Furans oxidative conversion

A catalytic, enantioselective approach towards the synthesis of polyol chains has recently been reported by Carreira et al. and has been applied in a synthesis of the polyol subunit of amphotericin B (Scheme 5) [13]. Aldol addition of the silyl dienolate 26 to furfural (27) catalyzed by the Tol-BlNAP-CuF -complex (2 mol-%) gives rise to the addition product 28 in 95 % yield and >99 % ee after one recrystallization. Spectroscopic evidence indicates that a copper dienolate is formed in situ from the silyl dienolate 26 and is actually the active nucleophile [14], Standard transformations including a j-yw-selective reduction of the ft-hydroxy ketone by the method of Prasad and the oxidative conversion of the furan ring to the car-... 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

Unexpected results were obtained when chromenone-based cyclophanes was treated with DBU to achieve HNO elimination, followed by DDQ oxidation to give benzo[fo]furan-based cyclophanes as shown below <05TL8789>. DDQ oxidative conversion of (Q-p-[2-hydroxyphenylethylene]benzeneethanol into 2-phenylbenzofuran was also reported . In addition, an unusual rearrangement of substituted 2-phenylbenzo[d]pyrrolo[3,2-fo]pyrylium perchlorate to 2-phenylfuro[23-c]isoquinoline was observed <05SL1036>. 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

The conversion of furans by oxidative acetylation or methoxylation to 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofurans respectively, and their subsequent hydrogenation to the corresponding tetrahydrofurans, provides a useful source of protected 1,4-dicarbonyl compounds capable of conversion inter alia into the other five-membered heterocycles [Pg.142]

Formation of a 6-hydroxydihydropyran-3-one by the oxidative rearrangement of a furan followed by its conversion to a pyrylium ylide forms part of a synthesis of the taxane skeleton <96T14081>. 

Maleic anhydride is an important industrial fine chemical (see original citations in [43]). The oxidation of C4-hydrocarbons in air is a highly exothermic process, therefore carried out at low hydrocarbon concentration (about 1.5%) and high conversion. The selectivity of 1-butene to maleic anhydride so far is low. The reaction is composed of a series of elementary reactions via intermediates such as furan and can proceed to carbon dioxide with even larger heat release. As a consequence, hot spots form in conventional fixed-bed reactors, decrease selectivity and favor other parallel reactions. 

The well-known acid-catalyzed conversion of sugars into furan derivatives obviously consists of a complex sequence of reactions, and the industrial heterophasic conversion of pentosans in plant tissues has been discussed in detail.11 The reactions themselves are still not well understood, although xylose and glucuronic acid in deuterium oxide afford 2-furaldehyde without uptake of isotope thus limiting the mechanistic possibilities to those not permitting reversible enolization.12 The bacterial sugar streptose yields... 

Epoxides are believed to be intermediates in the conversion of the enol ethers of 1,3-diketones (ketoaldehydes are less satisfactory) into 2,4-substituted furans by means of the trimethylsulfonium ylids. No epoxides could be isolated, however, nor was it necessary to use acid to effect cy-clization. Methoxydimethylsulfonium ylids were less efficient and tended to produce thiabenzene oxides instead, so Scheme 8 remains speculative.57 The use of thioenols instead of 1,3-diones is advantageous.233... 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

Substituted benzo[b]furan can be oxidatively converted to the lactone 57 in 81% yield, which is allowed to react with chloroformate containing a chiral trans-( R,2R)-2-phenylcyclohexyl group (R ) to give the benzo[ft]furan-based enol carbonate. When the carbonate is treated with DMAP, nearly complete conversion to the new benzo-fused y-lactone 58 is observed in dichloromethane or in THF with a diastereomeric ratio of 3 1 <00OL1031>. 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

The hydrogenation of HMF in the presence of metal catalysts (Raney nickel, supported platinum metals, copper chromite) leads to quantitative amounts of 2,5-bis(hydroxymethyl)furan used in the manufacture of polyurethanes, or 2,5-bis(hydroxymethyl)tetrahydrofuran that can be used in the preparation of polyesters [30]. The oxidation of HMF is used to prepare 5-formylfuran-2-carboxylic acid, and furan-2,5-dicarboxylic acid (a potential substitute of terephthalic acid). Oxidation by air on platinum catalysts leads quantitatively to the diacid. [32], The oxidation of HMF to dialdehyde was achieved at 90 °C with air as oxidizing in the presence of V205/Ti02 catalysts with a selectivity up to 95% at 90% conversion [33]. 

PM), aerosols or tars, oxides of nitrogen (NO ), oxides of sulfur (SO ), dioxins and furans, hydrocarbon (HC) gases, multiple metals, and carbon monoxide (CO). There are many strategies for controlling emissions from thermochemical conversion processes, and they are highly dependent on the process requirements of each individual facility. 

Oxidation of several 1,1-bisphenols 78 with IBD gives spirobenzofuran derivatives of general formula 79 (Eq. 21). This approach, when applied to benzylidine l.l -bisnaphthols 80, leads to a stereospecific cyclization, thereby forming the less hindered naphtho[2,l-fi]furan-2(l//)-spiro-r-(2//)-naphthalene-2 -ones (82) [80JCS(P1)1978,80JCS(P1)1986]. The conversion 80 to 82 probably occurs through intermediate 81 (Scheme 25). 

Oxidation of 2-(trimethylsilyloxy)furan (301) with iodosobenzene in the presence of boron trifluoride etherate and alcohols or acids results in the formation of 5-substituted 2(5//)-furanones 303. The first step of this conversion gives intermediate 302, which on nucleophilic substitution by alcohols or acids affords the products (89TL3019) (Scheme 75). 

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

Furan has served as a starting material for the synthesis of methyl rt-tetradeca-frans-2,4,5-trienoate (177), a compound presumed to be a sex attractant of the male dried bean beetle (72TL3777). Conversion of furan to 4,4-dimethoxy-frans-but-2-en-l-al (174) was accomplished in two steps by a previously reported procedure. Intermediate (174) was then ethynylated, acetylated and reacted with lithium dioctylcuprate to yield allene (176). Hydrolysis of the acetal and Mn02-NaCN oxidation of the aldehyde afforded the methyl ester (177 Scheme 37). 

An anionic technique by indirect grafting was proposed for N-metallation of Nylon by Yamaguchi (153-155), in which alcali metals dissolved in liquid ammonia displace the amidic hydrogen atoms. Nylon derivatives and graft copolymers can be synthetized from the N-metallated Nylon (153). For ethylene oxide as grafting monomer, the metallated fibers were soaked in a tetrahydro-furan solution of the monomer, at 60° C (154). Methyl methacrylate is grafted on Nylon with a conversion over 90% by this technique (155). Other procedures involve the use of sodium methoxide in methanol solution and subsequent anionic graft copolymerization of acrylonitrile in a tetrahydrofuran solution (156). 

Furans can be converted into N- alkylpyrroles by heating with primary amines and alumina. Similar thermal conversions of furans and benzo[6]furans to their sulfur analogues in the presence of alumina or other metal oxide catalysts and hydrogen sulfide are also known. l,3-Diphenylbenzo[c]furan is converted into the thiophene by heating with phosphorus pentasulfide. The mechanism of these reactions is obscure. 

Examples of the conversion of three-membered ring systems into furans may be found in the review (66AHC(17)377). A more recent example was the oxidation of cis (but not... 

An alternative approach to the oxidative rearrangement uses chlorine as the oxidant in a one-pot synthesis (80JOC1109). Not only does this reagent convert the furan to the pyran-3-one, but it takes the latter to the 4-halogeno derivative via the labile 3,4-dihalogeno compound. The pyran-4-one is obtained by an in situ acid-catalyzed hydrolysis. The overall yield of maltol is over 60%, and ethylmaltol is obtained in a similar conversion from ethylfurfuryl alcohol. 

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