Desorption Chemical Ionization Mass Spectrometry of Epimeric 3-Hydroxysteroids and Derivatives. Stereoselectivity and Nucleophilic Substitution with Ammonia

Article abstract of DOI:10.1002/oms.1210170608

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 Δ⁴- or Δ⁵-3α-benzoate moiety, stereospecific stabilization of the protonated molecular ion ⁺ is observed.This behaviour is rationalized in terms of interaction of the double bond and the protonated benzoate group at C-3.Nucleophilic substitution by NH3 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_N1 type reaction) rather than a S_N2 type mechanism for the formation of the substitution ion ⁺.