Palladium-catalyzed furan annulation

Vinyloxiranes 206 undergo palladium-catalyzed furan annulation with soft nucleophiles via two cyclization paths to give either 210 or 211 as a primary product, which undergoes further transformations, depending on the substituents (Scheme 11-55) [44,47]. At first, the allenylpalladium 207 is generated this is converted to two kinds of n-... 

A general synthetic methodology and mechanistic details for the palladium-catalyzed carbonylative annulation of the o-alkynylphenol to construct 2-substituted-3-arylcarbonyl-benzo[fo]furans was reported <02JOC2365>. 

The palladium-catalyzed carbonylative annulation of 3-(2-iodophenyl)-benzo[ ]furan 125 provided benzo[/5]-indeno[l,2-4 furan-6-one 126 in 81% yield, as can be seen in Equation (107) <2002JOC5616>. 

The palladium-catalyzed carbonylative annulation of o-hydroxyphenylacetylene was employed to generate methyl benzo[h]furan-3-carboxylate, a key intermediate for the total synthesis of the natural product wedelolactone <03JOC8500>. 

Hu, Y, Zhang, Y, Yang, Z. and Fathi, R. 2002. Palladium-catalyzed carbonylative annulation of o-alkynylphenols Syntheses of 2-substituted-3-aroyl-benzo[h]furans. J. Org. Ghem. 67 2365-2368. 

A palladium-catalyzed one-step synthesis of dihydrobenzo[fc]furan-based fused aromatic heterocycles from bifunctional bromoenoates or bromoalkyl indoles and iodoarenes was reported, and an example is provided in the scheme below <060L3601>. 2-Alkyl- or 2-aryl-substituted benzo[ >]furans were synthesized by a copper-TMEDA catalyzed intramolecular annulation from the corresponding ketones <06OL1467>. 

Palladium-catalyzed annulation reactions were involved in the syntheses of benzo[A furan-based spirocyclic compounds, 2-substituted-3-aroyl-benzo[A]furans, and 2-benzofuran-2-ylacetamides <07AGE7068 07JOC9278>. 

A palladium-catalyzed approach to substituted [3]-annulated furans is based on the use of a cyclic allylic dielectrophile of type 40 and a /3-ketoester which acts as a l,3-C,0-dinucleophile (Equation 64) <2007S39>. The reaction appears to be generally applicable for furan-annulation reactions. 

A palladium-catalyzed annulation of internal alkynes to o-iodophenol derivatives to afford 3-fluoromethyl benzo-furans has also been described (Equation 116) <2004X11695>. 

As depicted below, a procedure for the iodocyclization of acetoxy-containing 2-(l-alkynyl)anisole and subsequent direct palladium-catalyzed earbonylatiori/lactonization was reported as an efficient entry to naturally occurring eoumestan and coumestrol <05IOC9985>. A novel Pd(II)-mediated cascade carbonylative annulation of substituted o-hydroxyphenylacetylenes to give benzo[fo]furan-3-carboxylie adds was achieved in a one-pot reaction <050L2707>. 

One of the first palladium-catalyzed annulation protocols, in which the involvement of paUadium(IV) intermediates had been proposed, was published by Dyker in the early 1990s. A homocoupling of ort/jo-iodoanisole afforded 6//-dibenzo[ >, flpyran, and a cross-coupling of ort/io-iodoanisole with vinyl bromides provided benzo[6] furans (Scheme 2) [2, 3]. The mechanistic rationale put forward for these processes invoked an initial Csp -H activation induced by the paUadium(II) center, followed by the formation of paUadium(IV) intermediates via oxidative addition of aryl iodide or vinyl halide building blocks. Dyker s reports inspired synthetic chemists... 

A norbornene-mediated, palladium-catalyzed sequence was described to form annulated indoles (eq 155). Furans or thiophenes can undergo a similar cycUzation to provide annulated... 

Palladium-catalyzed annulation of propargylic carbonates is a convenient general strategy for five-membered heterocycle synthesis. For example, P-keto esters 114 were converted to furans 115 upon treatment with 5 mol % Pd(OAc)2 and 1,1 -bis(diphenylphosphino)ferrocene (dppf) in THF using K2CO3 as a base (Scheme 19.25) [41]. 

Scheme 36 Synthesis of 2,3-diarylbenzo[fc]furans via Sonogashira reaction and palladium-catalyzed annulation...

A conformationally restricted 2,3-diarylbenzo[i)]furan library was built up on solid-phase by the palladium/bipyridy-catalyzed annulation of o-alkynyl phenols with aryl halides <04JOC2235>. 

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