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The redox potential (E) of the couple Ce 4+ /Ce 3 + has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HC10 4 solutions with c HC104 decreasing from 7.45 to 0.023 mol kg" 1 . A constant potential of (1.741 V) 298 K , resp. (1.836 V) 368K , indicating the existence of pure unhydrolysed Ce 4+ was obtained at c Hcl04 > 6.05 m. At lower HC10 4 concentration, the potential as a function of the HC10 4 molality, as well as of the pH shows 4 further distinct steps. At 298 K, for instance, the potential became nearly constant at pH values of 0.103, 0.735,1.115, after which it drastically decreased, respectively at 1.679, just before the precipitation of Ce(OH) 4 occurred. The curves indicate obviously the stepwise formation of the Ce(IV) mono-, di-, tri-and tetrahydroxo complexes. The slope of the curves E vs. pH increased gradually with increasing temperature. AS and AH of the redox reaction were determined as functions of T at the different HC10 4 concentrations. AS is positive at c HC104 > 1.85 m and turns to be negative at lower concentrations. AH is negative at all HC10 4 concentrations studied. The cumulative formation constants ß, of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (K h ); were calculated. An unusual decrease of ß t with increasing temperature has been discussed.

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12-Carboxamido- and 12-carboxyl-11-spirostenes were synthesized from the corresponding 12-iodo-11-ene derivative in palladium-catalyzed carbonylation reactions under mild reaction conditions. The synthesis of the iodo-alkene substrate is based on the transformation of the 12-keto derivative (hecogenin) to hydrazone, which was treated with iodine in the presence of a base (1,1,3,3-tetramethyl guanidine). While various 12-carboxamides were synthesized in moderate to high yields by using simple alkyl/arylamines or amino acid methylesters as N-nucleophiles, low yields can be achieved with alcohols as O-nucleophiles. The homogeneous carbonylation reactions tolerate the 3-hydroxy substituent and the spiroacetal moiety.

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