with chemoselectivity and E-isomer selectivity and offers facile access to a wide range of ß-aryl/heteroaryl ethenesulfonyl fluorides from commercial boronic acids. Furthermore, we demonstrate a "one-pot click" reaction to directly transform the products to aryl-substituted ß-sultams.