Furan theoretical

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

The two intermediates of commercial furan resins are furfural and furfuryl alcohol. Furfural occurs in the free state in many plants but is obtained commercially by degradation of hemicellulose constituents present in these plants. There are a number of cheap sources of furfural, and theoretical yields of over 20% (on a dry basis) may be obtained from both com cobs and oat husks. In practice yields of slightly more than half these theoretical figures may be obtained. In the USA furfural is produced in large quantities by digestion of com cobs with steam and sulphuric acid. The furfural is removed by steam distillation. 

In agreement with the previously reported theoretical study, the results of semi-empirical calculations showed that the formation of the Dewar isomer is favored [99H(50)1115]. Probably, the observed formation of the azirine derives from a thermal isomerization of the first photoproduct, in line with that described in the case of furan and thiophene derivatives (Fig. 11). 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

The results of these four groups of investigators present, at least for the present author, a bewildering and scarcely understandable diversity in the interactions of furan with arenediazonium ions. Where is the physical organic or theoretical chemist who will accept the challenge to unravel this entanglement It is not surprising to... 

The curve for model XIX represents the appearance of the photographs very well, except that the sixth and seventh maxima actually appear to be equally strong. Several of the twenty theoretical curves calculated for furan are nearly as satisfactory, and one, for the model with C—O = 1.42, C=C = 1.34, C—C = 1.44, and a = 104°, is somewhat better, the sixth and seventh maxima on it being equally high. This improvement is probably not significant, especially since the temperature effect may... 

Thus Marvel and Levesque found that from 79 to 85 percent of the oxygen was removed in this process, to be compared with the theoretically calculated figure of 81.6 percent (fraction unreacted equal to 1/c) for intramolecular reaction of this type in a head-to-tail polymer. A head-to-head, tail-to-tail arrangement, consisting of 1,4-diketone structures, should be expected to yield furan rings... 

The anodic oxidation of furans has been studied from both synthetic and theoretical points.285 Because of the many possibilities for delocalization, tetraphenylfuran is a special case but it does allow certain basic processes to be monitored by cyclic voltammetry.286 In nitrobenzene two steps are seen ... 

Here we will present some basic theoretical photochemistry of a set of furocoumarin compounds, starting from the basic building blocks furan and pyrone, as shown in Fig. 4.13. Comparison is also made with the photochemistry of flavin, the active component of vitamin B2. 

The aromaticity of isoxazole has been reviewed in terms of theoretical and structural studies,138 139 and the conclusion is that it is slightly less aromatic than oxazole and furan. 

Friedrichsen and co-workers (135), along with Padwa, has utilized the carbonyl ylide cycloaddition to generate reactive furan moieties that can be further used in inter- or intramolecular Diels-Alder reactions to prepare aza- and carbocyclic compounds. Friedrichsen conducted a number of synthetic and theoretical studies on the reactivity, regioselectivity, and stereoselectivity of substituted furan formation and subsequent Diels-Alder reaction (Scheme 4.69). 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

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