Furan heterocycle

Metal-Induced Cycloadditions. The effect of coordination on the metal-iaduced cyclo additions of maleic anhydride and the isostmctural heterocycles furan, pyrrole, and thiophene has been investigated (47). Each heterocycle is bound to an Os(II) center in the complex... 

Despite its unfavorable NMR properties, the nO nucleus has attracted considerable interest, since its chemical shifts represent a discriminating probe for structural and molecular properties. In a study of some 5-membered heterocycles (furan and isoxazole methyl derivatives) (840MR(22)55) it was found that the nO chemical shifts are mainly determined by the p-electron density on the oxygen atom. A nO downfield shift of 222 ppm is observed on the formal aromatization of tetrahydrofuran to furan (61HCA865). 

The five-membered heterocycles (furan, thiophene and pyrrole), despite their... 

For the so-called jt-excessive heterocycles, furan, pyrrole, and thiophene, where the heteroatom contributes two electrons to the aromatic sextet, the HOMO is of relatively high energy, compared to that in benzene, which fact confers the familiar high reactivity of these species toward electrophiles. Correspondingly, ionization to give cation-radicals is facile however, such species are not persistent without stabilizing substitution or annelation (see Section III,D,3). Anion-radicals are also known, particularly for thiophene-containing systems (see Part Two this Series, Volume 27, in press). 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

Isotope exchange is a technique which gives reliable comparative data on the electrophilic and protophilic reactivity of the five-membered heterocycles furan, thiophene, and selenophene, revealing how the activity of their hydrogen atoms varies with the position of the hydrogen in the ring and with the number and position of substituents. The results from kinetic data on isotopic exchange in deuterated selenophenes, thiophenes, and furans, may be summarized as follows. 

Besides the Michael addition of heteroatomic nucleophiles initiating cyclocondensations, acceptor substituted unsaturated systems can also be reacted with carbon nucleophiles stemming from aldehydes in the sense of an umpolung, generally referred to as the Stetter reaction [244-246]. This process is organocatalytic and furnishes in turn 1,4-dicarbonyl compounds, intermediates that are well suited for Paal-Knorr cyclocondensations giving rise to furans or pyrroles. Among numerous heterocycles furans and pyrroles have always been the most prominent ones since they constitute important classes of natural products [247-249], of synthetic... 

The pyridine-sulfur trioxide adduct is a valuable reagent for the sulfonation of acid sensitive substrates like the heterocycles furan and pyrrole, which are polymerised by strong acids like sulfuric acid. Pyridine-sulfur trioxide thus converts furan (40) and pyrrole (41) into the corresponding 2-sulfonic acids (Scheme 21). 

Another example of 7s(r) — Vs(r) complementarity is provided by the heterocycles furan (1) and pyrrole (2). The locations of their computed 7S min and Vs min are given in Table 2 [85], Both have relatively high 7S min near the heteroatoms and lower ones near the a carbons of furan and the C(a) — C(j8) bond midpoints of pyrrole. The KS mm values are associated with the C(jS) — C(j8) midpoints and the oxygen of furan there is none by... 

Similar to other five-membered-ring heterocycles, furans can be reduced by ionic hydrogenation. While the choice of reaction conditions is critical in order to avoid polymerization, the yields of tetrahydrofurans can be good26. Nevertheless, this method has not been frequently used and catalytic hydrogenation is generally preferred. 

The TT-excessive five-membered ring heterocycles furan, pyrrole, and N-methylpyrrole add readily to C-7 of DNBF to yield the C(a) adducts 27... 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

The present review includes data on the synthesis and chemical properties of the fluorine containing derivatives of furan, thiophene, selenophene, phospholene, and hve-membered heterocycles with two or more oxygen and sulfur atoms pubhshed prior 2007. Every section starts with discussion on the synthetic methods for unsaturated heterocycles (furan, thiophene etc.) and is followed by synthesis and transformations of dihydro and tetrahydro derivatives. 

Ohta and co-workers carried out extensive studies on intermolecular arylation of various heterocycles (furan, thiophene, pyrrole, oxazole, thiazole, A-methylimi-dazole, benz[fe]oxazole, benzo[i>]furan, and benzo[ ]thiophene) with mainly chlo-ropyrazines as heteroaryl halides using Pd(PPh3)4 and AcOK [3]. 

Properties Colorless liq., brandy-like odor, sweet si. fruity flavor sol. in alcohol insol. in water m.w. 158.20 dens. 0.9697 b.p. 84.5 C (11 mm) flash pt. 173 F ref. index 1.4219 Uses Synthesis of heterocyclics (furanes, pyrazolones, isoxazoles, pyrimidines), drugs synthetic flavoring agent in foods and pharmaceuticals Features Fruity flavor... 

Keywords Aromatic heterocycles Furans Pyridines Pyrroles Quinolines Ring-closing... 

Five-Membered Heterocycles Furan, Pyrrole, and Thiophene... 

Pyranoses are formed by reaction of the hydroxyl group at C-5, with the carbonyl group. With some sugars, however, the hydroxyl group at C-4 reacts instead. In these cases, the cyclic hemiacetal that is formed has a five-membered ring. This type of cyclic monosaccharide is called a furanose, after the parent five-membered oxygen heterocycle furan. 

Like the other five-membered heterocycles, furan can also be regioselectively bor-ylated in high yield to afford the C2-borylated product using the Hartwig-Ishiyama protocol. ... 

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