Furans stannyl

Stannylation of furan can be performed by Bu3SnCl93, as illustrated by the following 5-substituted lithium derivatives ... 

A distannation product of 2-butyne-l,4-diol oxidatively cyclizes to provide 3,4-bis(stannyl)furan, which then undergoes palladium-catalyzed cross-coupling with an aryl iodide to give 3,4-diarylfuran (Scheme 31).152,153... 

The trialkylstannyl derivatives of furane are also frequently employed in Stille coupling. This reaction was utilised, for example, to introduce a chiral oxazoline moiety onto the furane core through the coupling of 2-trimethylstannylfurane and chiral 2-bromooxazoline derivatives (6.30.)40 The furylstannane can be conveniently prepared in a lithiation-stannylation sequence, which makes it an attractive reagent for the introduction of the 2-furyl moiety. 

Simple 3-acylfurans are not accessible by direct Friedel-Crafts-type substitution of furan, but a most unusual route to these compounds has been discovered. Thus, reaction of the stannylated cyclopropane derivative 1 with acid chlorides (RCOC1) in refluxing toluene for 5 hours gave the dihydrofurans 2 in 64-83% yield. Treatment of 2 with 1.2 equivalents of BF3,OEt2 in dichloromethane at -78°C followed by slow wanning of the reaction mixture to room temperature gave the 3-acylfurans 3 in 30-62% yield. 

The bis-iodonium acetylene 198 is even more reactive than 189 and undergoes Diels-Alder reaction with cyclopentadiene, furan and 1,3-diphenylisoben-zofuran in acetonitrile under very mild conditions (Scheme 71) [139]. All adducts (62,199) are isolated in the form of stable microcrystalline solids products 62 can be reacted further with nucleophiles or combined in a cross-coupling reaction with lithiated or stannylated alkynes [52,53,61]. 

The DFT study of the 3 + 2-cycloaddition between ketene and TV-silyl-, IV-germyl-, and TV-stannyl-imines shows that the TV-germylimine reaction is a two-step process the TV-stannylimine reaction is a competition between two- and three-step processes whereas the TV-silyl process follows a three-step process44 A new and convenient synthesis of functionalized furans and benzofurans based on 3 + 2-cycloaddition/oxidation has been reported. The cyclization of cyclic 1,3-bis-silyl enol ethers (48) with l-chloro-2,2-dimethoxyethane (49), via a dianion, produced 5,6-bicyclic 2-alkylidenetetrahydrofurans (50), which are readily oxidized with DDQ to 2,3-unsubstituted benzofurans (51) (Scheme 13)45 The Evans bis(oxazoline)-Cu(II) complex catalyses the asymmetric 1,3-dipolar cycloaddition of a -hydroxyenones with nitrones to produce isoxazolidines.46 The... 

Good regioselectivity was observed using silyl and stannyl furan derivatives. The reaction in this case occurred on the less hindered side of the molecule (Scheme 3.7) [30]. On the contrary, 2-furylmethanol and the corresponding silyl ether gave low regioselectivity [31, 32]. 

In the total synthesis of naturally occurring frondosin B, the palladium-catalyzed coupling reaction of C-3 stannylated benzo[. ]furan 94 with the vinyl triflate 95 was employed as a key step to build up the framework of the final target, as depicted in Equation (89) <2001JA1878>. 

Heterocyclic stannanes were also employed and yielded S-acetoxyfuran-2(S//)-ones and S-acetoxy-1-methyl-3-pyrrolin-2-one from 2-stannylfi s and 2-stannyl-A(-methylpyrTole, respectively (Scheme 8). In the furan system, it appears initial oxidation of the stannane provides the 2-furyl acetate, which is known to undergo further oxidation with LTA (Scheme 8). ... 

Jousseaume et al. reacted stannylfurans containing an aldehyde substituent in the 2-,3-, or 4-position with chloroforraates [86]. Liebeskind and Wang carried out annelations of furan, thiophene and benzofurans by coupling stannyl derivatives with chlorocyclobutenones [87] (Scheme 4-29). 

Reactions with aryl halides have been described [79-81] in one case a stannylated benzofuran was allowed to react with an aryl iodide containing sterically demanding trialkylsiloxy groups in positions 3 and 5, but yields remained good to excellent [82]. Yang and Wong have also reported coupling reactions of 3,4-bis(tributylstannyl)furan [83,84] (Scheme 4-28). 

V-protected 4-iodoimidazoles have been employed to synthesize an assortment of imidazole-heteroaryls [53, 54]. For example, coupling between stannyl furan and imidazole 84 proceeded efficiently to produce 85 [31]. 

Due to the lack of chemoselectivity the addition of aldehydes to unsymmetrical furans is seriously limited. However, silyl- and stannyl-substituted furans have been used in natural product synthesis, for example, the synthesis of a ginkgolide-B-dasurenone hybrid [potential antagonist of the platelet activating factor (PAF)]77. 

Mono- through tetrasubstituted furans can be prepared by the reaction of ot-bromoketones with a-stannylated carbonyl compounds (Equation (30)) <83CC989>. 

The same methodology has also been applied to the synthesis of stannylated furans. Heating bis(tributylstannyl)acetylene and 55 in a sealed mbe at 185°C for 10 days provided 3,4-bis(tributylstannyl)furan 139 and 3-(tributylstannyl)furan 140 in 22 and 10% isolated yield, respectively (Fig. 3.41). These stannanes underwent palladium-catalyzed couplings with aryl, vinyl, and benzylic halides as well as acid chlorides. 

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