Fused benzofurans

Coumestans,21 although they have a fused benzofuran ring, will be surveyed as being 8-lactones of 2-(2-hydroxyphenyl)-3-benzofuran carboxylic acids [coumestrol (4)22 is the 8-lactone of 2-(2,4-dihydroxy-phenyl)-6-hydroxy-3-benzofurancarboxylic acid]. The synthesis of some rotenoids 23 from benzofuran derivatives will also be reviewed, but dibenzofuran derivatives, such as usnic acid and its derivatives, will not be treated. 

This occurs for 2-hydroxyphenylacetones, hydroxylated deoxy-benzoins, and, as in the simplest case (R=H), o-hydroxyarylacetal-dehydes. The reaction is especially suitable for the synthesis of polyheterocyclic compounds with fused benzofuran rings. 

In the fused benzofuran series, the same method, applied to bromoacyl-o-hydroxycoumarins502,549 and bromoacyl-o-hydroxyxan-thones321,433,550 allows the synthesis of the corresponding oxo-dihydrofuro derivatives. The nitrogenated analogs of compounds 237 (2,3-dihydro-3-oxofuro[2,3-6] and -furo[3,2-c] pyridines) have also been obtained.551,552... 

The fast fade rate shown by 2-(4-trifluoromethyl)-2-phenyl-5-trifluorophenyl-[l]benzofuran[2,3-(]naphtho[l,2-6]pyran is attributable to the bulky 5-substituent rather than to the fused benzofuran ring <01WOP36424>. Its synthesis follows from the preparation of 9-hydroxy-7-trifluoromethylbenzo[6]naphtho[ f]furan from 4-chloromethyldibenzofuran (Scheme 18). 

Following these reports, SanMartin and Dominguez developed a recyclable CuI-TMEDA catalytic system for the ring-closing transformation of a variety of 2-bromoaryl ethanones 39, which allowed facile assembly of mono- and tri-substi-tuted, and even fused benzofurans 40 in aqueous media, normally, in good to excellent yields (Scheme 9.16) [107, 108]. 

Willis reported that alkenyl triflates 43 could be employed in the Cu(I)-catalyzed synthesis of benzofurans 44 as surrogates of the respective 2-bromoaiyl ketones analogs 37 used by Chen (Scheme 9.18) [109]. This reaction was proposed to proceed via the initial hydrolysis of triflates 43 to give reactive enolates which, upon Cu-catalyzed intramolecular arylation, were transformed into various polycyclic fused benzofuran derivatives 44. 

A convenient synthesis of 3-spiro-fused benzofuran-2(3//)-ones was reported via Pd-catalyzed chemoselective carbonylation of vinyl triflates in the presence of o-iodophe-nols, followed by a regioselective intramolecular Heck reaction (Scheme 20). ... 

Gonzalez JP, Edgar M, Elsegood MRJ, Weaver GW (2011) Synthesis of Huorinated fused benzofurans and benzothiophenes smiles-type rearrangement and cyclization of perfluoro(het)aryl ethers and sulfides. Org Biomol Chem 9(7) 2294-2305... 

Y. Moon, Y. Kim, H. Hong, S. Hong, Synthesis of heterocycUc-fused benzofurans via C-H functionalization of flavones and coumarins, Chem. Commun. 49 (2013) 8323-8325. 

The reaction of phenol-linked 1,6-diyne with isocyanate afforded 2-pyridone-fused benzofuran in good yield as a single regioisomer (Scheme 4.76) [48]. 

Rodriguez-Garcia et al. extended the TST-RCM to the pterocarpans, which have a cis-fused benzofuran-benzopyran skeleton (Scheme 8.19) [43]. These agents are... 

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