Fast Atom Bombardment (FAB) Mass Spectrometry of Diterpene Resin Acid Derivatives

The behaviour of methyl dehydroabietate and its ring C-substituted derivatives has been investigated by fast atom bombardment (FAB) mass spectrometry conditions. Similar FAB mass spectra were obtained in a 3-nitrobenzyl alcohol matrix using argon or xenon as the target gas, which has been attributed to the hydrophobic character of these compounds. Significant [M – H]⁺, M^+• and [M + H]⁺ peaks and minor abundant [M – 3H]⁺, [M – 2H]^+• and [M + 2H]^+• ions have been observed to occur in the spectra. The intense M^+• peaks generally observed have been rationalised in terms of the relatively aprotic and hydrophobic character of the matrix 3-nitrobenzyl alcohol and the hydrophobic nature of the compounds studied. The molecular ion represents the base peak in the molecular mass region of the amino abietates when 3-nitrobenzyl alcohol or triethyltetramine are used as matrices, presumably reflecting the stability of these radical cations. The relatively aprotic character of the matrix has been suggested to favour the ionization to M^+• ions of the amino derivatives. The electrophilic nitro groups and/or a proton acceptor carbonyl group, have been found to play an important role in the formation of [M + H]⁺ with respect to M^+• ions. The enhanced abundance of [M + H]⁺ with respect to M^+• ions, when HCl was added to the matrix, as well as in a thioglycolic matrix, has been suggested to derive from a contribution of solution chemistry for the compounds bearing one to two unsubstituted amino groups. An important role played by the common hydrocarbon skeleton has been suggested to account for the significantly abundant [M – H]⁺ ions observed.