Trimethylsilyl group migration during electron impact and chemical ionization mass spectrometry of the trimethylsilyl ethers of 20-hydroxy-5a-pregnan-3-ones and 20-hydroxy-4-pregnen-3-ones

The electron impact and chemical ionization (isobutane) mass spectra of the TMS derivatives of 20-hydroxy-5a-pregnan-3-ones and 20-hydroxy-4-pregnen-3-ones include ions [M ? 44]+? attributable to loss of a CH3CHO fragment from C-17 with migration of the TMS group to the charge-retaining fragment. Mass spectra of isotopically labelled ([3-18O], [20-18O] and [2H9-TMS]) analogues are consistent with this mechanism; [2H9-TMS] labelling further indicates that subsequent loss of a methyl group from [M ? 44]+? ions does not involve the TMS group. Corresponding ions are not observed at significant abundance in the spectra of 20ß-trimethyl-silyloxy-5a-pregnane and 20ß-trimethylsilyloxy-4-pregnen-3-one 3-O-methyloxime. In the electron impact mass spectrum of the t-butyldimethylsilyl ether of 20ß-hydroxy-4-pregnen-3-one, an ion is observed corresponding to loss of 44 atomic mass units from the intense [M ? C4H9]-ion.