Polarographic analysis of corticosteroids: Part 6. Mechanism of polarographic electroreduction of some Δ4-3-ketosteroids and Δ1,4-3-ketosteroids

Prednisolone and dexamethasone give waves in d.c. and normal pulse polarography and peaks in differential pulse polarography which correspond to a one-electron uptake. The transfer of the first electron is reversible and at pH < 7 (i₁) is preceded or at pH >7 (i₃) is followed by a proton transfer. Protonation occurs on the carbonyl group and pinacol is formed. At pH >7, wave i₃ is followed by wave i₄ which involves transfer of another proton and electron on the carbonyl group and yields the unsaturated alcohol. At pH <3, the radical formed in the first electron uptake is further protonated and is reduced in wave (i₁₊₂). In tetramethylammonium hydroxide solution, reduction of the side chains on C-17 occurs at more negative potentials. Reduction of testosterone and hydrocortisone follows a similar mechanism (A)—(G), but the second electron uptake in wave i₄ at pH >7 is not observed.