Stereocontrolled Synthesis of the JKLM Ring Fragment of Ciguatoxin

A highly stereocontrolled synthesis of the JKLM ring fragment of ciguatoxin has been achieved. The present synthesis starts with methyl 4,6-O-benzylidene-2-deoxy-2-C-methyl-α-d-altropyranoside, whose configurations and substituents at C2−C5 correspond to those at C46−C49 of ciguatoxin, and involves as the key step base-induced 7-endo selective cyclization of a hydroxy epoxide for the efficient construction of the fully substituted oxepane ring K. Construction of the spiroether ring M was achieved by introduction of an allyl group to the ring L lactone followed by asymmetric dihydroxylation to install the C54 stereogenic center and acid-induced spiroketalization to furnish the protected KLM ring system. Finally, ring J was constructed by the method of Nicolaou to complete the synthesis of the JKLM ring fragment. The synthesis of a carboxylic acid derivative and its conjugation to carrier proteins to give an artificial antigen for development of an immunoassay system for the parent toxin molecule are also reported.