Dibenzofuranic derivatives

Regioselective cleavage of dibenzofuran derivatives has been achieved with lithium metal, as exemplified by the preparation of 3-methyl-2-phenylphenol (108) (80S634). 

Biquinones may also be converted to dibenzofuran derivatives by a variety of methods. Thus the biquinone 176 (Scheme 45) is converted to the dibenzo-furanquinone 177 on treatment with acid. " The same transformation of 176 and its analogs into dibenzofuranquinones may be effected thermally and... 

Coumestans,21 although they have a fused benzofuran ring, will be surveyed as being 8-lactones of 2-(2-hydroxyphenyl)-3-benzofuran carboxylic acids [coumestrol (4)22 is the 8-lactone of 2-(2,4-dihydroxy-phenyl)-6-hydroxy-3-benzofurancarboxylic acid]. The synthesis of some rotenoids 23 from benzofuran derivatives will also be reviewed, but dibenzofuran derivatives, such as usnic acid and its derivatives, will not be treated. 

The acid catalyzed dehydration of 2,2 -dihydroxybiphenyls or their methyl ethers is the classical method for synthesis of the dibenzofurans. p-Benzoquinone is transformed by mineral acids into a series of complex dibenzofuran derivatives (79T535). 

In the case of enaminoketone (77) and /j-benzoquinone, there are two parallel reactions in the /3-position of the enamine 77 and in the 8-position of the tautomeric dienamine 77a [93MC40 94KFZ(2)36]. The first pathway (common in the Nenitzescu reaction) gives a dibenzofuran derivative (78), the second affords an unusual product—a jpiro[benzofuran-3,l cyclohexene] derivative (79). The likely pathways are indicated. The... 

The formation of the substituted dibenzofuran-l,4-dione (405) from 5,5 -dimethyl-2,2 -di-p-benzoquinone (406) can be achieved thermally or by photolysis in hydroxylic solvents such as methanol or acetic acid,421 and a related cyclization has been observed in dinaphtha-quinones.422 A dibenzofuran derivative (407) is also obtained on irradiation of the conjugated dione (408) the intermediate (409) can be isolated by maintaining a temperature of 6° in the reaction flask.423 A mechanism for the photorearrangement of 2-acetyl-3-furyl-p-benzoquinones to derivatives of isobenzofuran has been proposed424 [Eq. (109)]. 

Photochemical cyclization reactions of iodinated diphenyl ethers to produce dibenzofuran derivatives have also been studied545. l,3-Diiodo-2-(4 -methoxyphenoxy)-5-nitrobenzene affords 4-iodo-8-methoxy-2-nitrodibenzofuran upon irradiation in acetone. 

Polychlorinated dibenzofuran derivatives, analysis in environmental objects 90UK1799. 

Dibenzofuran-derived cyclohexamers were also detected in these syntheses. See Chapter 7 for a discussion of these compounds. 

Thermal decomposition of the polymer generates fragments of the polymeric chain and dibenzofuran derivatives. At higher temperatures CO, CO2 and aromatic hydrocarbons are formed. In the presence of potassium carbonate and heating around 340° C, PEKEKK undergoes transesterification reactions forming polymers with ether-ketone ether-ketone-ketone sections. Some results reported in literature regarding thermal properties of these polymers are indicated in Table 9.3.11. 

Oxidation of 2-iodo-4,6-di(terr-butyl)phenol (702) with K3Fe(CN)e in aq. KOH (room temp., 20 min) also provided in 82% yield the corresponding benzoxete 703 probably through o-diphenoquinone. In contrast, similar oxidation of three 2-halo-4,6-di(ferf-butyl)phenols (704, 705 and 706) mainly afforded a dibenzofuran derivative 707 (81%), a mixture of 708 and 709 (23 and 53%, respectively) and a diaryl ether 710 (63%), respectively (Scheme 138). Clearly, iodine and bromine substituents promote the diaryl formation, while chlorine and fluorine substituents prefer to produce diaryl ethers. These results seem to be in good agreement with the ab initio calculations . 

Demethylation of methoxyarenes NaOMe, and the demethylation occur dinone at 180 C. Certain phenolates i dibenzofuran derivatives. ... 

Demethylation of methoxyarenes. Me.iSiSNa is prepared from (MejSiljS and NaOMe, and the demethylation occurs in the presence of l,3-dimethyl-2-imidazoli-dinone at 180°C. Certain phenolates thus generated can undergo cyclization to give dibenzofuran derivatives. ... 

However, 2-t-butyl-4-methoxyphenol was oxidized in the same system to give dibenzofuran derivatives concomitantly with dimer. It was useful to use copper(II)compounds in order to get these phenols by one step synthesis. 

CigHiftO, Mr 344.32, yellow prisms, mp. 150-152 °C, [a]o +490° or-490° (dioxan). A dibenzofuran derivative from lichen the (+)-form occurs in Cladonia mitis and Sphaerophorus ramulifer, the (-)-form in Cladoniapleurota. I. is considerably less common than usnic acid. The biosynthesis involves phenol oxidation of methylphloracetophenone. 

CigHisO, Mr 344.32, yellow prisms, mp. 203 °C, [a] +495° and -495° (CHCl,), respectively. A biologically highly active dibenzofuran derivative, occurring both as the (+)- and the (-)-forms in numerous lichens (especially in Usnea species) antibiotic, antileukemic, growth regulatory (in plants), and antifeedant (in insects) activities have been described. It is used as the free acid and as the sodium salt in pharmaceutical preparations. The biosynthesis presumably proceeds via phenolic oxidation. Other dibenzofuran derivatives from lichens are didymic acid, pannaric acid, pla-codiolic acid, and schizopeltic acid. 

In most cases, the nature of the spacer is the governing factor as far as optical properties are concerned. Pacman porphyrins, with rigid spacers such as anthracene and biphenylene, have received much attention due to the electro-catalytic properties of their cobalt complexes. Variations of both the dihedral angle and the distance between the porphyrm rings in these architectures have afforded examples of cofacial porphyrins linked by xanthene and dibenzofurane derivatives. Recent examples are detailed hereafter. 

In 2011, Yoshikai and Liu developed a Pd(ii)-catalyzed oxidative cyclization reaction of a 2-arylphenol to dibenzofuran derivatives respectively (Scheme 2.27). ° The catalytic system features a simple combination of a palladium(ii) salt and a pyridine ligand and the use of pero g benzoate as an inexpensive and convenient oxidant. In their mechanistic experiments, they showed that the reaction involves Pd(ii)-mediated C-H bond cleavage as the rate-limiting step, which is followed by oxidation to a high-oxidation-state palladium species and subsequent C-O bond-forming reductive elimination. 

Maetani, S., Fukuyama, X, Ryu, I. (2013). Rhodium-catalyzed decarbonylative C-H arylation of 2-aryloxybenzoic acids leading to dibenzofuran derivatives. Organic Letters, 15, 2754-2151. 

Connolly JD, Freer AA, Huneck S (1984a) Crystal structure of (-)-placodiolic acid, a dibenzofuran derivative from the hchen Rhizoplaca chrysoleuca. Phytochemistry 23 702... 

Cyclisation to the aromatic ring of the methylphloroacetophenone precursor of the usnic acids is basically different from the orsellinic acid cyclisations leading to precursors of dibenzofuran derivates, but the oxidative coupling and cyclodehydration reactions of pairs of units are similar in the two cases. With the usnic acids, a C-methyl substituent prevents rearomatisation of one ring. The structurally isomeric usnic acids are related by the orientation of the A ring about the carbon-carbon bond. In the two types of dibenzofuran derivates, the second unit also remains fixed but the A ring is reoriented with respect to the ether linkage. 

The orientation of the second phenolic unit in the usnic acids resembles that of all the known dibenzofuran derivates in that the position involved in the carbon-carbon coupling is meta to a carbonyl substituent and ortho and para to 0-substituted positions. It differs from the dibenzofurans in having a second ortho-hydroxyl substitoent instead of an alkyl group. Dibenzofuran derivates could form from the methylphloroacetophenone precursor of usnic acid by a change in the orientation of the C ring with respect to the ether linkage, and such an orientation would still allow a para-quinoid radical. 

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