Mass spectral behaviour of some semisynthetic derivatives of vinblastine-type alkaloids

The mass spectral fragmentation of the biomedically important vinblastine (1a) and its eight semisynthetic derivatives obtained by selective oxidation have been studied using in‐beam electron impact ionization, low and high resolution, as well as linked scanning techniques. The mass spectra were obtained without any thermal decomposition and exhibited abundant molecular ions and several fragment ions of structural importance. It was found that decomposition of these indole–indoline dimers having a velbanamine, or a ψ‐aspidosperma, or a ψ‐eburnane type indole moiety connected with the aspidosperma skeleton of vindoline (sets 1–3, respectively) led to several fragments of common types due mainly to the presence of the vindoline skeleton. However, sets 1–3 possessing various indole parts give rise to significant and characteristic fragments of these subunits, too. Varying the N(1) substituent of the vindoline part (CH3 → H → CHO), a strong influence was observed in the relative importance of the main reaction routes. It can be rationalized in terms of the effect of substituent on the localization of the positive charge.