Bicyclic 5-5 Systems Two Heteroatoms

Svoboda and co-workers studied substitution reactivity of thieno[3,2- ][l]benzofuran 61 1997CCG1468 in comparison with the isomeric [l]benzothieno[3,2-. ]furan 60, which was already described 1993CCC2983 . They discovered that under electrophilic substitution conditions, the system 61 is more stable than 60. 

Electrophilic substitution and metallation reactions of [l]benzothieno[3,2-3][l]benzofuran 63 were studied 2000CCC58 . Bromination, acetylation, benzoylation, formylation, and nitration usually afforded inseparable mixtures of 2- and 7-substituted derivatives as the main products. Disubstitution reactions preferably led to 2,7-disubstituted derivatives. [l]Benzothieno[3,2-3][l]benzofuran-10,10-dioxide 64 and [l]benzothieno[3,2-3][l]ben-zofuran-lO-oxide 65 can be selectively obtained by oxidation of 63. Mononitration of 64 and 65 led selectively to corresponding 7-nitro derivatives, respectively. Only sulfoxide 65 was successfully reduced. 

The influence of catalysts (AICI3 and SnCU) acid chlorides, and solvents (dichloroethane, nitromethane) in the acylation of methyl 2-methyl-4/f-thieno[3,2- ]pyrrole-5-carboxylate 75 was studied. Conditions for the regioselective acylation processes were found and four types of compounds 76a-f, 77a-e, and 78 were obtained. 

The possibility of acetylating the pyrrole nitrogen atom of 9a was investigated. The reaction was performed in the presence of different bases, and the highest yield of the corresponding compound 9b was achieved for potassium tert-butoxide. As should be expected, even trace amounts of this compound are absent from the acylation products of thienopyrrole 9a under Friedel-Crafts conditions 20020L387 . 

For the preparation of [l]benzothieno[3,2- ][l]benzofuran-10,10-dioxide 64, hydrogen peroxide in acetic acid, 3-chloroperoxybenzoic acid, and also the urea-hydrogen peroxide adduct with phthalic anhydride were employed. All three methods provided 64 in excellent yields (88-94%). Monitoring the course of the reaction showed the 

The l3C NMR spectrum of compound (10) (84TL5669) shows for C-3 and C-6 a deviation of 16.7 ppm from the chemical shift of pyrrole at the -position this suggests an electron density increase due to the electron-donating character of the adjacent nitrogen atom. An analogous upheld shift of C-3 and C-6 was observed in the furo[3,2-6]pyrrole (8). 

The signal 173.8 ppm upfield from that of selenophene in selenolo[2,3-c]thiophene (19) is observed also in this region 8lizvi285 . To determine the charge distribution in compound (19), calculations of the electron densities in thiophene, selenophene, thieno[3,4-6]thiophene (25), selenolo[3,4-6] selenophene (26) and selenolo[2,3-c]thiophene (19) were carried out 8lizvi285 . These calculations showed that in the selenolo[2,3-c]thiophene (19) and its derivatives, the sulfur atom is more positively charged than the selenium atom. The charge density is not determined by the character of the heteroatom but by its position in the fused heterocyclic system. 

Mass spectra of the classical and nonclassical thienothiophenes as well as thieno[3,4-c]pyrrole derivatives were discussed in the first edition 84CHEC-i(4)i037 . Other A,B-diheteropentalenes were not mentioned. 

The UV spectral data of known parent A,B-diheteropentalenes are summarized in Table 9. The analogous thienothiophenes and selenolothiophenes show more absorption maxima in their UV spectra than the other systems. The known progressive shift of absorption to longer wavelengths which is observed in the simple five-membered heterocycles in the sequence furan pyrrole thiophene selenophene is missing since the rings are fused. 

There has been a great deal of interest since the mid 1970s in photoelectron spectrometry and the ionization potentials of various A,B-diheteropentalenes have been summarized 84CHEC-I(4)1037 . The photoelectron spectrum of thieno[3,2-6]thiophene (12) was measured in the solid state as well as in the gas phase and has been investigated in comparison with linearly polycondensed poly thiophenes, (66) and (67) 92JCS(P2)765 . 

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