Exchange in the spirostan side chain

The side chain in spirostan derivatives (44) can undergo certain exchange reactions which are characteristic for this skeleton. In general, this spiro-ketal moiety is quite stable under alkaline or neutral conditions, but in acidic media it undergoes a reversible opening of ring F (45 and 46). In 

When the exchange is carried out on a (25S)-spirostan derivative (50) the compound undergoes epimerization at C-25. During this inversion about 20 % deuterium is incorporated at C-25 [(51), R = D]. This observation is consistent with the various mechanisms which have been proposed for this 

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