Synthesis of Spirolactones via a BF3·Et2O-Promoted Cascade Annulation of α-Keto Acids and 1,3-Enynes

Mechanochemical Synthesis of Indene-Containing Spirolactones via Tf2O-Promoted Cascade Cyclization of 2-Benzoylbenzoic Acids with Arylalkynes

The Tf2O-promoted cascade cyclization of 2-benzoylbenzoic acids with arylalkynes was achieved under solvent-free ball-milling conditions. This process renders rapid entry to the rare indene-containing spirolactones via intramolecular cyclization of 2-benzoylbenzoic acids and subsequent annulation with arylalkynes, which resulted in one C-O and two C-C bond formations. As the reactions utilize easily available starting materials and proceed in the absence of organic solvents, the present protocol constitutes an expedient and environmentally friendly access to indene-containing spirolactones.

BF3·OEt2-catalyzed/mediated alkyne cyclization: a comprehensive review of heterocycle synthesis with mechanistic insights

The quest for efficient and versatile methods for heterocycle synthesis continues to drive innovation in organic chemistry. In this context, the cyclization of alkynes catalyzed or mediated by boron trifluoride diethyl etherate (BF3·OEt2) has emerged as a powerful and widely applicable strategy. This review provides a comprehensive and authoritative overview of BF3·OEt2-catalyzed/mediated alkyne cyclization reactions, covering the scope, mechanisms, and applications of these processes. We discuss the synthesis of a diverse range of heterocyclic compounds, including dihydropyrans, quinolines, dehydropiperidines, oxindoles and others, and highlight the unique advantages of BF3·OEt2 as a catalyst/mediator. Recent advances, challenges, and future directions in this rapidly evolving field are also addressed. This review aims to serve as a valuable resource for synthetic chemists, inspiring further research and applications in this exciting area.

Substrate-Controlled Regioselective Cascade Reactions of Deconjugated Butenolides and Cinnamaldehydes: Access to Structurally Diverse Spirobutenolides

Two different cascade pathways to access spirobutenolides were achieved based on the substrate-controlled regioselectivity of deconjugated butenolides. A new class of functional deconjugated butenolides was designed and exhibited superior γ-regioselectivity in the vinylogous Michael/Michael cascade reactions with cinnamaldehydes. The aryl-substituted deconjugated butenolides and cinnamaldehydes underwent a Michael/Michael/aldol/dehydration cascade process induced by double α-regioselectivities. Both conjugated and deconjugated spirobutenolides could be obtained in good yields with excellent enantioselectivities.

The Construction of Novel Spirocyclic Frameworks with Cyclobutane through Rh(III)-Catalyzed [3 + 2]-Annulation between Quinoxalines and Alkynylcyclobutanols

An effective synthesis strategy for the preparation of 1'H-spiro[indene-1,2'-quinoxaline] has been developed. This involves a Rh(III)-catalyzed [3 + 2]-annulation of quinoxalines with alkynylcyclobutanols. The developed protocol offers a straightforward method for the preparation of versatile heterocyclic compounds with a four-membered ring and is compatible with a wide range of functional groups.

Concise and Free-Metal Access to Lactone-Annelated Pyrrolo[2,1-a]isoquinoline Derivatives via a 1,2-Rearrangement Step

Here, An efficient approach to obtaining previously unknown furo[2',3':2,3]pyrrolo[2,1-a]isoquinoline derivatives from readily available 1-R-1-ethynyl-2-vinylisoquinolines is described. The reaction features a simple procedure, occurs in hexaflouroisopropanol and does not require elevated temperatures. It has been found that the addition of glacial acetic acid significantly increases the yields of the target spirolactone products. Using trifluoroethanol instead of hexaflouroisopropanol results in the formation of pyrido[2,1-a]isoquinolines.

Diastereoselective Synthesis of Allenes through Phosphine-Catalyzed Cascade Isomerization/Annulation

Phosphine-catalyzed cascade isomerization/annulation has been developed to realize a diastereoselective synthesis of allenes installed on the hexahydropentalene skeleton, which contains five chiral centers (and one axial chirality). This reaction tolerated a broad range of allenoates and enynes. The allene products were transformed to various halogen-substituted fused-ring compounds.

Metal-free three-component assemblies of anilines, α-keto acids and alkyl lactates for quinoline synthesis and their anti-inflammatory activity

A new and metal-free three-component method for the synthesis of 2,4-disubstituted quinolines via the reactions of anilines, α-keto acids and alkyl lactates is reported. The reactions proceed in the presence of p-toluene sulfonic acid (p-TSA) and tert-butyl peroxybenzoate (TBPB) to provide diverse quinoline products via the construction of new CC double, C-C single and CN double bonds without producing any organic mass-based side product. Notably, the anti-inflammatory activity of the quinolines has been investigated by measuring their ability to inhibit NO release by lipopolysaccharide (LPS) induced RAW264.7 cells, leading to the identification of 4i, 4t and 4x as potent anti-inflammatory compounds in vitro.

Synthesis of Spirocyclopropylpyrazole Derivatives via the Cascade Reaction of Alkylidenecyclopropanes with Pyrazolidinones and Trifluoroethanol

A novel synthesis of spirocyclopropylpyrazole derivatives tethered with a trifluoromethyl unit through an unprecedented cascade reaction of alkylidenecyclopropanes with pyrazolidinones and trifluoroethanol (TFE) has been developed. Mechanistically, the reaction involves...

BF3·Et2O-Mediated annulation of α-keto acids with aliphatic ketones for the synthesis of γ-hydroxy-butenolides and γ-alkylidene-butenolides

Annulation reaction of α-keto acids with cyclic or acyclic aliphatic ketones is reported herein to divergently access γ-hydroxy-butenolides and γ-alkylidene-butenolides depending on the amount of BF₃·Et₂O. This protocol features good functional tolerance and ease of operation, to open a route to access butenolides via an annulation and dehydration process.

An efficient approach to divergently access γ-hydroxy-butenolides and γ-alkylidene-butenolides via annulation reaction of α-keto acids with simple aliphatic ketones is reported herein.

Synthesis of Indolyl-Tethered Spiro[cyclobutane-1,1'-indenes] through Cascade Reactions of 1-(Pyridin-2-yl)-1H-indoles with Alkynyl Cyclobutanols

Presented herein is an efficient and unprecedented synthesis of indolyl-tethered spiro[cyclobutane-1,1'-indenes] through the cascade reaction of 1-(pyridin-2-yl)-1H-indoles with alkynyl cyclobutanols. Mechanistic experiments implicate a sequential process in which 1-(pyridin-2-yl)-1H-indole first undergoes an alkenylation with alkynyl cyclobutanol followed by an intramolecular Friedel-Crafts reaction to give the title products. The utility of this novel protocol was reflected by the ample substrate scope, high chemo- and regioselectivity, removable directing group, and scalable preparation. In addition, the product thus obtained can be further derivatized quite efficiently.