Furan continued

SYNTHESIS OF, furanes (Continued) phosphorane. Palladium( II jchloride. 4-PhenyloxazoIe. 

Decarbonylations of furfuraldehyde to furan continue to be of commercial interest and various new catalysts have been recommended.253 Decarboxylations are still occasionally useful,254 and the selective decarboxylation of furan-3,4-dicarboxylic acid to furan-3-carboxylic acid is said to be much improved by omitting any solvent.255 The easy decarboxylation of furan acetic acid derivatives is formulated in structure 139, although the acidic conditions need not preclude ring opening.2553... 

Furans continue to be useful synthons for pyridazine syntheses. A detailed investigation of the reaction between ethyl 2-methoxy-2-methyl-3-oxofuran-... 

Furans continue to be useful synthons for pyridazine syntheses. A detailed investigation of the reaction between ethyl 2-methoxy-2-methyl-3-oxofuran-4-carboxylate (47) and hydrazines revealed that, depending upon reaction conditions, compound 49, a mixture of 48 and 49 or, in addition to these, also 50, is obtained.172,173 Similarly, ethyl 5-nitrofuran-3-carboxylate and hydrazine yield a pyridazine, probably by addition, ring opening of the furan, and recyclization [Eq. (12)].174 Another synthesis involves electrolytic methoxylation of a furan derivative and subsequent treatment with hydrazine.175... 

Highly substituted furans play an important role in organic chemistiy not only as the key structural units in mai r natural products, common subunits in pharmaceuticals [30], and flavors [31] but also as useful building blocks in synthetic chemistiy [32]. They have also found utilities as synthetic intermediates or synthons for numerous functional groups, caiboxyhc acids, a-keto esters, and aromatics [33]. For this reason, the efficient syntheses of highly substituted furans continue to attract the interest of synthetic chemists. 

This chapter is a continuation of the one on organometallic chemistry of furans and thiophenes [01AHC(78)1]. It starts with the trends of coordination of... 

Emissions from hazardous waste combustors are regulated under two statutory authorities RCRA and the CAA. The MACT standards set emission limitations for dioxins, furans, metals, particulate matter, total chlorine, hydrocarbons/carbon monoxide, and destruction and removal efficiency (DRE) for organics. Once a facility has demonstrated compliance with the MACT standards by conducting its comprehensive performance test (CPT) and submitting its notification of compliance (NOC), it is no longer subject to the RCRA emission requirements with a few exceptions. RCRA-permitted facilities, however, must continue to comply with their permitted emissions requirements until they obtain modifications to remove any duplicative emissions conditions from their RCRA... 

This review of furan chemistry is meant to continue the earlier survey by Bosshard and Eugster1 and concentrates upon the period 1968 to the end of 1979. Like the earlier review, this one is limited to the chemistry of the monocyclic furan nucleus and does not deal, except incidentally, with fused rings such as benzofuran or its quinones. Nor does it deal in detail with dihydro- or tetrahydrofurans, nor with compounds like furylpyridine that contain some other heterocyclic nucleus as well. Some butenolides and tetronic acids are admitted to consideration since they are the carbonyl equivalents of hydroxyfurans regarded as enols, but side-chain reactions are wholly excluded unless the furan nucleus clearly affects them in some important way. 

Monitored by ESR spectroscopy, the continuous radiolysis of furan derivatives in water leads, in effect, to the addition of the hydroxy group at the 2-position ring opening of the resultant radical is rapid.2Sla... 

Furans and benzofurans continue to play an important role in the field of heterocyclic chemistry because their skeletons are present in many naturally occurring molecules, and they serve also as extremely useful precursors or intermediates towards the realization of many complex molecules. The authors of the present chapter have placed their attention on the more interesting applications and syntheses of these families of compounds, instead of executing an exhaustive literature search of all the relevant papers that were recorded in 2000. 

Ketoses should react under a similar scheme. Indeed they do but an important problem in the chemistry of ketoses consists on the lack of selectivity due to (1) the complexity of their tautomeric equilibria and (2) their tendency to form tertiary oxocarbenium ions under acidic conditions. Thus, mixtures of open-chain, cyclic and dehydrated products are frequently obtained.7 The discussion about OZT structures obtained from D-fructose as proposed by Zemplen, Wickstrom and more recently by Grouiller et al. continues today. In fact, the first authors claimed the fusion of OZT on a pyran form of D-fructose, while Grouiller suggested the formation of a mixture of fused OZTs with /1-pyran (major) and p-furan (minor) forms (Scheme 22).18... 

Me2S04 or Et,S04 (17.3 mmol) is added with vigorous stirring to the TBA-tetronate (16.5 mmol) prepared by the addition of aqueous TBA-OH (0.5 M, 3.5 ml) to the appropriate tetronic acid (16.5 mmol) and evaporation to dryness under reduced pressure, in CH2C12 (70 ml). The mixture is stirred until the reaction is complete and the solvent is then evaporated. H,0 (50 ml) is added to the residue and the aqueous solution is continuously extracted with Et20 for ca. 3 h. The ethereal extract is washed with aqueous NaHCO, dried (MgS04), and evaporated to give the 4-alkoxy-5H-furan-2-one (R = H,... 

Pure decarbonylation typically employs noble metal catalysts. Carbon supported palladium, in particular, is highly elfective for furan and CO formation.Typically, alkali carbonates are added as promoters for the palladium catalyst.The decarbonylation reaction can be carried out at reflux conditions in pure furfural (165 °C), which achieves continuous removal of CO and furan from the reactor. However, a continuous flow system at 159-162 °C gave the highest activity of 36 kg furan per gram of palladium with potassium carbonate added as promoter. In oxidative decarbonylation, gaseous furfural and steam is passed over a catalyst at high temperatures (300 00 °C). Typical catalysts are zinc-iron chromite or zinc-manganese chromite catalyst and furfural can be obtained in yields of... 

Treatment of a furan with ortho amino and cyano groups continues to be a viable route to fused pyrimidines. Thus, 2-amino-4,5-di-(2-furanyl)furano-3-carbonitrile produces 5,6-di-(2-furyl)furo[2,3-rf pyrimidin-4(//)-ones in 72% yield upon cyclization in acetic anhydride/formic acid <2006JFIC1129>. 

To a cold (-30°C) solution of furan (1.8 g, 27.5 mmol) in EtjO (10 mL) was added, under argon, the solution of n-BnLi in Et2() (1.27 M, 4.8 mL, 6 mmol). The reaction mixture was allowed to reach room temperature, stirred for 1 h, and cooled to -78°C. The solution of aldehyde 69 (1.17 g, 4.1 mmol) in THF-n-hexane (20 mL, 1 1 v/v) was added dropwise and the stirring was continued for 2 h. The reaction mixture was washed with saturated aqueous NH4C1 (10 mL), water (10 mL), brine (10 mL), and dried (MgS04), then concentrated in vacuo. Flash chromatography (hexanes-EtjO-CHjClj, 10 2 1) of the oily residue afforded... 

A theoretical understanding of furan and its congeners followed in the wake of the progress with the benzene problem, but for the five-membered heterocyclic compounds Bamberger s centric formula was always the most reasonable one, as Kekule structures could not be written at all. It is not now clear to what extent the heterocycles were really regarded as benzene analogues, but a remark made by Hantzsch, that coumarone is the furfurane of the naphthalene series was certainly percipient. Furan is variously said to be aromatic, superaromatic or not aromatic at all, for the debate continues. 

Benzo[c]furans generated from acetals (473) (80JOC4061) or benzalphthalans (474) (80JOC1817) continue to provide a source of polycyclic aromatics. 

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