Fused benzothiophenes

A related approach has been used for the synthesis of poly(thiaacene)s, which are novel helical aromatic polymers comprised of fused benzothiophene rings, poly(thiaheterohelicene) <20050L755>. 

The literature on conductivity in poly(p-phenylene sulfide) is confused. According to Shacklette et al.74) heavy doping with AsF5 causes reduction of the polymer with the formation of fused benzothiophene structures which are responsible for conjugation. This would more properly place poly(p-phenylene sulfide) in the category of precursor polymers, discussed later. On the other hand, Friend and Giles 75) proposed an intrinsic conduction mechanism, based on optical measurements and Tsukamoto et al. 76) have presented XPS and 13C NMR measurements to support this view. 

The fluorescence excitation and emission spectra of the electrogenerated fused benzothiophene oligomers [poly(39) and poly(41)] show the existence of dramatic red shifts of the fluorescence maxima and important increases of the fluorescence intensity relative to the parent monomers. These results suggest the existence of extended electronic conjugation in the oligomer chains. Poly(39) and poly(41) showed a well-structured excitation band with maxima at about 335 nm and 395 nm, respectively. These excitation maxima are strongly red shifted by about 50 and 108 nm, respectively, against the 39 and 41 excitation spectra. The emission spectra are characterized by a relatively wide, poorly structured band, centered at 410 nm and 445 nm, respectively. These emission maxima also present dramatic red shifts relative to the emission spectra of the parent monomers. 

T. Iwasaki, Y. Kohinata, H. Nishide, Poly(thiaheterohelicene) a Stiff Conjugated Helical Polymer Comprised of Fused Benzothiophene Rings, Org. Lett. 2005, 7, 755-758. 

This approach is used for the syntheses of fused heterocycles which have a [l,3]-heteroatom juxtaposition in one of the rings <1996CHEC-II(7)49>. The unambiguous synthesis of tetrahydro-3//-benzothieno[2,3- / imidazole 87, as a target for the potential treatment of anxiety disorders, was accomplished starting from commercially available ethyl 2-amino-4,5,6,7-tetrahydro-benzothiophene-3-carboxylate 85 the key formation of the bicyclic heterocycle occurred through a final acid-mediated cyclization of 86 (Scheme 17) <1997SC473>. 

The synthesis and preliminary biological activity of substituted 7-alkylseleno-l,4-dihydro[l,6]naphthyridines have been reported <2000DOC218> along with their further synthetic ttansformations <2001RCB122>. There have also been reports of the preparation of selenium-containing fused heterocycles. The C=0 function in benzothiophen-3-ones, 3,4-dihydrothiopyrano[3,2-. ]benzothiophen-4(2//)-ones, and 3,4-dihydro-2//,5//-thiopyrano[2, 3 4,5]thiopyrano[3,2-/ ]ben-zothiophen-4-ones reacts with selenium dioxide and thionyl chloride to give fused 1,2,3-selena and thiadiazoles via their semicarbazones <1999IJB308>. 

Equilibrium and rate constants for the keto-enol tautomerization of 3-hydroxy-indoles and -pyrroles are collected in Table 32 (86TL3275). The pyrroles ketonize substantially (103-104 times) faster than their sulfur or oxygen analogues, and faster still than the benzo-fused systems, indole, benzofuran, and benzothiophene. The rate of ketonization of the hydroxy-thiophenes and -benzothiophenes in acetonitrile-water (9 1) is as follows 2-hydroxybenzo[b]thiophene > 2,5-dihydroxythiophene > 2-hydroxythiophene > 3-hydroxybenzo[/ Jthiophene > 3-hydroxythiophene. 3-Hydroxythiophene does not ketonize readily in the above solvent system, but in 1 1 acetonitrile-water, it ketonizes 6.5 times slower than 2-hydroxythiophene (87PAC1577). 

In addition to procedures for pyridine ring closure based on the use of 3-amino-thiophene derivatives, there are alternative methods for the construction of thieno [3,2-Z>]pyridines. One approach made use of cyclic (3-keto sulfones, which proved to be convenient synthons for the modified Hantzsch synthesis of fused pyridines (1986KGS1563, 1990JHC1453, 2000MI1, 2002USP6191140). For example, the reactions of benzothiophene 1,1-dioxide 168 with enamines 169 or methylene-active compounds 170 in the presence of NH4OAc produced fused dihydropyridines 171 (1990JHC1453). 

Equilibrium and rate constants for the keto-enol tautomerization of hydroxy heterocycles are summarized in Table 38 <1986TL3275>. The pyrroles ketonize (i.e., 226 — 230) substantially faster (103-104 times) than their sulfur or oxygen analogues, and still faster than the benzo-fused systems (indole, benzofuran, and benzothiophene). 

In the case of fused aromatic nuclei (e.g., p-naphthols, 5- and 6-hydroxyindoles or -hydroxy benzothiophenes, 6- and 7-hydroxycoumarins and analogous derivatives - ), however, the ortho/ot position (77) is still predominantly aminomethylated, even when it is appreciably hindered. - In this connection, it has been discov-ered- - - that the ortho/p position of 5- and 6-hydroxyindoles 78 is not active at all. 

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