Other Annelated Furans

Isobenzofurans and other Annelated Furans. - 1-Methoxyisobenzofuran (72 R = Me) is formed when the acetal (70) is heated in toluene that contains a trace of acetic acid. The acetoxy-analogue (72 R = Ac) has been generated by the action of 3,6-di-2 -pyridyl-l,2,4,5-tetrazine on the adduct 

Fisera, M. Dandarova, J. Kovac, P. Mesko, and A. Krutosikova, Collect. Czech. Chem. Commun., 1981,46, 2421 (Chem. Abstr., 1982, 96, 68 873). 

A modified mechanism for the formation of the cycloheptafuranone (83) by pyrolysis of phenyl propiolate (82) has been proposed.The action of copper(I) phenylacetylide on the azulene ester (84) leads to the azuleno[2,l-Zi]furan(85). 2  

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Isobenzofurans and other Annelated Furans.- The oxonium salt (63), prepared by the reaction of trimethyloxonium tetrafluoroborate with phthalide, can be reduced... 

Benzofurans and Other Annelated Furans.-A new synthesis of benzofurans (73) is by the reaction of the phosphonium salt (72) with acid chlorides RCOCl in the presence of triethylamine. The benzofuran (74) results from the condensation of hexafluorobenzene with acetylacetone. Three instances of the formation of benzofurans from O-aryl-hydroxylamines, i.e. the oxygen analogue of the Fischer indole synthesis, have been reported O-phenylhydroxylamine hydrochloride and benzenesulphonylacetone give a 2 1 mixture of compounds (73 R = CH2S02Ph) and (75), the oxime ether (76) is converted into the aldehyde (77 ... 

The lability of thieno[3,4-6]thiophene (3) and other iso-annelated systems, such as benzo[c]thiophene and benzole] furan, may be due to the strain effect (Mills-Nixon effect see also Zwanenburg et alP and references therein) in the condensed five-membered ring. The stability of the iso-annelated dithienothiophenes 7—9 is noteworthy. Simple LCAO MO method calculations on benzo[c]thiophene indicate that its instability is due to low specific delocalization energy and high free valence index at position 1. 

Linear benz-annelation enhances the reactivity and decreases the stability of benzo[c]furan, as with other o-quinonoid heterocycles.1,3-Diphenyl-naphtho[2,3-e]furan (299) has been obtained from 297 via lactol 298 as deep red glistening plates with mp 148-154°C in 89% yield.Although dried crystals of 299 could be stored for several months in the absence of light and air, solutions in organic solvents decolorized slowly on standing... 

The furan annelation reaction represents a new method for the preparation of this important class of compounds. Furans are widely found in nature, and highly functionalised furans are desirable targets in total synthesis work, either as products themselves, or as intermediates. Commercially available or readily accessible furans are usually limited to substitution at the more reactive 2- and 5-positions of the ring. The need for the preparation of furans with substitution at other positions on the ring, or with sensitive functional groups present, led to the development in 1987 of a palladium-catalysed method for their preparation. ... 

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