Furan, 2,5-diaryl

Furan undergoes phenylation rather than diazo coupling on reaction with ben-zenediazonium salts, and thiophene similarly yields 2- or 2,5-diaryl derivatives rather than coupled products. However, 2,5-dimethylfuran and 2-/-butylfuran give coupled products with 2,4-dinitrobenzenediazonium ion (Scheme 26). 

The most common use of the Paal-Knorr condensation begins with a 1,4-diketone and yields a 2,5-disubstituted furan. This method has been used to produce dialkyl and disilyl furans however, the most popular use of this strategy is for the production of 2,5-diaryl furans. In addition to their utility as synthetic intermediates, these compounds are under investigation for novel electronic and pharmaceutical applications. 

Not surprisingly, based on the bioactivity of diaryl furans discussed in section 4.2.4.1, triaryl furans have also been investigated as enzyme inhibitors. De Laszlo prepared several 2,3,5-triarylfurans via the Paal-Knorr reactions (for example 44 to 45) and tested these compounds for their activity toward P38 kinase. ... 

Ryder reported the preparation of an interesting alkyl diaryl furan that was subsequently polymerized and studied as a conducting polymer. The monomer furan 49 was available fron the acid catalyzed cyclization of dione 48. ... 

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

In contrast with the relatively facile thermal rearrangement of sulfinates to sulfones discussed in the preceding section, the reverse process is relatively, rarely encountered and is usually observed only at elevated temperatures. One of the first thermal sulfone to sulfinate isomerizations has been invoked by Fields and Meyerson to occur during the pyrolysis of dibenzothiophene S, S-dioxide (26) to dibenzofuran, through elimination of sulfur monoxide from the sultine intermediate 27 (equation 27). More recently, the flash vapor-phase pyrolysis of various 2,5-dialkyl and diaryl thiophene-S, S-dioxides has also been shown to involve SO extrusion and formation of the corresponding furans in good yields . 

In addition, complexes of P(/-Bu)3 have been shown to catalyze the formation of diaryl heteroarylamines from bromothiophenes.224 Aminations of five-membered heterocyclic halides such as furans and thiophenes are limited because their electron-rich character makes oxidative addition of the heteroaryl halide and reductive elimination of amine slower than it is for simple aryl halides. Reactions of diarylamines with 3-bromothiophenes occurred in higher yields than did reactions of 2-bromothiophene, but reactions of substituted bromothiophenes occurred in more variable yields. The yields for reactions of these substrates in the presence of catalysts bearing P(/-Bu)3 as ligand were much higher than those in the presence of catalysts ligated by arylphosphines. 

Highly enantioselective Mannich-type reactions of A-(2-hydroxyphenyl) aldi-mines with ketene trimethylsilyl acetals and of A-Boc-aldimines with acetyl acetone or furan are catalyzed by chiral phosphonic acids 9 derived from 3,3 -diaryl-(l )-BlNOL and POCI3 (Scheme 12.7). ... 

Diaryl- and tetraaryl-substituted furans such as tetraphenylfuran (384) for example, yield generally cw-diaroylethylenes such as 380, probably via intermediate ozonide formation.257 Secondary reactions seem to depend very much on the nature of the solvents. Thus, cis-dibenzoylstilbene (380) has been observed by direct photooxygenation of 384 in CS2 as well as by methylene blue-sensitized photooxygenation of 384 in methanol.257,258 However, when the latter reaction was carried out in acetone, epoxide 386 and the enolbenzoate 387 were obtained.258... 

Benzo[c]furan (4) exhibits a long-wave absorption band of medium intensity in the region of 340 nm. Lack of solvent dependence together with mirror relationship to the fluorescence spectrum signifies a tt-ti band a rotational analysis of the vapor phase spectrum led to an assignment as 82 <- Ap 1,3-Diaryl-substituted benzo[c]furans show a strong absorption band in the region of 415 nm in sterically hindered compounds, this... 

Reaction of Grignard reagents on 3(2H)- and 2(3/7)-benzofuranones yields 3-alkyl, 2-aryl (and 2,3-diaryl)benzofurans. A number of 3-substituted benzofuran derivatives result from the reaction of Grignard reagents on 3(2H)-benzofuranones.239,276 The method has also been applied in the naphtho[2,3-6] series.685 Substituted 2,3-diphenylbenzo-furans (352) have been obtained in the same way from 3-phenyl-2(3//)-benzofuranones (350) by dehydration (in boiling AcOH) of the intermediate hemiacetals (351).762,763... 

Phenylbenzo[c ]furan-3-carbonitrile and the corresponding carboxylic acid are catalyti-cally reduced in a cis manner to the expected 1,3-dihydro compounds. 1,3-Diaryl-benzo[c]furans are also reduced to phthalans with sodium amalgam. 

The numbering system for benzo[c]furan (isobenzofuran) is as shown for (230). The parent compound, benzo[c]furan (230), is unstable in solution and reacts almost instantaneously with typical dienophiles to give the corresponding Diels-Alder adducts. A series of 1,3-diaryl substituted benzo[c]furans (231a-e) had been prepared previously and were later identified (32JA1446). 

Bis(trimethylsiloxy)furan (361) was reacted with a number of substituted benz-aldehydes with activation by titanium tetrachloride to give 2,6-diaryl-3,7-dioxobicyclo[3.3.0]octan-4,8-diones (362), the ring system occurring in natural lignan (80TL3427). 

Benzo[c]furan, l-cyano-3-phenyl-synthesis, 4, 683 Benzo[c]furan, 1,3-diaryl-cycloaddition reactions, 4, 635 reduction, 4, 614 synthesis, 4, 701 Benzo[c]furan, 1,3-di-t-butyl-synthesis, 4, 703 Benzo[c]furan, 1,3-dihydro-NMR, 4, 572-574. structure, 4, 549 synthesis, 4, 683 Benzo[c]furan, 4,7-dihydro-synthesis, 4, 682... 

Direct irradiation of 2,5-dihydrofuran (175) results in the formation of furan, tetrahydrofuran, and the isomeric oxiran (176).146 Certain 2,5-dihydrothiophen derivatives are similarly converted to the corresponding vinylthiirans,147 whereas 5,5-diaryl-2,5-dihydrofurans are reported to undergo di-7i-methane rearrangement.148 An unexpected transannular phototransformation has been observed on irradiation of the l,3-dioxolen-2-one (177)... 

The reaction is best carried out using the Meerwein reaction conditions (acetone-water/CuCl2). Analogous reactions were described for diazoarsanilic acid and furan.160 The course of the reaction with 2-furylacrylic acid is more complex, as decarboxylation occurs and diarylated compounds form161 ... 

Sulfides with a hydrogen atoms like dimethyl sulfide, dimethyl disulfide, and tetrahydrothiophene form diols under UV irradiation (795). In the tetrahydrothiophene-2HFA2 adduct the hydroxyhexafluoropropyl groups are in trans positions and the trifluoromethyl groups show nonequivalence (795). Aromatic thio compounds like thiophenols (77), diaryl sulfides (779, 180, 207), and thiophene (95) add HFA in an ortho position, as does furan (95). 

They were formulated by him as anthranols. Also, von Pechmann described a monooxyphenylanthranol which presumably was a mixture of7f and 9-(p-hydroxyphenyl)anthrone (see Section IV,A,4). Now many 1,3-diaryl benzo[c]furans are known (Section IV,A,6) the preparation of the parent compound (4) and of simple derivatives has also been described (Section... 

Butene-l,4-diones and 2-butyne-l,4-diones were converted into 2,5-diaryl- and 2,3,5-triarylfurans in high yields in the presence of HCOOH and a catalytic anaount of Pd on carbon under microwave-irradiation conditions <03JOC5392>. This procedure provides a new approach to the starting material used in the Paal-Knorr furan synthesis, as unsaturated diones are reduced to saturated diones in situ by formic acid and palladium. The solid-phase synthesis of 2,3,5-trisubstituted furans from 1,4-diketones was also reported <03SL711>. 

Cyclopropene and its deuterium-labelled derivatives can be obtained by the photo-decarbonylation of the corresponding furan at 254 nm but the method is of strictly limited value because of the photolability of many cyclopropenes (Section IV.B.2). West and his coworkers have shown that aryltrihalo- and diaryldihalocyclopropenes are available from classical Friedel-Crafts aromatic substitution reactions employing the cyclopropenyl cation as electrophile. Thus tetrachlorocyclopropene is converted to its derived cation which is then allowed to react with an aromatic compound. The exothermic reaction provides monoarylcyclopropene at low ( 0°C) temperature and the diaryl derivative at higher ( > 50° C) temperature (equation 26). In this way 2-phenyl-1,3,3-trichlorocyclopropene can be obtained in 58 % yield and the p-fluorophenyl analogue in... 

Reduction of the semicarbazone of a phenacyl bromide in dimethylformamide (DMF) yields the bis-semicarbazone of 1,4-diaryl-1,4-butanedione, which in DMF forms 3,6-diarylpyridazine [4] or, after treatment with aqueous acid, 2,5-diary 1-furan [5], Reduction of phenacyl bromide in DMF is reported to give 2,4-diarylfuran [6]. 

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