SYNTHESIS OF MACROCYCLIC OR LINEAR PYRIDINIUM OLIGOMERS FROM 3-SUBSTITUTED PYRIDINES - MODEL SYNTHETIC STUDIES TOWARD MACROCYCLIC MARINE ALKALOIDS

Refluxing of 3-substituted pyridine 6 in acetonitrile resulted in the formation of a series of oligomers 8, which with an average value of n=14, were found to possess cytotoxicity comparable with those of natural toxins 1 extracted from sponges. By contrast, refluxing of 6 in the presence of potassium iodide afforded the bispyridinium macrocycle 7 in fairly good yield. The macrocyclic compound 7, whose structure was established by X-ray analysis, afforded in four steps an intermediate 13 which proved to be a stable precursor of bis 5,6-dihydropyridinium salt 12 or unstable bis 1,6-dihydropyridine 14. These compounds are expected to be suitable models for studying intramolecular additions of species 4 which are considered to be key intermediates in the biosynthesis of a number of recently discovered macrocyclic pyridine alkaloids.