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Five new polyketide, biselides A, B, C, D, and E, were isolated from the Okinawan ascidian Didemnidae sp. Their structure were determined by spectroscopic analysis. Interestingly, the structurally related haterumalide NA, haterumalide B, and oocydin A were isolated from an Okinawan sponge, an Okinawan ascidian, and a South America epiphyte, respectively. Biselides A and C showed cytotoxicity against human cancer cells NCI-H460 and MDA-MB-231. Although biselides A and C are less cytotoxic than haterumalide NA, they are not toxic toward brine shrimp. In order to develop a new type of anti-cancer drug, further investigation of the biological activities of biselides is in progress. We planned the convergent synthesis of biselides by Suzuki-Miyaura cross coupling between the C1-8 and C9-19 segments. Synthesis of the C5-C15 segment started from a known glycal. The glycal was converted to an α,β-unsatureted ester in 3 steps. The α,β-unsatureted ester was treated with Triton B to form a tetrahydrofuran unit with a desired stereostructure. After 3 steps, we prepared a terminal olefin compound. Treatment of the terminal olefin under hydroboration conditions with 9-BBN dimer to form a borane, followed by addition of a catalytic amount of PdCl_2(dppf) and an alkenylsilane, gave the desired coupling compound. For preparation of the chloroolefin, we modified the procedure for conversion of an alkenylsilane to a bromoolefin and found that the addition of a catalytic amount of K_2CO_3 was important for the reaction to be reproducible. Futher approach to the synthesis of bidelides is currently underway in our group and will be reported in the symposium.