Furanes, benzofuranes

For commonly encountered heterocycles, the chemical shifts of trifluo-romethyl substituents will depend somewhat upon where in the heterocycle they are located. Examples of trifluoromethyl derivatives for a number of common heterocycles, including pyridines, quinolines, pyrroles, indoles, thiophenes, benzothiophenes, furans, benzofurans, imidazoles, and uracils are given below. 

This article aims to review papers that were published in 2006 on reactions and syntheses of furans, benzofurans and their derivatives. Two reviews have summarized the syntheses of furans <06OBC1627> and tetrahydrofurans <06EJO1627>. Another review <06001613> records the progress of transition metal-catalyzed asymmetric ring opening of oxabenzonorbornadienes. 

Oxazoles and benzoxazoles are viable participants in the heteroaryl Heck reactions. In their monumental work published in 1992, Ohta and colleagues demonstrated that oxazoles and benzoxazoles, along with other rc-sufficient aromatic heterocycles such as furans, benzofurans, thiophenes, benzothiophenes, pyrroles, thiazole and imidazoles, are acceptable recipient partners for the heteroaryl Heck reactions of chloropyrazines [22b]. Therefore, treatment of 2-chloro-3,6-diethylpyrazine (27) with oxazole led to regioselective addition at C(5), giving rise to adduct 28. By contrast, a similar reaction between 2-chloro-3,6-diisobutylpyrazine (29) and benz[fc]oxazole took place at C(2) exclusively to afford pyrazinylbezoxazole 30. 

A great deal of work has been reported on the synthesis of furans, benzofurans, and related compounds. For the sake of clarity, the subject matter covered in this section has been divided into four parts that are based on different synthetic approaches as presented in Section II.A. 

As an extension of this work, photoinduced [2+2]-cycloadditions of 1-acetylisatin (13) with cyclic enolethers (furan, benzofuran, 2-phenylfuran, 8-methoxypsoralen), and acyclic enolethers (//-butyl vinyl ether and vinyl acetate) were investigated which afforded the spiro-oxetanes in high yields (82-96%) and with high regio- and diastereoselectivity (Sch. 4) [19]. Treatment of the furan-derived oxetane 15 with acid resulted in oxetane ring opening and yielded the 3-(furan-3-yl)indole derivative 16. 

When organic solvents are used for anodic fluorination, anode passivation (the formation of a nonconducting polymer film on the anode surface that suppresses faradaic current) takes place very often, which results in low efficiency. Moreover, depending on the substrates the use of acetonitrile can yield an acetoamidation byproduct. To prevent acetoamidation and anode passivation, Meurs and his coworkers used an EtaN 3HF ionic liquid as both a solvent and supporting electrolyte (also a fluorine source) for the anodic fluorination of benzenes, naphthalene, olefins, furan, benzofuran, and phenanthroline. They obtained corresponding partially fluorinated products in less than 50% yields (Scheme 8.2) [11]. 

Alcohols add to alkynes under certain conditions to give vinyl ethers. In an excess of alcohol, and in the presence of a platinum catalyst, internal alkynes are converted to ketals. " The alcohol to alkyne addition reaction is quite useful for the preparation of heterocycles. Dihydrofurans, " furans, " benzofurans, " and pyran... 

Furstner, A., Jumbam, D. N. Titanium-induced syntheses of furans, benzofurans and indoles. Tetrahedron 1992,48, 5991-6010. 

Similar to furans. benzofurans can be reduced by several noble metal catalysts in a highly diastereoselective cis fashion. Palladium is the catalyst of choice for the selective hydrogenation to dihydrobcnzofurans49 50, while platinum gives significant amounts of cleavage products. Rhodium can be used if fully saturated, cw-fused octahydrobenzofurans are the desired products51. 

We aim to review articles that were published in 2(X)4 on applications and syntheses of furans, benzofurans and their derivatives. 

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively re viewed and include their 4ir heterodiene Diels-Alder reactions with olefins, " vinyl ethers,enamines, selected dienes, " diphenyl Iketenimines, ketenes, fulvenes, " and selected heterocycles including furan, " - benzofuran, - - indoles, azepines, and 1,2-diazepines. The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cy-cloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products." With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 + 2] cycloadditions [Eq. (52)J, ... 

GMIN, GDZ, HF, AMI Furan, benzofuran Geometry, cycloaddition 91JCS(P2)1243... 

GMIN, GDZ, HF, CNDO/S, Furan, benzofuran. Electronic spectra, IP 83JST(105)375... 

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