Structural elucidation studies of polyketide tetrasubstituted delta-lactones by gas chromatography/tandem mass spectrometry and electrospray mass spectrometry

A series of tetrasubstituted polyketide delta-lactones were used to evaluat e whether gas chromatography/tandem mass spectrometry (GC/MS/MS) and electr ospray mass spectrometry (ESI-MS) are useful techniques for probing the str ucture and stereochemistry of such highly functionalised molecules. Analyse s were performed with two commercially available mass spectrometers: a Finn igan/MAT GCQ instrument (CI source) and a Q-TOF Hybrid quadrupole time-of-f light instrument (ESI source). The analyses revealed that a range of variat ion in the structure and stereochemistry of the lactones did not affect the fragmentation pathway common to these molecules. By accurate mass determin ation (ESI-MS), the first two fragmentations were assigned to losses of wat er. Although it was anticipated that the initial dehydration would include the hydroxyl group at the 3-position of the lactones, evidence from deuteri um- and O-18-labelling studies suggests that the losses of water instead in volve the oxygen atoms in the ester bond. Attempts to identify further the structures of daughter ions by GC/MS/MS were complicated by extensive rearr angements and non-specific hydrogen/deuterium migrations within the lactone s, Together, these results illustrate the limitations of mass spectrometry in the structural elucidation of complex molecules. Copyright (C) 1999 John Wiley & Sons, Ltd.