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Polymerisation of thiophen with phosphoric acid gave two major products, 2.4-di-2-thienyltetrahydrothiophen (I) as cis-and,"trans-isomers, and 4,7-di-2-thienyl-l,2,3,4-tetrahydrobenzo[b]thiophen (IV). The structures of the isomers of (I) were established by dehydrogenation to 2,2':4',2"-terthienyl (11) and a detailed analysis of the lH n.m.r. spectrum. Physical methods were used to elucidate the tetrahydro[b] benzothiophen structure (IV) from which 4,7-di-2-thienylbenzo[b] thiophen (VI) was obtained by dehydrogenation.Acm-catalysed polymerisation of thiophen was first reported by Victor Meyer.l Since then the polymerisation of thiophens 293 and, more recently, di-and trichlorothiophens by various acidic catalysts has been extensively investigated; in most cases amorphous, highly insoluble products were obtained. However, Hartough et aL5 showed that thiophen and alkylthiophens with 100 yo orthophosphoric acid and various synthetic silica-alumina catalysts or montmorillonite clays at elevated temperatures gave liquid polymers. A ' trimer ' structure (I) and an undefined ' pentamer ' structure were assigned to the two principal components of the liquid polymer obtained from thiophen.The major component, an oil C1,H1,S,, was assigned structure (I) on the basis of spectral evidence and highpressure destructive hydrogenation-desulphurisation which gave a product containing at least 20% of 5methylundecane. This evidence could be equivocal and the correctness of structure (I) has been questioned,6 especially as pyrrole is known to produce a linear trimer under acid conditions.' No further investigations have been reported although 2,2':4',2"-terthienyl(I1) has been obtained in reasonable yield by dehydrogenation of the crude polymeric product.8

As part of a study, by high-resolution n.m.r.

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Übersichtsreferat (56 Literaturzitate) über neueste Untersuchungen zur Chemie cyclischer Peptide und Depsipeptide.