Furan from 1,4-diketone

The Paal-Knorr synthesis of furans as well as thiophenes and pyrroles has been carried out on solid support and a library of heterocyclic compounds has been prepared <2003SL711>. It has been shown that the acid-catalyzed synthesis of 2,3>4-substituted furans from 1,4-diketones can be assisted by microwave irradiation <20040L389>. 

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>. 

Amberlyst 15 O-Heterocyclics by cyclodehydration Furans from -diketones ... 

Han and Widenhoefer have developed a Pd-catalyzed alkoxidation protocol to furnish 2,3,5-trisubstituted furans from easily accessible 2-allyl-l,3-diketones (Equation 24). Electron-rich and electron-poor aromatic groups as well as heteroaryl substituents give comparable good results. Symmetric 1,3-diketones with two identical alkyl or aryl substituents performed equally well. The reactions of homoallyl and 4-hexenyl-substituted 1,3-diketones also gave rise to furans, albeit in moderate yields <2004JOC1738>. 

Bicydic furan (11) Comes from diketone (12) and the ring-chain disconnection gives simple starting materials. The enamine method of control (Chapter 20) gives good results and the cyclisation needs no added oxygen nucleophiles as there are two oxygen atoms available in (12). 

Carbon sub oxide/sulfuric acid Furans from y-diketones... 

Reactions of carbon suboxide with oxo compounds 5-Alkylidene-l,3-dioxane-4,6-diones from aldehydes Furans from y-diketones... 

Other methods were also used for the synthesis of trifluoromethylfurans. Thus, flash vacuum thermolysis of silyl enol ether of 1,3-diketone 148 at 800 °C afforded furan 69 (70 %) via intermediate allenic ketone 149 [110], Another synthesis of trifluoromethylated furan from 1,3-diketone comprised reaction of diazomethane with 2-(trifluoroacetyl)dimedone 150. This approach produced dihydrofuran 152 in a mixture with methylated product 151. Compound 151 underwent aromatiza-tion into 153 under heating withp-toluenesulfonic acid [111]. 

Diketones are readily transformed to cycHc derivatives, such as cyclopentanones and furans. In this manner, the fragrance dihydrojasmone (3-meth5l-2-pentyl-2-cyclopenten-l-one) is prepared by the base-catalyzed aldol condensation of 2,5-undecanedione. 2,5-Undecanedione is itself prepared from heptanal and methyl vinyl ketone in the presence of thiazoHum salts (329). i7j -Jasmone can be similarly prepared (330,331). 

Julid investigated the behavior of terfuran 22 and bis(thienyl)furan 23 by cyclic voltammetry as well as the EPR spectra of the radical cations derived from these two compounds. Condensation of the diketone 20 with sulfuric acid furnished furan 22 in 18% yield, while reaction of diketone 21 with hydrochloric acid produced 23 in 84% yield.In a related report, Luo prepared oligomeric bis(thienyl)furans via similar methodology. ... 

A variety of 2,5-dialkylfurans are available via the Paal-Knorr condensation cyclization is possible for both hindered and unhindered 1,4-diketones. Fleming prepared 2-cyclohexyl-5-methylfuran (31) in 91% yield via treatment of dione 30 with catalytic p-toluenesulfonic acid in refluxing benzene. Using the same methodology, Denisenko synthesized furan 33 in 35% yield from the corresponding dione (32). ... 

Thus Marvel and Levesque found that from 79 to 85 percent of the oxygen was removed in this process, to be compared with the theoretically calculated figure of 81.6 percent (fraction unreacted equal to 1/c) for intramolecular reaction of this type in a head-to-tail polymer. A head-to-head, tail-to-tail arrangement, consisting of 1,4-diketone structures, should be expected to yield furan rings... 

Disubstituted furans were synthesized from 1,4-diketones, which were prepared from the reaction of methyl vinyl ketones with arylboronic acids in the presence of CO using rhodium catalyst as illustrated below <06T11740>. 

Addition of acyl anions generated from acylsilanes to a,(3-unsaturated ketones using N-heterocyclic carbenes (NHCs) derived from thiazolium salts as catalyst produced 1,4-diketones, which cyclized to form the corresponding furans in good yields under an acidic condition <06JOC5715>. 

The synthesis of a new benzo[6]furan-containing cyclophane has been achieved by irradiation of the starting diketone 113 via a 6-hydrogen abstraction from the o-alkoxybenzophenone. X-ray analysis shows that the cyclophane has a well-defined rectangular cavity <00TL1393>. 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

A survey of Wacker-type etherification reactions reveals many reports on the formation of five- and six-membered oxacycles using various internal oxygen nucleophiles. For example, phenols401,402 and aliphatic alcohols401,403-406 have been shown to be competent nucleophiles in Pd-catalyzed 6- TZ /fl-cyclization reactions that afford chromenes (Equation (109)) and dihydropyranones (Equation (110)). Also effective is the carbonyl oxygen or enol of a 1,3-diketone (Equation (111)).407 In this case, the initially formed exo-alkene is isomerized to a furan product. A similar 5-m -cyclization has been reported using an Ru(n) catalyst derived in situ from the oxidative addition of Ru3(CO)i2... 

Vitexilactone, obtained from Vi tex cannabifolia (Verbenaceae), has been shown to have the structure (16). It has been correlated with rotundifuran. An interesting iron(ii)-catalysed decomposition of unsaturated cyclic peroxides derived from butadienes leads to 3-alkylfurans. This procedure has been used to convert the peroxide (17) from ds-biformene into the furan (18). Some diterpenoid furans are amongst the constituents of Austroeupatorium inulaefolium (Compositae). These include the diketone austrofolin (19), the corresponding 12-alcohol, and the 15-alcohol (20). A triol, austroinulin (21), was also identified. 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

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