Armatol F (1), isolated from the red alga Chondria armata by Ciavatta et al., is an unusual triterpene compound characterized by the presence of an oxepane (A-ring), a 6/7/7 fusea-tricyclic ether (BCD-ring), six quaternary carbons, and two bromine atoms in its structure. It is also notable that 1 has a cis ring-fusion between C- and D-rings, which is a rare mode of ring-fusion in polycyclic ether natural procudts. The absolte configuraton of the A-ring and relative stereostructure of the BCD-ring have been elucidated by NMR and degradation studies, but the full absolute stereochemistry of 1 remains unclear. Although the bioactivity of 1 has not been reported, it is strongly expected that 1 would have some biological properties because of its striking structural similarity to bioactive thyrsiferol and the congeners. Since we have been interested in the structural complexity and the potential bioactivity of 1, we have been engaged in studies toward the total synthesis of 1 and the determination of its full absolute configuration. As a part of the studies, the model syntheses of the A and BCD-ring skeletons were performed. In the synthesis of A-ring model 16, a chirality transferring Ireland-Claisen rearrangement and a ring-closing olefin metathesis (RCM) were employed as key reactions for the construction of the oxepane skeleton, and the final bromination at C3 is now under investigation. BCD-ring model 18 was designed to be constructed from B-ring model 36 via a chirality transferring Ireland-Claisen rearrangement, an RCM, and Morimoto's intramolecular bromoetherification. At present, BC-ring 50, possessing a carbon chain corresponding to the D-ring, has been synthesized. The details of the model syntheses will be presented in the poster session.
