Dibenzofurans, synthesis

When 2 -methoxy-2-biphenylamine is diazotized in fluoroboric acid, an 85% yield of solid 2 -methoxy-2-biphenylyldiazonium tetrafluoroborate is produced. On heating in dry benzene, this salt loses nitrogen and provides 0-methyldibenzofuranium tetrafluoroborate as an unstable solid. The latter salt, on account of the low basicity of dibenzofuran, reacts with a range of nucleophiles that undergo methylation with the release of dibenzofuran. Pyridine and pentachloropyridine are N-methylated, benzo[fc]thiophene is S-methylated, and tetrahydrofuran and phenetole are 0-methylated. ° Thus it is no surprise that diazotization of 2 -methoxy-2-biphenylamine, followed by boiling of the diazonium salt in acidic solution, provides dibenzofuran in 90% yield.The scope of this method of dibenzofuran synthesis (Scheme 18) has been little explored probably because of the difficulty of preparing the requisite biphenyls, which is usually accomplished by a crossed Ullmann reaction. A similar synthesis of xanthones from 2-amino-2 -methoxybenzophenones is known. ° ... 

Scheme 1.34 Palladium-catalyzed intramolecular direct arylation for a dibenzofuran synthesis...

Interestingly, an intramolecular palladium-catalyzed oxidative arylation for a dibenzofuran synthesis was reported earlier Shiotani, A. and... 

Della Rosa, C., Sanchez, J.P., Kneeteman, M., Mancini, P.M.E. (2011). Diels-Alder reactions of nitrobenzofurans A simple dibenzofurans synthesis. Theoretical studies using DFT methods.Tetrahedron Letters, 52,2316-2319, ISSN0040d039 Domingo L. R., Aurell M. J. (2002). Density Fimctional Theory Study of the Cycloaddition Reaction of Furan Derivatives wth masked o-Benzoquinones. Does the Furan Act as a Dienophile in the Cydoaddition Reaction .Jourrml of Organic Chemistry, 67, 959-965, ISSN 0022-3263... 

As a metal-mediated approach, intramolecular Ullman-type reaction of 2-(2-haloaryl)phenols with copper salts has been used for dibenzofuran synthesis [35]. However, harsh reaction conditions are generally required. On the other hand, Liu et al. developed the copper-mediated intramolecular O-arylation of 2-(2-haloaryl)phenols under milder and neutral conditions [36]. The reaction of 2-(2-chloroaryl)phenols 65 with electron-withdrawing substituent(s) gave the corresponding dibenzofurans 66 in high yields (Scheme 23.25). They also demonstrated the... 

Dibenzofuran, 1,2,3,4,6,7,8,9-octahydro-synthesis, 4, 693 Dibenzofuran, polychloro-occurrence, 4, 709 Dibenzofuran, 2,4,6,8-tetra-t-butyl-synthesis, 4, 682... 

These dienes are valuable for the Diels-Alder based synthesis of dibenzofurans, dibenzothiophenes, carbazoles and other classes of complex polycyclic heterocyclic compounds. Scheme 2.32 summarizes some of the cycloadditions [81] of 2-vinylbenzofurans (80). 

Kuzuhara S, H Sato, N Tsubouchi, Y Ohtsuka, E Kasai (2005) Effect of nitrogen-containing compounds on polychlorinated dibenzo-p-dioxin/dibenzofuran formation through de novo synthesis. Environ Sci Technol 39 795-799. 

Zhao and Larock have described the synthesis of carbazoles, indoles, and dibenzofurans 118 via a Ic type cyclization that follows a sequence of Pd-catalyzed cross-coupling of alkynes and aryl iodides 116, then nitrogen-directed palladium migration to an arylpalladium intermediate 117 that undergoes an intramolecular Mizoroki-Heck ring closure <06JOC5340>. 

I. M. Muslimova, F. F. Hizbullin.- Problem of Accumulation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Sludge Reservoirs of the Enterprises of Chlororganic Synthesis. Ecological Chemistry.-2001.-Vol. 10.- No. 04, p.269-274. 

From limited data the generalization has been made that this method fails for the synthesis of 1-substituted dibenzofurans and for the synthesis of 4-substituted dibenzofurans unless the amino group and the potential 4-substituent are in the same ring. These generalizations are not substantiated in the synthesis of polychlorodibenzofurans. Polychloro-2-phenoxy-anilines are poorly soluble and nonbasic so that the usual method is precluded. In these cases aprotic diazotization with isopentyl nitrite in tetrachloroethylene at 80 C is the method of choice. Yields, as determined by GLC, are usually of the order of 40-50%. The yields of pure isolated... 

There are a number of syntheses of dibenzofurans of the type shown in Scheme 11 that are usually carried out under basic conditions where X is a variety of leaving groups. A convenient synthesis of 2-dibenzofuranols... 

Baker and co-workers observed during the synthesis of the biphenyl 74 (Scheme 19) from the iodo compound 73 by the Ullmann reaction at 230°C with copper bronze in nitrobenzene that the dibenzofuran 75 (1%)... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

Ebel s method is an adaptation of the Stoermer synthesis of benzo-[h]furans and involves the 0-alkylation of a phenolate anion (229, Scheme 58) with a 2-halocyclohexanone (230). The resultant 2-phenoxycyclo-hexanone 231 is then cyclized by poly phosphoric acid, usually at 100°C, or sometimes by concentrated sulfuric acid, to afford a 1,2,3,4-tetrahydrodi-benzofuran (232). Dehydrogenation to the dibenzofuran is often effected with palladized charcoal, but 2,3-dichloro-5,6-dicyano-l,4-benzoquinone ... 

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. 

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