Synthesis and Stereochemistry of the C30−C63 Section of Karlotoxin 2

Karlotoxin 2 (KmTx2) is a marine natural product isolated from the dinoflagellate Karlodinium veneficum. The absolute configuration of KmTx2 was determined by NMR analysis in combination with degradation of the natural product, and the absolute configuration at C49 was revised by computational analysis. On the other hand, we reported revision of the absolute configuration of amphidinol 3 (AM3), whose structure is similar to that of KmTx2, by comparing NMR data of the synthesized partial structures and total synthesis of AM3. In this paper, we report the synthesis of the partial structures of KmTx2 corresponding to the C30−C63 section, and the diastereomer at C37, C41−C43, and C45−C49, which is hypothetically proposed on the basis of the revised structure of AM3. Comparison of the NMR data of the synthetic specimens with those of the natural product suggested that the absolute configuration of KmTx2 is revised to be 37R, 41R, 42R, 43R, 45R, 46R, 47R, 48S, and 49S.