The electrospray ionization (ESI) mass spectrometric behavior of five Stemona alkaloids, stemokerrin, oxystemokerrin, oxystemokerrilactone, oxystemokerrin N-oxide and stemokerrin N-oxide, was studied using an ESI tandem mass technique (MS(n)). These compounds, isolated from Stemona saxorum endemic in Vietnam, represent a class of alkaloids containing a pyrido[1,2-a]azepine A,B-ring core with a 1-hydroxypropyl side chain attached to C-4. Their fragmentation pathways were elucidated by ESI-MS(n) results and the elemental composition of the major product ions was confirmed by accurate mass measurement. In order to rationalize some fragmentation pathways, the relative Gibbs free energies of some product ions were estimated using the B3LYP/6-31+G(d) method. Based on the ESI-MS(n) results of five reference compounds, a reversed-phase high-performance liquid chromatography with tandem mass spectrometry (RP-HPLC/MS(n)) method was developed for the characterization of Stemona alkaloids with a pyrido[1,2-a]azepine A,B-ring core from the extract of S. saxorum. A total of 41 components were rapidly identified or tentatively characterized, of which 12 compounds were identified as Stemona alkaloids with a pyrido[1,2-a]azepine A,B-ring core, including four new compounds. This method is convenient and sensitive, especially for minor components in complex natural product extracts.
Copyright 2009 John Wiley & Sons, Ltd.