-New antitunor polyether macrolides were successfully isolated fron a marine sponge, Halichondria okadai Kadota. One of them, halichondrin B exhibited remarkable in vivo antitumor activity. Physiological properties and structures of these compounds are reported herein. The structures have been characterized by a long-straight carbon chain, a polyether macrolide, and a novel 2,6,9-trioxatricyclo[3.3.2.03'7]decane system which is the first example in natural products as far as we know.

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