The current-controlled cathodic cyclisation of N-(oxoalkyl)pyridinium salts in aqueous sulphuric acid using a mercury cathode in a divided cell has been optimised by variation of temperature, charge, current density and acid concentration to yield indolizidines and quinolizidines. Furthermore, the scope of the cyclisation with regard to the nature and position of substituents at the pyridine ring has been investigated. A mechanism involving reduction of the hydrogen bonded aldehyde group and a reversible addition of the formed hydroxyalkyl radical to the pyridinium ring is proposed. The important indolizidine alkaloid (±)-swainsonine was synthesised in only 8 steps from 3-methoxypyridine using the new cyclisation in the key step.
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