Transition Metal Catalyzed Furans Synthesis provides an overview of Furans, describing properties of these heterocycle compounds and covering traditional synthetic procedures for them. This book then explores catalyzed procedures for Furans synthesis in greater detail and depth then is currently available in published Reviews. Finally, this useful short work discusses natural products and bio-active compounds containing Furans, information of particular interest for their applications to medicinal and pharmaceutical chemistry.
The short series Transition Metal Catalyzed Heterocycles Synthesis Series, authored by Xiao-Feng Wu, summarizes recent achievements on heterocycles synthesis with transition metal as the catalysts, with each volume dedicated to one heterocycle compound.
- Brief, focused review of this active research area, Furans synthesis via transition metal catalysis
- Useful coverage of furans properties and procedures, as well as relevant Furan-containing natural products
- First volume in short work series, 'Transition Metal-Catalyzed Heterocycles Synthesis'
Xiao-Feng Wu was born in China in 1985. He studied chemistry in Zhejiang Sci-Tech University (China) where he got his bachelor's degree in science (2007). In the same year, he went to Rennes 1 University (France) and earned his master's degree in 2009. Then he joined Matthias Beller's group in Leibniz-Institute for Catalysis (Germany) where he completed his PhD thesis in January 2012. Subsequently, he started his independent research at ZSTU and LIKAT. His research interests include carbonylation reactions, heterocycles synthesis, and the catalytic application of cheap metals. He also was a fellow of the Max-Buchner-Forschungsstiftung. On academic record, Xiao-Feng has authored more than 120 publications in international journals, and authored or edited five books, 15 chapters, and two patents.
Synthesized by [3+2] Cyclization Reactions
The procedures based on transition metal-catalyzed [3+2] cyclizations to prepare furans have been discussed here.
Furan; cyclization; organic synthesis; synthetic methodology
In the reported [3+2] cyclization for furans synthesis, 1,3-diketones or propargyl alcohol derivatives are usually applied as the core component. In 1985, Tsuji and co-workers reported a palladium-catalyzed cyclization of propargyl carbonates with 1,3-diketones to produce the corresponding 4-methylfurans . In the presence of palladium catalyst under neutral conditions, good yields of the desired furans can be isolated (Scheme 3.1). For the reaction mechanism, the generation of 1,2-propadienylpalladium carbonate complex was proposed. Scheme 3.1
Palladium-catalyzed synthesis of furans under neutral conditions.
Trost and McIntosh reported a tandem process for the synthesis of furans in 1995 . Terminal alkynes and ?-hydroxy ynoates were applied as the substrates. Depending on ?-hydroxy ynoates applied, furans or butenolides can be selectively produced.
Cadierno, Gimeno, and their co-workers developed a ruthenium-catalyzed simple and efficient procedure for the preparation of tetrasubstituted furans . Starting from readily accessible propargylic alcohols and commercially available 1,3-dicarbonyl compounds, good yields of the expected furans were formed (Scheme 3.2a). The reaction proceeded in a one-pot manner, involves the initial trifluoroacetic acid-promoted propargylation of the 1,3-dicarbonyl compound, and subsequent allyl-ruthenium(II) complex [Ru(?3-2-C3H4Me)(CO)(dppf)][SbF6]-catalyzed cycloisomerization of the ?-ketoalkyne. In the substrates testing, they found that 6,7-dihydro-5H-benzofuran-4-ones can be formed when 1,3-cyclohexanediones were applied as the coupling partner. Soon later, Zhan and co-workers explored the possibility of applying base metal as the catalyst. With FeCl3  or Cu(OTf)2  as the catalyst, propargylic alcohols or acetates and 1,3-dicarbonyl compounds or enoxysilanes as the substrates, the desired substituted furans can be formed in good yields (Scheme 3.2b).
To a 5 mL flask, propargylic alcohols or propargylic acetates (0.5 mmol), 1,3-dicarbonyl compounds (2.0 mmol), toluene (2.0 mL), and FeCl3 (0.025 mmol, 4 mg) were successively added. The reaction mixture was stirred at reflux and monitored periodically by TLC. Upon completion, toluene was removed under reduced pressure by an aspirator, and then the residue was purified by silica gel column chromatography (EtOAc-hexane) to give the pure product.
To a 5 mL flask, 1-(naphthalen-1-yl)-3-(trimethylsilyl)prop-2-yn-1-ol (127 mg, 0.5 mmol), ethyl acetoacetate (195 mg, 1.5 mmol), toluene (2.0 mL), and Cu(OTf)2 (9 mg, 0.025 mmol) were successively added. The reaction mixture was stirred at reflux and monitored periodically by TLC. Upon completion, the toluene was removed under reduced pressure by an aspirator, and then the residue was purified by silica gel column chromatography (EtOAc/ hexane) to afford the pure product. Scheme 3.2
Ru/Fe/Cu-catalyzed synthesis of furans.
In 2003, a novel ruthenium- and platinum-catalyzed sequential reaction to afford the corresponding tri- and tetrasubstituted furans or pyrroles was reported . Starting from propargylic alcohols with ketones, or with ketones and anilines, furans or pyrroles were formed in moderate to good yields with high regioselectivities (Scheme 3.3). For the reaction pathway, the reaction started with transforming propargylic alcohols and ketones into ?-ketoalkyne by ruthenium catalyst. Then, followed by PtCl2-catalyzed hydration of the alkyne moiety by the produced H2O slowly gives the 1,4-diketone which will produce the desired furans after intramolecular cyclization.
[Cp*RuCl(µ2-SMe)2-RuCp*Cl] (38 mg, 0.06 mmol), NH4BF4 (12 mg, 0.12 mmol), and PtCl2 (31 mg, 0.12 mmol) were placed in a 50 mL flask under N2. Anhydrous acetone (30 mL) was added, and then the mixture was magnetically stirred at room temperature. After the addition of propargylic alcohol (0.60 mmol), the reaction flask was kept at reflux temperature for 36 h. The solvent was concentrated under reduced pressure by an aspirator, and then the residue was purified by TLC (SiO2) with EtOAc-hexane (1/9) to give the pure product. Scheme 3.3
Ru and Pt-catalyzed synthesis of furans.
Willis and co-workers demonstrated a rhodium-catalyzed intermolecular hydroacylation and applied to the synthesis of di- and trisubstituted furans in 2011 . The reaction proceeds through hydroacylation and followed by a simple dehydrative cyclization; moderate to good yields of the desired furans can be isolated (Scheme 3.4). Scheme 3.4
Rh-catalyzed synthesis of furans via
Bach and Nitsch reported a methodology for the synthesis of 2-alkenylfurans in 2014 . The reaction relies on a successive twofold SN'-type substitution reaction at methoxy-substituted propargylic acetates. The furan C3-C4 bond is presumably established by silyl enol ether attack at a propargylic cation intermediate. The resulting a-methoxyallene is intramolecularly substituted, leading to cyclization by displacement of the methoxy group (O-C2 bond formation) and to simultaneous formation of the exocyclic alkene double bond. With bismuth(III) triflate as the catalyst, a variety of 2-alkenylfurans were isolated in good yields (Scheme 3.5). Scheme 3.5
Bi-catalyzed synthesis of 2-alkenylfurans.
Balme's group reported an efficient one-step synthesis of furofurans and furopyrroles from the easily available propargyl alcohols (or amines) and arylidene (or alkylidene) ß-ketosulfones in 2000 . Moderate yields of the furans can be produced (Scheme 3.6). Scheme 3.6
Pd-catalyzed synthesis of furofurans.
In 2003, a palladium-catalyzed reaction of acylchromates and propargylic tosylates was developed . Substituted furans are prepared in moderate to good yields (Scheme 3.7). From mechanistic point of view, the reaction was initiated by the oxidative addition of propagylic tosylates to palladium(0) complexes to give 1,2-propadienylpalladium(II) complexes, which then reacted with acylchromates to form 1,2-propadienyl ketones. And then the 1,2-propadienyl ketones were transformed to furans by the action of in situ generated Cr(CO)5. Scheme 3.7
Pd-catalyzed synthesis of furans from acylchromates.
In 2009, Jiang and co-workers developed a novel and efficient method for the regiospecific synthesis of polysubstituted furan aldehydes/ketones . The reaction proceeded via a copper(I)-catalyzed rearrangement/dehydrogenation oxidation/carbene oxidation sequence of 1,5-enynes which in situ formed from alkynols and diethyl but-2-ynedioate under atmospheric pressure. Highly functionalized furans were produced in moderate to good yields (Scheme 3.8). Later on, they found nano-Cu2O can be the catalyst as well. With the same substrates, different selectivity can be observed when silver was applied as the catalyst. An interesting methodology for the synthesis of 3-alkylidene furans from a-alkynyl epoxides and ß-keto esters with palladium as the catalyst was reported as well . Scheme 3.8
Cu-catalyzed synthesis of polysubstituted furans.
In 1994, Pirrung and co-workers developed a rhodium-catalyzed dipolar cycloaddition of cyclic rhodium carbenoids to diagonal carbon for the synthesis of furans . By using diazocyclohexanediones and terminal alkynes as the substrates, the corresponding furans were formed in moderate to good yields (Scheme 3.9). Interestingly, fluorobenzene was applied as the solvent and reactions were run at room temperature. Later on, they further explored this transformation and succeeded in extending the substrates to terminal alkenes. Lee's group studied the using of 3-diazo-2,4-chromenediones and 2-diazo-1H-1,3-phenalenedione as the substrates in this rhodium-catalyzed cyclization with terminal alkynes . Meanwhile, isocyanates, ketones, and olefins were applied as coupling partners as well. Scheme 3.9
Rh-catalyzed synthesis of furans from diazocyclohexanediones.
In 2009, a Rh2(OAc)4-catalyzed process for the cyclopropenations of ynamides was developed by Hsung and Li . Highly substituted 2-amido-furans were formed in good yields by this formerly [3+2] cycloaddition. Interestingly, the products can go [4+2] cycloadditions to give the corresponding dihydroindoles and tetrahydroquinolines under thermal conditions (Scheme 3.10). In this procedure, in addition to diazo malonate and ethyl a-diazoacetate, the corresponding phenyl iodonium ylide was tested as well. Scheme 3.10
Rh-catalyzed synthesis of furans from diazo malonate.
Wang and co-workers reported a copper-catalyzed cascade coupling/cyclization of terminal alkynes with a-alkyl-substituted diazoesters in 2011 . 2,3,5-Trisubstituted furan...