The assigned structure of the dinoflagellate-derived toxin belizentrin was prepared by total synthesis in form of the corresponding methyl ester for stability reasons. The successful route features an unusual solution for the preparation of a recalcitrant ylide on a C-glycosidic segment; moreover, it involves an asymmetric hetero-Diels-Alder reaction en route to the tertiary hemiacetal substructure, a Negishi cross-coupling of two elaborate building blocks, and a macrocyclization based on an intramolecular aminolysis of a spirolactone. A modified Kocienski olefination ultimately allowed the polyol side chain to be attached to the macrocycle although this transformation faced the exceptional base sensitivity of this polyunsaturated target compound.

Preparation of the Tetrahydrofuran Segment (S)-5-(Hydroxymethyl)dihydrofuran-2(3H)-one (S-2). conc. aq. HCl (25 mL) was slowly added to a stirred solution of L-glutamic acid 3 (25.0g, 170 mmol) in water (60 mL) at ambient temperature. The resulting solution was cooled to 0 °C before a solution of NaNO 2 (15.2 g, 221 mmol) in water (80.0 mL) was added dropwise over the course of 45 min, causing a gentle evolution of N 2 gas. Once the addition of NaNO 2 was complete the colorless mixture was warmed to ambient temperature and stirring was continued for 23 h. The solvents were evaporated and the resulting white solid was triturated with EtOAc (100 mL) and filtered off. The filter cake was washed with EtOAc (2 x 100 mL) and the combined filtrates were dried over anhydrous Na 2 SO 4. The drying agent was filtered off and the solvent was evaporated to give acid S-1 as a white solid (17.5 g, 79%) which was used in the next step without further purification. BH 3 •SMe 2 (15.2 mL, 170 mmol) was slowly added to a stirred solution of the crude carboxylic acid S-1 (17.5 g, 170 mmol) in THF (280 mL) at 0 °C over the course of 15 min. Once the addition of BH 3 •SMe 2 was complete, the resulting mixture was allowed to reach ambient temperature and stirring was continued for 18 h. The mixture was cooled to 0 °C and the reaction was quenched with MeOH (70 mL). The solvents were evaporated and the resulting oil was passed through a short plug of Celite ® , eluting with EtOAc. Evaporation of the combined filtrates afforded compound S-2 as a white solid (10.8 g, 69%). 1 H NMR (400 MHz,

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