Furan cores

A Dotz benzannulation reaction was utilized in the synthesis of the furo[2,3- >]furan core of aflatoxin B2 as illustrated below <06TL2299>. Synthesis of polynuclear aromatic compounds was achieved by using [5+5] cycloaddition of 2-alkynylarylcarbene complexes and enyne-aldehyde derivatives <06TL5303>. 

Furfural. Furfural is readily obtainable from dehydration of pentoses. Reduction of furfural can lead to a variety of products that are more volatile, more stable and possibly also more useful than furfural itself. Selective reduction of the aldehyde moiety leads to furfuryl alcohol (Scheme 15), whereas further reduction of the furan core will lead to tetrahydrofurfuryl alcohol. Reductive deoxygenation can result in the formation of either 2-methylfuran or 2-methyltetrahydrofuran, which can be used as liquid fuels or solvents. 

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. 

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 following scheme, the benzo[Z>]furan core of antibiotic erypoegin H was built up by a PtCL-catalyzed cycloisomerization of or// o-alkynylphenyl-0,(9-acetals <07AGE4760>. A similar type of reaction was applied to the syntheses of 2,3-disubstituted bcnzo b furans <07T8670>. A metal-catalyzed cyclization was also used in the syntheses of 5,6-disubstituted furo 2,3-c/]pyrimidines from alkynyl-pyrimidinols and aryl iodides <07T1931>. 

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 phalarine, the dihydrobenzo[Z>] furan core shown below was constructed by treatment of the substrate with TFA, followed by a CSA-catalyzed rearrangement <07AGE1448>. Asymmetric total synthesis of bisabosquals was also achieved via an epoxide-ring opening reaction <07T10018>. 

It is well known that enolates act as O-nucleophiles in Sisr -type cyclizations to furnish alkylidene tetrahydrofurans (Equation 72) <2004JOC6715>. A double intramolecular Sn reaction with O-nucleophiles to yield bis-tetrahydro-furan cores of acetogenins has been achieved <1999JOC2259>. 

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>. 

Cu-Bisoxazoline-catalyzed asymmetric cyclopropanation of methyl 2-furoate with ethyl diazoacetate was a key step in the synthesis of the cw-fused 5-oxofuro[23-fc]furan core of spongiane diterpenoids <05OL5353>. An interesting example of rtiodium-catalyzed intramolecular addition of a diazoketone to furan affording a strained cyclobutenone, is illustrated below. Iodine-induced isomerization of the product provided the fused tricyclic dihydrofuran compound <05HCA33Q>. 

Acetals incorporated in aimelated five-membered rings are prominent examples in natural products such as, for instance, aflatoxins. A new route to this structural motif weaving the different reactivities of a furan core into a reaction pathway was... 

Among many various formal [3 + 2] approaches towards assembly of the furan core described in theUterature, special attention was given to the Rh- or Cu-catalyzed reactions between alkynes and a-diazocarbonyl compounds [196-200]. This method quickly became very popular as a highly convenient and general tool for the construction of diversely substituted furans, and is often referred to as the... 

Flynn and coworkers have used a palladium-catalyzed MCR to construct the benzo[ ]furan core of frondosin B 49 (Scheme 6.21) [23], a strategy that they have already established for the synthesis of some analogues of the anticancer compound combretastatin A-4 [49]. Bromide 157... 

Scheme 14.10 Regioseiective opening of zirconacyciopentadienes proposed one-pot synthesis of the furan core of the ieupyrrins.

Braddock DC, Bhuva R, Millan DS, Perez-Fuertes Y, Robers CA, Sheppard RN, Solanki S, Stokes ESE, White AJP. A biosyntheticaUy-inspired synthesis of the tetrahydro-furan core of Obtusallenes II and IV. Org. Lett. 2007 9(3) 445-448. 

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