Stereochemical control in electron-transfer induced reactions. Cyclic voltammetry and cathodic reduction of epimeric 3-halogeno-6-nitro-5-cholestenes.

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The cathodic electrolyses and cyclic voltammetric measurements were carried out for epimeric 3-halogeno-6-nitro-5-cholestenes. The stability and the fate of anion radicals were found to be highly dependent on the stereochemistry of homoallylic halogen groups. Half-lives of the anion radical from 3α-chloro- and 3α-bromo derivatives were found to be 12.5 and 1.6 s, respectively, while those of 3β-halogeno derivatives were much shorter. On cathodic reduction 3β-derivatives gave a cyclosteroid, 6β-nitro-3α,5α-cyclo-5α-cholestane in a two-electron process, whereas 3α-derivatives gave a dimer, 6β,6′β-dinitro-3α,3′α-cholesta-4,4′-diene, as the major product <I>via</I> a one-electron process. Detailed CV studies were carried out for 6-nitro-5-cholestene as a model. The stereochemistry of 3,3′-dimer was established by the <SUP>1</SUP>H NMR measurements.
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