Pergamon PII: S0040-4020(97)00745-X Tetrahedron, Vol. 53, No. 34, pp. 11529-11540, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0040-4020/97 $17.00 + 0.00 Extended Heterocyclic Systems 2.1 The Synthesis and Characterisation of (2-Furyl)pyridines, (2-Thienyl)pyridines, and Furan-Pyridine and Thiophene-Pyridine Oiigomers R. Alan Jones and Pervin U. Civcir School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ UK Abstract: The Stetter procedure has been adapted to produce simple 2-furyl- and 2-thienyl- pyridines and their related oligomers, which have been characterised by 13C NMR spectroscopy, o values for the 2-furyl and 2-thienyl rings have been deduced from the pKa values of the conjugate acids of the 2-furyl- and 2-thienylpyridines. © 1997 ElsevierScience Ltd. Following our recent report I describing the synthesis of oligomeric and polymeric alternating pyrrole:pyridine systems, we now report the synthesis and characterisation of the corresponding oligomeric furan:pyridine and thiophene:pyridine systems. Prior to the commencement of our work, of the relatively few methods described for the synthesis of thienylpyridines 2 many involved the reaction of thienyl Grignard reagents or lithiothiophenes 3-5 with halogenopyridines and the yields were generally poor (2 - 35%). Higher yields (60 - 80%) have been attained using palladium or nickel catalysed coupling reactions 6 and heteroarylboron derivatives.7,8 Photochemical9 and Gomberg-type 10 coupling reactions have also been reported. However, although the higher yielding procedures are acceptable for the synthesis of simple thienylpyridines, they are not readily extended to the synthesis of oligomeric thiophene:pyndine systems. Photochemical9 and Gomberg-type10,11 and borane- mediated8 coupling reactions have also been utilised for the synthesis of furylpyridines. Isomeric (2- furyi)pyridines have also been obtained in good yield via a Hansch-type reaction from pyridinoylacetates.12 Utilising procedures analogous to those described in our earlier paper, 1 we have now obtained furylpyridines and thienylpyridines from formyipyridines, via the Paal reaction on the intermediary pyridyl- 1,4-diketonesl, 13 1 using polyphosphoric acid or Lawesson's reagent 14, respectively (Scheme 1). The ketonic precursor 14 for the 2,6-bis(2-furyl)- and 2,6-bis(2-thienyl)pyridines, 15 and 16, was readily obtained by the Stetter reaction on 2,6-diformylpyridine. As with the corresponding pyrrole:pyridine systems, the simple furyl and thienylpyridines were best characterised by their t3c NMR spectra (Table 1) and the data could be used in the analysis of the polymeric systems. In general, the chemical shifts for the substituted pyridine rings, relative to the unsubstituted ring, suggest that the mesomeric effect of the five-membered rings contributes less to the interaction between the two rings than does their inductive effects. Significantly, it would appear that the thiophene ring is more electron-withdrawing in its overall electronic effect than is the furan ring (see also the pKa data, Tables 2 and 3). 11529