A novel fragmentation of trimethylsilyl ethers of 3β‐hydroxy‐Δ⁵‐steroids

An ion formed by loss of 56 mass units from the molecular ion is often seen in mass spectra of trimethylsilyl ethers of C₁₉ and C₂₁ steroids having a 3β‐hydroxy‐Δ⁵ structure and an oxo group at C‐17 or C‐20. The nature of this fragment was investigated by the use of perdeuteriotrimethylsilyl ether derivatives and of [4‐¹⁴C], [3‐¹⁸O], [4,4‐²H₂] and [2,2,4,4‐²H] labelled derivatives of 3β‐hydroxy‐5‐androsten‐17‐one and 3β‐hydroxy‐5‐pregnen‐20‐one. Evidence is presented to show that the neutral fragment of mass 56 is composed of carbon atoms 1, 2 and 3, the oxygen at C‐3 and four hydrogen atoms. During the fragmentation process, the trimethylsilyl group and one of the hydrogens at C‐2 are transferred to the fragment that carries the charge.