3- substituted furans

A one-pot synthesis of furan 2-substituted-3-carboxylic and 2-substituted-3,4-dicarboxylic esters was reported. Thus, reaction of an acyl isocyanate with trimethylsilyldiazomethane, a safe replacement for hazardous diazomethane, gave 2-substituted oxazoles, which were treated with dimethyl acetylenedicarboxylate or ethyl propiolate to afford the corresponding di- and trisubstituted furans in good yields <04S1359>. 

Treatment of o-alkynylphenols with alkenyl triflates under the same conditions—potassium acetate and Pd(PPh3)4—under an atmosphere of carbon monoxide produces 2-substituted-3-acylbenzo[i ]furansf (Scheme 9). Depending on the substitution pattern of the reagents, variable amounts of 2-substituted-3-alkenylbenzo[( ]furans, 2-substituted-benzo[fc]furans, and 0-acyl derivatives of the starting alkyne have also been isolated. 

As we found that furan and thiophene substituted oximes can be used as substrates for the INOC reactions (Eq. 5) [29b] similarly, furan substituted nitro alkane 134 is also a good substrate for INOC reactions (Eq. 13) [40]. The furfuryl derivative 134, prepared via Michael addition of furfuryl alcohol to 4-methoxy- -nitrostyrene, was subsequently transformed without isolation of the intermediate nitrile oxide 135 to the triheterocyclic isoxazoline 136 as a 5 1 mixture of isomers in high yield. 

Acetoxylation. Although furans are readily oxidized, furans substituted by a triisopropylsilyl (TIPS) group when treated with DDQ in toluene-acetic acid at 0° undergo acetoxylation at an adjacent a-methylene group.3... 

Diels-Alder reactions have been used to synthesize and functionalize polymers, as reported by several groups. Rotello and coworkers112, for example, covalently attached [60]fullerene to furan and cyclopentadiene substituted resins. The reaction with the furan substituted resin proved reversible. The resin was recovered by heating the fullerene functionalized resin. 

Furan-substituted ethanol amines are easily oxidized affording chiral 2-hydroxymethylpyridinones 67 as precursors to azasugars <99TA3649> or piperidine alkaloids <9981889, 99TL6869>. 

Furans substituted with electron releasing groups usually undergo polymerization with mineral acids due to facile protonation at the 2-position. Aluminum chloride also causes resinification of furan but benzo[6 ]furan and compounds with electron withdrawing groups are, however, more stable. The reversion to type so characteristic of the electrophilic substitution of benzene is by no means prevalent in the chemistry of furan and benzo[6]furan and many apparent electrophilic substitutions in reality proceed by addition. 

Oxidation of alkylfuranes.5 Furanes substituted by alkyl groups at C2 and C5 arc oxidatively cleaved by PCC to /ran.v-enediones, which are useful precursors to cyclopcnlcnoncs. The ra-isomers are formed originally and are then isomerized by the acidic reagent. 2-Alkylluranes are also oxidized in this manner. 

Xylose-based chemicals analogous to some of the glucose derivatives discussed in the next section are commercially available. An example is xylitol, the pentahydric alcohol that is manufactured by hydrogenation of the aldehyde group in xylose. One of the unique reactions of the pentoses known since the nineteenth century is cyclodehydration to yield furfural. Xylose, for example, is dehydrated by acid treatment through one or two intermediates to form the furan-substituted aldehyde in quantitative yield, or 64.0 wt % of the xylose... 

Evers, W.J., Heinsohn, H.H. Jr., Mayers, B.J., Sanderson, A. Furans substituted at the three position with sulfur. In Phenolic, sulfur and nitrogen compounds in food flavours, Charalambous, G., Katz, I. (Eds.), American Chemical Society, Washington, DC, 1976, 184-193. 

Cyclopropanation reactions with alkyl 2-diazobut-3-enoates (entries 3-6) occur with a remarkably high diastereoselectivity. When 1,3-dienes (acyclic 1,3-dienes,cyclopen-tadiene, furan, " substituted pyrroles ) are cyclopropanated with these reagents, the CK-divinylcyclopropane derivatives are formed with high or complete diastereoselectivity, but they usually undergo a spontaneous Cope rearrangement under the reaction conditions. 

Carboxylic acid anhydrides or halides normally require the presence of a Lewis acid (often boron trifluoride) for Friedel-Crafts acylation of furans, though trifluoroacetic anhydride will react alone. Aluminium-chloride-catalysed acetylation of furan proceeds 7 x lO times faster at the a-position than at the P-position. 3-Alkyl-furans substitute mainly at C-2 2,5-dialkyl-furans can be acylated at a P-position, but generally with more difficulty. 3-Bromofuran is efficiently acetylated at C-2 using aluminium chloride catalysis. ... 

Since Reichstein and Staudinger, we have been aware that furan compounds containing one or more sulfur atoms play an essential role in the aroma of roasted coffee. A review of the sulfur volatile constituents of coffee, showing the importance for coffee flavor of furans substituted on the 3-position by a sulfur atom, has been published by Flament and Chevallier (1988). As can be seen in the values given for the furfuryl sulfur compounds, the concentrations are higher in robustas than in arabicas when there is no methyl substituent on the ring. When there is a 5-methyl the concentrations are lower overall but higher in the arabicas than in the robustas (compare for example 1.128 and 1.129). 

This thiol exhibits an intense meat-like aroma and, as with other furans substituted by a sulfur atom in the 3-position, it contributes undeniably to the character of roasted coffee (Holscher and Steinhart, 1994). Its flavor is described as sweetish, fried meat, beef broth (Golovnya and Rothe, 1980), burnt, meaty, onion, thiol at a concentration of 0.5 ppm (Chemisis, 1982). Gasser and Grosch (1988, 1990) gave an odor threshold of 0.005-0.01 ppb and of 2.5-10 xg/m3 air (probably a printing error gave 0.0025-0.001 ng/L). 

Why is the thermal stability of diarylethene derivatives enhanced by replacing phenyl groups with furan or thiophene groups In molecular orbitals calculation, the photochromic reaction is treated as a typical electrocyclic reaction between hexatriene and cyclohexadiene. The thermal reaction proceeds disrotatorily and the photoreaction, conrotatorily. Disrotatory cyclization of A to B requires an increase in free energy larger than 138 kJ/mol, and hence no thermal ring-closure occurs in the case of either phenyl- or furan-substituted molecules (see... 

A new method for the synthesis of furan-substituted allenes via Cu(I)-catalyzed coupling of conjugated ene-yne-ketones with terminal alkynes was developed. A copper carbene migratory insertion was proposed as the key step in this transformation, with conjugated ene-yne-ketones as carbene precursors (14OL4082). 

The synthesis of benzo[i ]furan derivatives has become a very active field because these molecules have been identified as having a variety of biological aaivities. Many syntheses of interesting benzo[f ]furans substituted with carbon, oxygen, nitrogen, or sulfur functional groups at the C-2/C-3 position were reported in 2014. 

Jiang et al. [38] reported syntheses of furan-substituted rhodanine derivatives by Suzuki-Miyaura cross-coupling, followed by Knoevenagel condensation reaction. The derivatives XXXrV and XXXV have shown excellent potency against primary HIV-l strains with effective concentration 50% (EC50) at low nanomolar level of all the synthesized derivatives. The SAR data indicated that these derivatives also inhibit the HIV-l-mediated cell-cell fusion and the glycoprotein 41 (gp41) six-helix bundle formation. 

In 1968, Tobey and Law reported on the intriguing reaction of furan, substituted furans, and cyclopentadiene with both tetrachloro- and tetrabromocyclo-propene (TBCP). The two unsaturated systems reacted smoothly to directly produce cycloheptanoid systems, the products of a formal [4-i- 3]-cycloaddition reaction. The authors proposed in their early manuscripts that the tetrahalocy-clopropenes underwent an initial thermal Diels-Alder cycloaddition to produce the cyclopropyl norbornene derivatives la and lb (Figure 2). However, these workers were unable to isolate or characterize the primary cycloadducts and instead observed a product which had spontaneously rearranged by way of a halogen atom migration to yield the bicyclo[3.2.1]octadiene nucleus 2a and 2b. 

Further internal rearrangement to 1,4 oxygen bridged molecule (IV) (1,4-anhydro-P-D-glucopyranose) is followed by conversion of the aldehyde (V) to the furan-substituted enediol (VI) ... 

Pyrolysis of biomass is carried out under inert atmosphere and forms, depending on the residence time and temperature, char, oil, and gas. Pyrolysis with long residence time at low temperamre (400°C) produces a black solid (charcoal), while fast pyrolysis at high temperarnre (500°C) favors the formation of a black liquor (bio-oil). The short contact times (<2s at ca. 500" C) thus maximize the liquid yield. Fast pyrolysis is preferred by the chemical industry because of the relative ease of handling liquids. However, bio-oil produced by pyrolysis of bulk biomass contains more than 400 different components like carboxylic acids, ketones, aldehydes, sugars, furans, (substituted) phenols, aromatics, and tar (Table 1). Separation of useful chemicals from this complex pool is very difficult. As an alternative, pyrolysis can also be used as a first step for generating heat or electricity, followed by combusting the pyrolytic products. Excellent papers and reviews that describe fast pyrolysis in more detail are available [27-32]. 

Furans substituted at the 2-position efficiently undergo coupling to give 2-alkenylated furans in good yields (Scheme 2.20). Selected alkenes as the coupling partner are acrylates, acrylamides, methylvinylketone, acry-laldehyde, acrylonitrile, and styrenes. 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

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