| Abstract
| - A convergent synthetic route to the (E)FGH ring system 4 of ciguatoxins, the causative toxins forciguatera fish poisoning, has been developed. The synthesis features convergent coupling to formdioxane acetal, regioselective acetal cleavage by diethylaluminum phenylthiolate or diisobutylaluminum phenylselenolate followed by intramolecular radical cyclization to construct the oxepanering G, and a ring-closing metathesis reaction to form the hexahydrooxonine ring F. Thehexahydrooxonine ring F of tetracyclic model system 4 existed as a 5:1 equilibrium mixture of twoconformers (UP and DOWN conformers), with the UP one predominating. This is the first illustrationthat reproduces the preference for the UP conformer over the DOWN one, which preference wasobserved for natural ciguatoxins.
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