Fragmentation study of clerodane diterpenes from Casearia species by tandem mass spectrometry (quadrupole time‐of‐flight and ion trap).

Rationale: Clerodane‐type diterpenes from Casearia species show important pharmacological activites such as antitumor, antimicrobial and anti‐inflamatory. There are several mass spectrometry (MS)‐based methods for identification of diterpenes; however, there is still a lack of MS procedures capable of providing characteristic fragmentation pathways for a rapid and unambiguous elucidation of casearin‐like compounds. Methods: Casearin‐like compounds were investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS). The fragmentation studies were carried out by tandem mass spectrometry in space (quadrupole time‐of‐flight (QTOF)) using different collision energies and also by tandem mass spectrometry in time (QIT) by selective isolation of product ions. Results: Casearin‐like compounds presented a predominance of sodium‐ and potassium‐cationized precursor ions. Both QIT and QTOF techniques provided sequential neutral losses of esters related to the R1 to R5 substituents linked to the nucleus of the clerodane diterpenes. The fragmentation pathway is initiated with a cleavage of the ester moieties R2 followed by the elimination of the ester groups R3, both losing neutral carboxylic acids. Using QIT, it was also possible to observe the cleavage of the ester groups R1 or R5 by MS4 experiments. Conclusions: Through a rational analysis of the fragmentation mechanisms of Casearia diterpenes it was possible to suggest an annotation strategy based on the sequential cleavages of the ester groups related to the R2, R3 and R5 substituents. These results will assist studies of the dereplication and metabolomics involving casearin‐like compounds present in complex extracts of Casearia species. [ABSTRACT FROM AUTHOR]