Characterization of 27-hydroxy-13-desmethyl spirolide C and 27-oxo-13,19-didesmethyl spirolide C. Further insights into the complex Adriatic Alexandrium ostenfeldii toxin profile.

Alexandrium ostenfeldii is a widespread toxic dinoflagellate that has recently bloomed across the Adriatic Sea, seriously threatening both shellfish consumers and aquacultures. In 2007 we reported on preliminary studies carried out on field samples and cultures of A. ostenfeldii. At the time, along with three major spirolides - among which 27-hydroxy-13,19-didesmethyl spirolide C (3) proved to be a novel compound - a number of new minor spirolides were detected. Unfortunately, for all of them only Mass Spectrometry-based structural hypotheses could be ventured due to their very small amount. In the present paper we report on isolation and High Resolution Mass Spectrometry- and NMR-based structural elucidation of two of those minor spirolides detected in our previous study.