Metal‐Free Oxidative Cyclization of Alkynyl Aryl Ethers to Benzofuranones

Abstract

Readily available phenols can be converted into substituted aryl alkynyl ethers, which react with an N‐oxide as an oxidant and catalytic amounts of a Brønsted acid to provide benzofuranones. If non‐terminal alkynyl ethers are applied, a 1,2‐hydride shift takes place and phenyl acrylates are obtained. Thus activated alkynes can serve as α‐oxy carbene precursors even in the absence of a metal catalyst.

A Gold(I)‐Catalyzed Oxidative Rearrangement of Heterocycle‐Derived 1,3‐Enynes Provides an Efficient and Selective Route to Divinyl Ketones

“…[5] The regioselective oxidation of internal, nonsymmetrical alkynes was first reported by Zhang and co-workers. [6] Interestingly, similar transformations were realized in the absence of gold catalyzed by HBF 4 ⋅OEt 2 [7] and N-iodosuccinimide. [8] As a result of our interest in the chemistry of heterocycle-derived triflates and phosphates [9] as key intermediates in the synthesis of natural compounds, [10] we have previously demonstrated that the gold(I)catalyzed cyclization reaction of N-tert-butoxycarbonyl (Boc) protected 6-alkynyl-3,4-dihydro-2H-pyridines affords synthetically useful vinylogous amides (Scheme 1, b).…”

Gold‐Catalyzed Redox Synthesis of Imidazo[1,2‐a]pyridines using Pyridine N‐Oxide and Alkynes

“…On the other hand, no product was formed in the absence of gold catalysts (entry 4). [20] Interestingly slow formation of the desired compound was observed when 0.5 equivalent of TFA was used (entry 7), and therefore 1 equivalent of TFA was chosen for further studies. Dichloromethane proved to be the solvent of choice; toluene, chlorobenzene and 1,2-dichloroethane did not give any product and other solvents were screened but none produced a better conversion than dichloromethane.…”

Für die Energiewende studieren

“…Further studies revealed that elevating the equivalent of oxidant 6 c and maintaining it slightly excess to MsOH could lead to a further improved yield, and 3‐coumaranone 4 a was isolated in 86 % yield using 2.7 equivalents of 6 c and 2.4 equivalents of MsOH (Table 1, entry 19). Notably, in the absence of the gold catalyst, the desired product was not formed under the acidic reaction conditions,14 and PtCl 2 and AgNTf 2 could not catalyze this reaction.…”