The electron impact induced cleavage of the C-17--C-20 bond and D-ring in trimethylsilyl derivatives of C21 steroids. Reciprocal exchange of trimethylsilyl groups.

The cleavage of the C-17--C-20 bond in the trimethylsilyl derivatives of C21 steroids has been investigated with the aid of isotopic labelling. A significant amount of reciprocal exchange was observed between trimethylsilyl groups or hydrogen atoms on the 20-O- and 17-O-positions. The loss of positional identity of these groups was found to influence the specificity of fragmentation reactions associated with the 17 beta-sidechain and the D-ring. It is supposed that the interchange of these silyl groups is facilitated by donation of the non-bonding electrons on oxygen to the empty d orbitals of silicon.