Synthesis of the CDE/FG ring models of prymnesins: Reassignment of the relative configuration of the E/F ring juncture

Synthesis of two diastereomeric models (3a and 3b) corresponding to the CDE/FG ring of prymnesins, polycyclic ether toxins isolated from the red tide phytoflagellate Prymnesium parvum, is described. Comparison of the H-1 and C-13 NMR data for each compound with those reported for prymnesins suggests that the earlier stereochemical assignment of the E/F ring juncture needs to be revised.