Benzofurans and Other Annelated Furans

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  

R = CHO) by the action of the dimethylformamide-phosphorus oxychloride complex, and 0-(p-nitrophenyl)hydroxylamine and ethyl benzoylacetate yield, inter alia, the benzofuran (77 R = COaEt). Sodium salts of 2-hydroxyben-zofuran-esters, e.g. (78), are produced by the action of dimethyl chloromalonate on hydroquinones.  

Pyrolysis of tri-, tetra-, and penta-chlorobenzenes affords mixtures of chlorinated dibenzofurans and dibenzo-p-dioxins, and chlorinated diphenyl ethers cyclize in the presence of palladium acetate to polychloro-dibenzofurans/ The action of malononitrile on tetrahydroxy-p-benzoquinone leads to the benzo-difuran (79), contrary to a previous report/ The acid-catalysed condensation of quinones with phenols has been studied p-benzoquinone and resorcinol, for example, afford compound (80)/ Naphtho[2,3- ]furan-4,9-diones (81 R = Me or Ph R = Ac, Bz, C02Et, or CN) are obtained from 2,3-dichloronaphtho-1,4-quinone and compounds R COCHaR in DMF that contains potassium fluoride/ The action of mineral acids on p-benzoquinone produces a mixture of complex benzofurans, which includes (82) and (83)/ Bases convert the ben-zofuranone (84) into the tetrameric compound (85), contrary to an earlier 

The tosylhydrazone (88 X = O) decomposes in the gas phase to the carbene (89), which cyclizes to a mixture that contains mainly the dihydrobenzofuran (90) and dihydrobenzopyran (91) and in which the former predominates in the case of the sulphur analogue (88 X = S) the ratio of the two types of product is reversed. The lithium salt (92) of the 2-methyleneaIlyl dianion reacts with o-phthalaldehyde to yield the dihydroisobenzofuran (93), together with the methylenebenzocycloheptanediol (94). A mixture of the positional isomers (96  

= Me) and (96 R = Me, R = H) is obtained by oxidative decarboxylation of the bicyclo[4.1.0]hept-3-ene-l,6-dicarboxylic acid (95) with lead tetra-acetate. The amino-acid (97), prepared from C- (o-hydroxyphenyl)glycine and t-butyl azidoformate, cyclizes to the aminobenzofuranone (98) this type of product exhibits chemiluminescence when exposed to oxygen in the presence of a base. Treatment of 2,2 -di(bromomethyl)benzil (99) with potassium t-butoxide results in an unusual intramolecular nucleophilic substitution to give the spiro-compound (100). 

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