Structures of agelasines, diterpenes having a 9-methyl-adeninium chromophore isolated from the Okinawan marine sponge Agelas nakamurai Hoshino - Wu, H; Nakamura, H; Kobayashi, J; Kobayashi, M; Ohizumi, Y; Hirata, Y (Bull Chem Soc Jpn, 1986)