Furans coupling

From time to time, synthetic transformations are reported that appear to be particularly convenient One such is the procedure described by Paul Hanson of the University of Kansas (Tetrahedron Lett. 44 7187,2003) for the conversion of an alcohol to the amine. The iminc 2, easily prepared by the addition of maleimide to furan, couples under Mitsubobu conditions with the 1 to give the imide 3, contaminated with the usual impurities from the condensation. The crude imide 3 smoothly polymerizes under Ru metathesis conditions to give polymeric 3, from which the impurities are easily washed away. Exposure of the washed polymer to hydrazine then liberates the pure free amine 4. 

Indium enolates, prepared conveniently by transmetalation of hfhium enolates with IriCl j, react wifh aldehydes to give fhe corresponding -hydroxy esters [80]. Ultrasound irradiation promotes fhe Reformatsky reaction of aldehydes and ethyl bromoacetate wifh indium [81]. Indium-mediated Reformatsky reaction of phenyl a-bromoalkanoates wifh ketones or aldehydes gives di-, tri-, and tetrasubstituted -lactones (Scheme 8.57) [82]. Indium-mediated reaction of imines with ethyl bromoacetate gives 3-unsubstituted -lactams (Scheme 8.58) [83]. An indium-Refor-matsky reagent prepared from 2-(chlorodifluoroacetyl)furan couples with aldehydes (Scheme 8.59) [84]. 

The furan coupling constants are much smaller in magnitude than the corresponding ortho and meta coupling constants of the benzene derivatives. It is characteristic of the furans and the other heteroaromatic compounds that the "ortho" couplings, J2 3 and J3 4 are not the same. 

A C-H insertion was also employed in the synthesis of 3-substituted furans. Coupling of 2,5-dimethylfuran with 4-methystyrene using palladium(II) acetate gave the tra/w-alkenyl furan selectively and in modest yield. ... 

The oxidative coupling of thiophene, furan[338] and pyrrole[339,340] is also possible. The following order of reactivity was observed in the coupling of substituted furans[338] R = H > Me > CHO > CO Me > CH(OAc)i > CO2H. The cross-coupling of furans and thiophenes with arene is possible, and 4-phenylfurfural (397) is the main product of the cross-coupling of furfural and benzene[341]. 

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

The cross-coupling of aromatic and heteroaromatic rings has been carried out extensively[555]. Tin compounds of heterocycles such as oxazo-lines[556,557], thiophene[558,559], furans[558], pyridines[558], and seleno-phenes [560] can be coupled with aryl halides. The syntheses of the phenylo.xazoline 691[552], dithiophenopyridine 692[56l] and 3-(2-pyridyl)qui-noline 693[562] are typical examples. 

Corrosion Resistant Fiber-Reinforced Plastic (FRP). Fiber glass reinforcement bonded with furfuryl alcohol thermosetting resias provides plastics with unique properties. Excellent resistance to corrosion and heat distortion coupled with low flame spread and low smoke emission are characteristics that make them valuable as laminating resins with fiber glass (75,76). Another valuable property of furan FRP is its strength at elevated temperature. Hand-layup, spray-up, and filament-win ding techniques are employed to produce an array of corrosion-resistant equipment, pipes, tanks, vats, ducts, scmbbers, stacks, and reaction vessels for industrial appHcations throughout the world. 

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

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). 

C- F coupling constants, 4, 569 Benzo[b]furan-5-carboxylic acid, 4-hydroxy-2-isopropyl-synthesis, 3, 698... 

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