Oxa-[3+3] annulation of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones to access 2H-pyrans

Directing Group-Controlled Regioselective [4 + 3] Annulation of Indole-2-Carboxamides with MBH Carbonates toward Highly Substituted Indole-1,2-Fused Diazepanones

A Lewis base-catalyzed [4 + 3] annulation between dinucleophilic indole-2-carboxamides and Morita-Baylis-Hillmann (MBH) carbonates was developed to access densely substituted indole-1,2-fused diazepanones. This reaction is initiated by a Lewis base-catalyzed N-allylic alkylation of the indole scaffold with MBH carbonates, followed by intramolecular Michael cyclization. Notably, the selectivity of this process is controlled by a removable o-methoxyphenyl (OMP) directing group attached to the indole-2-carboxamides. The wide scope of substrates, high regio- and stereoselectivity, and diverse transformations highlight the potential synthetic utility of this method in drug discovery.

Synergistic Rh(II)- and Zn(II)-Catalyzed [3 + 3] Annulation of Diazoenals and α-Hydroxy Ketones for the Direct Synthesis of 2H-Pyrans, A Gateway Toward γ-Pyrones

A new synergistic Rh(II)/Zn(II)-catalyzed [3 + 3] annulation has been developed between diazoenals and α-hydroxy ketones enabling the direct synthesis of 4-formyl-2H-pyrans. The reaction involves the formation of protic oxonium ylides from highly electrophilic Rh-enalcarbenoids followed by Zn-templated regioselective intramolecular aldol condensation. Subsequent investigations demonstrated that 4-formyl-2H-pyrans are unique precursors of γ-pyrones. The γ-pyrones were obtained via Et3N-mediated oxidative deformylation. Further, the triflic-acid-promoted double Schmidt reaction of 4-formyl-2H-pyrans provides synthetically important carboxamide-substituted 4-cyano 2H-pyrans.

Organocatalytic allylic alkylation of alkyne-substituted MBH carbonates: access to quaternary carbon-containing 1,4-enynes

A DABCO-catalyzed allylic alkylation of tertiary propargylic alcohol-derived MBH carbonates with nitromethane was developed. A series of substituted 1,4-enynes with an all-carbon quaternary stereocenter were efficiently obtained in moderate to high yields. The synthetic utility of the product was demonstrated by facile synthesis of 1,4-enyne-embedded 2-pyrrolidinones.

Domino Aza-Annulations of Enynyl-/(Alkynyl)aryl-acetonitriles to Access Nitrogen-Enriched Heterocycles

Unprecedented domino aza-annulations of (E)-2-en-4-ynyl-acetonitriles (generated from the Morita-Baylis-Hillman acetates of propiolaldehydes for the first time) with sodium azide under metal- and oxidant-free conditions for the assembly of triazolo-pyridines are accomplished. The developed strategy offers broad substrate scope, extending to (2-alkynyl)aryl and indolyl-acetonitriles to provide the corresponding triazolo-fused isoquinolines and β-carbolines, respectively, in good yields. Additionally, the synthetic utility of the products is demonstrated via denitrogenative coupling of fused triazoles with different nucleophiles.

FeCl3-mediated selenylative benzannulation of aryl alkynones to 3-selenyl β-naphthols

An efficient selenylative cyclization of aryl-alkynones with diselenides in the presence of iron(III)chloride at room temperature to prepare 3-seleno-2-naphthols in good yields has been described. Furthermore, the resulting products were transformed into selenyl-naphthofuran and selenyl-1,2-naphthoquinone derivatives.

Organocatalytic Regio- and Enantioselective [3 + 2]-Annulations of Ninhydrin-Derived Morita-Baylis-Hillman Carbonates with 3-Methyleneoxindoles

A type of Morita-Baylis-Hillman (MBH) carbonates has been developed from ninhydrin. These MBH carbonates have been successfully employed as 3C-synthons in the organocatalytic asymmetric [3 + 2]-annulations of the isatin-derived electron-deficient olefins, affording structurally diverse spirooxindoles in high yield with excellent stereoselectivity. In particular, the regioselectivity of MBH carbonates was controlled by the reaction partner, 3-methyleneoxindoles with carbonyl groups (R = ArCO), affording β-selective products and 3-methyleneoxindoles with ester groups (R = CO₂Me) furnishing γ-selective products. The representative scale-up reactions and transformation of product were examined. The reaction mechanism was expounded by control experiments.

Iodo- and Chalcogenoannulation of Morita-Baylis-Hillman Alcohols of Propiolaldehydes: Entry to Functionalized 2-Pyrones

An efficient intramolecular annulation of Morita-Baylis-Hillman (MBH) alcohols of propiolaldehydes is developed in the presence of ICl or PhSeSePh/PhSSPh-CuCl₂. This cyclization offers access to a wide variety of iodinated or chalcogenated 3-(chloromethyl)-2-pyrones in good yields. The chloromethyl group of the obtained 2-pyrones has been easily converted to introduce other handy functionalities, which allowed for further transformations to synthesize diverse 2-pyrone containing molecules.

Phosphine-Catalyzed Chemoselective Reduction/Elimination/Wittig Sequence for Synthesis of Functionalized 3-Alkenyl Benzofurans

An efficient protocol for the construction of functionalized 3-alkenyl benzofurans is demonstrated under metal-free conditions using catalytic amount of phosphine proceeding an intramolecular Wittig reaction. This one-pot reaction initiated by the phospha-Michael addition of phosphine to O-acylated nitrostyrene, in which phosphine was in-situ-generated from the chemoselective reduction of phosphine oxide with PhSiH₃, would provide the phosphorus ylide to result in the aforementioned multifunctionalized benzofuran via O-acylation/nitrous acid elimination/Wittig reaction.

Facile access to [1,2]-oxazine derivatives via annulations of aminoxy-tethered 1,7-enynes

An efficient approach for the highly diastereoselective construction of functionalized cyclopenta[d][1,2]oxazines via sequential oxyamination and Pauson-Khand reaction of readily accessible propargylic alcohols has been developed. Furthermore, the ring closing metathesis of these N-O linked 1,7-enynes afforded vinylated-[1,2]oxazines in good yields. The reduction of the N-O bond of the obtained cyclopenta[d][1,2]oxazine is accomplished to access cyclopentenone-based amino alcohols.