Synthesis of the FGHI Ring System of Azaspiracid

As a causative agent of food poisonings, azaspiracid (1), a novel marine toxin isolated from the mussel Mytilus edulis, challenges the synthetic chemist with an intricate array of acetal and ketal structures. The construction of the FGHI ring system of azaspiracid was achieved by means of a key BF3⋅Et2O-catalyzed cyclization to form the HI spirocycle. A final chemoselective intramolecular ketalization with Hg(OAc)2 formed the G ring, to afford the N-protected FGHI as a single diastereomer.