An Access to Benzo[a]fluorenes, Benzo[b]fluorenes, and Indenes Triggered by Simple Lewis Acid

Abstract

This report illustrates BF 3 •OEt 2 promoted intramolecular cascade cycloaromatization of 1,7-ynones toward synthesizing structurally diverse benzofluorene scaffolds. Remarkably, the present protocol promotes the formation of two consecutive C−C bonds intramolecularly and undergoes aromatization under mild reaction conditions to afford the tetracyclic benzo[a]fluorene frameworks. Besides, the formation of indenes was observed when 1-bromo-2-iodoarenes are relatively more electron-rich when compared with the one originating from the terminal arylacetylenes, under controlled conditions, wherein triple bond polarity has been just reversed due to the change of electronic effects exerted by the strong +M group of 1-bromo-2iodoarenes, which is in conjugation to the connected triple bond. The same concept to generate indenes has also been extended by using aliphatic alkyne tethered ynones. Further, it was noticed that 1,7-ynones bearing the more electron-rich 1-bromo-2-iodoarenes than the arene ring arriving from the terminal arylacetylenes lead to benzo[b]fluorenes, under thermodynamic conditions, instead of delivering the benzo[a]fluorenes. In addition, this method features metal-free conditions, easily accessible starting materials, operational simplicity, gram-scale synthesis, and a wide range of substrate scopes.

Acid-Mediated Cascade Cyclization Pathway to Indeno[2,1-c]chromen-6(7H)-ones

“…GP-1(b) was followed with 1-(2-bromo-3,5-dimethoxyphenyl)ethan-1one 8n (0.5 mmol, 129.5 mg), Pd(OAc) 2 (3 mol %, 4 mg), P(o-tol) 3 (6 mol %, 9 mg), ethyl acrylate (1.5 mmol, 3.0 equiv, 150 mg), triethylamine (1.5 mmol, 152 mg), and toluene (2 mL) at 110 °C for 24 h for the Heck coupling reaction. Purification of the crude material using silica gel column chromatography (85:15 to 75: 25 Methyl (E)-3-(2-Butyryl-4,5-dimethoxyphenyl)acrylate (3p). GP-1(b) was followed with 1-(2-bromo-4,5-dimethoxyphenyl)butan-1one 8p (0.5 mmol, 143.5 mg), Pd(OAc) 2 (3 mol %, 4 mg), P(o-tol) 3 (6 mol %, 9 mg), methyl acrylate (1.5 mmol, 3.0 equiv, 129 mg), triethylamine (1.5 mmol, 3.0 equiv, 152 mg), and toluene (2 mL) at 110 °C for 24 h for the Heck coupling reaction.…”

“…For instance, the Lewis/Brønsted acid-mediated cascade or domino organic transformation reaction for synthesizing valuable carbocyclic and heterocyclic organic frameworks from reasonably simple starting materials is a powerful tool in organic chemistry. ,,− In this context, in 2012 and 2013, the Dattatraya group reported the iron­(III) chloride-induced synthesis of indene skeleton-based molecules starting from the o -alkene-tethered cinnamates, and in 2015, the same research group described the synthesis of indanone molecules from ortho -carbonyl-tethered cinnamates via iron­(III) chloride-mediated cyclization. As a continuation, Balamurugan et al in 2021 presented the triflic acid-catalyzed domino cyclization pathway for synthesizing indene derivatives.…”

Dual Proton/Silver-Catalyzed Serial (5 + 2)-Cycloaddition and Nazarov Cyclization of (E)-2-Arylidene-3-hydroxyindanones with Conjugated Eneynes: Synthesis of Indanone-Fused Benzo[cd]azulenes

“…It first involves the activation of the hydroxyl of 1o by p -TsOH·H 2 O and AgSbF 6 to provide the allylic carbocation A stabilized by SbF 6 – anion. Then, the formal (5 + 2)-cycloaddition of the allylic carbonium ion A with 1-ethynylcyclohex-1-ene 2a leads to the dibenzo­[ a , f ]­azulen-12-one intermediate B along with the release of a proton . The intermediate B is electrophilically added by proton to form the carbocation C .…”

A two-step access to fused-/spiro-polycyclic frameworks via double Heck cascade and acid-driven processes

“…In continuation of our ongoing research interest in developing efficient synthetic processes towards polycyclic frameworks either via transition-metal catalysis 31 or acid-triggered cyclization pathways, [32][33][34][35] we envisioned that a rapid construction of fused-/spiro-polycyclic frameworks would indeed become possible by adopting a two-step synthetic process. Specifically, it was presumed that an intermolecular and intramolecular double Heck cascade starting from simple 2-bromocinnamate esters and diphenylacetylenes would afford indenes bearing an enoate ester moiety, which upon acid-mediated cyclo-condensation in the presence of external arenes/phenols would deliver fused and spiro-polycyclic frameworks.…”