Palladium -catalyzed furan synthesis

An example of palladium-catalyzed furan synthesis utilizing allenes as starting materials was reported, in which 2,4-disubstituted-2,3-butadienoic acids and 1,2-propadienyl ketones were used and 2,4-disubstituted furans were produced. The reaction may proceed via a matched double oxypalladation-reductive elimination process <04CEJ2078>. In a similar cycloisomerization of substituted allenes to tri- and tetrasubstituted furans with regioselectivity, the allenes were produced in situ from acyloxy-, phosphatyloxy- and sulfonyloxy-substituted alkynylketones via a 1,2-migration of such substituents catalyzed by CuCl or AgBF <04AG(E)2280>. 

Furthermore, Jiang and co-workers developed a powerful palladium-catalyzed furan synthesis (12T3135), in which the cyclization step is catalyzed by palladium(II) chloride (Scheme 9). Instead of a direct oxidation of... 

Scheme 1.15 (a, b) Palladium-catalyzed carbonylative synthesis of furans. 

Scheme 1.17 Palladium-catalyzed carbonyiative synthesis of furans and pyrans.

A useful palladium-promoted reaction on an intact pyrrolidone is illustrated in Scheme 27 in which a 1-vinyl derivative is arylated to give both 1-and 2-isomers.55 Compounds of type 24 may prove to be useful precursors for the synthesis of pharmacologically valuable 2-arylethylamines. A related palladium-catalyzed process is described in the section on furans. (see Eq. 14 in Section IV,B,1). 

The palladium-catalyzed arylation of alkenes by haloarenes, and applications in furan synthesis have been described earlier (see Eq. 14 in Section IV,B, 1). By employing o-aminoiodoarenes and appropriately substituted (Z)-alkenes it has proved possible to develop an efficient synthetic route to quinolin-2-ones (Scheme 140).213... 

A palladium-catalyzed one-step synthesis of dihydrobenzo[fc]furan-based fused aromatic heterocycles from bifunctional bromoenoates or bromoalkyl indoles and iodoarenes was reported, and an example is provided in the scheme below <060L3601>. 2-Alkyl- or 2-aryl-substituted benzo[ >]furans were synthesized by a copper-TMEDA catalyzed intramolecular annulation from the corresponding ketones <06OL1467>. 

Chloro-P-carboline (25) has served as a common intermediate in palladium-catalyzed cross-coupling reactions, offering easy access to several pyridine alkaloids. In Bracher s total synthesis of perlolyrine (27), a P-carboline alkaloid, the Suzuki reaction of 25 with 5-formylfuranyl-2-boronic acid (26) formed the C-C bond between the pyridine and the furan rings <92LA1315>. Reduction of the resulting Suzuki adduct with NaBIL subsequently... 

In the following scheme, the benzo[Z>]furan core of ( )-frondosin B was built up by the palladium-catalyzed quinone reduction, followed by Lewis acid-mediated benzo[Z>]furan formation <07OL3837>. In the total synthesis of bisabosquals, the core structure of benzo[ ]furan was constructed by an epoxide-ring opening reaction <07T10018>. 

In the total synthesis of (-)-conocarpan, the chiral 2,3-dihydrobenzo[b furan core was constructed by radical based intramolecular cyclization as shown below <07CC2151>. In addition, chiral 2-isopropeny 1-2,3-dihydrobenzo[6J furans and 2-amido-bcnzo b furans were be made by palladium-catalyzed reaction in the presence of Trost ligand, and the rhodium(I)-catalyzed cyclization, respectively <07JOC2857 07OL2361>. 

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>. 

The palladium-catalyzed Suzuki-Miyaura reaction of 3,5-dibromo-2-pyronc 100 with benzo[, ]furan-2-boronic acid 101 was applied to the synthesis of 3-(benzo[ ]furan-2-yl)-5-bromo-pyrone 102 in 50% yield (Equation 92) <2004SL2197>. 

The palladium-catalyzed hydrofuranylation of alkylidenecyclopropane 105 with unfunctionalized benzo[ ]furan was utilized in the synthesis of a 2-allylbenzo[ ]furan derivative 106, as illustrated in Equation (96) <2000JA2661>. 

Chloromercurio-benzo[ ]furans 107 were key intermediates for the syntheses of natural product XH14 and its analogs. The synthesis proceeded by the palladium-catalyzed carbonylation reaction as a pivotal step. The 3-chloromercurio-benzo[ ]furan 107 was also reduced to form its hydride derivative by NaBH4 reduction, as illustrated in Scheme 54 <2002JOC6772>. 

The palladium-catalyzed carbonylative annulation of o-hydroxyphenylacetylene was employed to generate methyl benzo[h]furan-3-carboxylate, a key intermediate for the total synthesis of the natural product wedelolactone <03JOC8500>. 

Cationic palladium-catalyzed addition of arylboronic acids to nitriles for the formation of benzo[h]furans was reported <06OL5987>, an example of which is illustrated in the following scheme. The palladium-catalyzed cross coupling of alkynes with appropriately substituted aryl iodides for the synthesis of substituted dibenzofurans in moderate to excellent yields was also achieved <06JOC5341>. The benzo[fc]furan core of heliannuls G and H were constructed by a palladium-catalyzed Ji-allyl cyclization reaction <06TL7353>. The palladium-catalyzed oxidative activation of arylcyclopropanes was applied to the synthesis of 2-substituted benzo[Z>]furans <06OL5829>. 

A previously reported palladium-catalyzed preparation of 2,3,5-trisubstituted furans from epoxyalkynyl esters was extended successfully to the synthesis of 2,3,4,5-... 

The intramolecular Fujiwara-Moritani/oxidative Heck reaction was applied to the synthesis of functionalized benzo[fe]furans and dihydrobenzo[h]furans in 50-80% yields, and 15 examples are given in the article <04AG(E)6144->. Ionic liquid ([bmimJBF4) was found to be an effective solvent for the PdCl -catalyzed intramolecular Heck reaction to realize benzofurans <04TL6235>. Another palladium-catalyzed intramolecular Heck reaction between vinyl triflate and benzo[(j]furan was utilized to construct the seven-membered ring based (-)-frondosin B <04T9675>. 

In connection with a total synthesis of frondosin B, the key intermediate shown below was synthesized by a sequential reaction of the phenol, the enyne and the bromide in a one-pot operation as shown <04OL457>. The palladium-catalyzed intramolecular C-0 bond formation between aryl halides and enolates was employed to make 2,3-disubstituted benzo[i> furans <04OL4755>. 

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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