API-Mass Spectrometry of Polyketides. II

Fragmentation Analysis of 6-Deoxyerythronolide B Analogs

Abstract

The API-MS spectra of 6-deoxyerythronolide B (6-dEB) and a number of its analogs have been studied to gain information into the fragmentation patterns of 6-deoxyerythronolides under atmospheric pressure ionization conditions. The API-MS spectrum of 6-dEB shows five major families of fragments. The spectra of a series of desmethyl-6-dEBs allow assignment of these fragment families to structural subunits as well as provide information regarding the fragmentation mechanisms. The spectrum of [9-¹⁸O]-6-dEB is consistent with loss of the ketone oxygen during the first dehydration, and the spectra of other oxygen-modified analogs implicate the non-obligate formation of a 5, 9-hemiacetal in the initial stages of fragmentation. These results taken together are used to propose a model for the fragmentation of 6-dEB and its analogs under API conditions.