Desorption chemical ionization mass spectrometry of epimeric 3‐hydroxysteroids and derivatives. Stereoselectivity and nucleophilic substitution with ammonia

The desorption chemical ionization mass spectra, using ammonia as reagent gas, of several epimeric 3‐hydroxysteroids and their ether and carboxylic acid ester derivatives are reported. In the case of steroids possessing a Δ 4 ‐ or Δ 5 ‐3α‐benzoate moiety, stereospecific stabilization of the protonated molecular ion [M+H] + is observed. This behavior is rationalized in terms of interaction of the double bond and the protonated benzoate group at C‐3. Nucleophilic substitution by NH 3 is observed when a double bond is present in the vicinity of the substitution center. The nature and the stereochemistry of the leaving group influence this substitution reaction. Our results seem to indicate the operation of a two‐step mechanism (e.g. S N 1 type reaction) rather than a S N 2 type mechanism for the formation of the substitution ion [MOR+NH 3 ] + .