Differentiation of 4α‐ and 4β‐phorbol diesters by fast atom bombardment mass spectrometry

Fast atom bombardment (FAB) mass spectra of a series of stereoisomeric pairs of 4α‐ and 4β‐phorbol diesters were determined in order to evaluate the possibility of differentiation between both isomers. A key feature in differentiating 4α‐ isomers ( cis fusion of A/B ring in a tigliane nucleus) from 4β‐ isomers ( trans A/B) is that the m/z 271 ion is only present in the FAB mass spectra of 4α‐ isomers, whereas this ion is virtually absent in the 4β‐isomers. Analogously, the m/z 255 ion is only seen in the spectra of 4‐deoxy‐4α‐phorbol diesters. The formation of m/z 271 ( m/z 255) is characteristic with FAB ionization, but not with electron impact or chemical ionization. High‐resolution, metastable decomposition and collision‐activated dissociation experiments were carried out to elucidate the ion structure of m/z 271, and the mechanistic rationale for the formation was discussed.