Sponges of the genus Latrunculia are rich sources of biologically active compounds. Polycyclic alkaloids such as discorhabdins [1][2][3][4][5][6][7][8][9][10][11] and macrolides [12,13], peptides [14][15][16], and norsesterterpene peroxides [17][18][19][20][21] have been found previously in them. Until now, the chemistry of Far-East sponges of the genus Latrunculia has not been studied. We isolated for the first time compound 1 from the new sponge species Latrunculia oparinae (Demospongiae, Poecilosclerida, Latrunculiidae) that was collected during an expedition of the SRS Academic Oparin near the shores of Kuril Islands (Ushishir Island, Rikord Straits, 49°22.10' N, 154°09.5' E).L. oparinae (dry weight 300 g) was extracted three times with EtOH. The extract was concentrated in vacuo. The solid was partitioned between EtOH (90%) and hexane. The aqueous EtOH layer was diluted with water (to 70% EtOH) and extracted with CHCl 3 . The resulting CHCl 3 extract was concentrated in vacuo to a moist oily residue and chromatographed over a column of Al 2 O 3 with elution by CHCl 3 :NH 3 (100:0.1) and CHCl 3 :EtOH:NH 3 (140:1:0.1), then three times over a column of Sephadex LH-20 with elution by CHCl 3 :EtOH (1:1), and then over a column of silica gel with elution by CHCl 3 :EtOH (14:1) to afford 1 (100 mg, 0.03% of dry sponge weight).The molecular formula of 1 was C 18 H 14 79 BrN 3 O 2 S and was confirmed by the HR-ESI mass spectrum: found, m/z 416.0110; calcd, m/z 416.0063. A comparison of the PMR, UV (EtOH), and IR (KBr) spectra with the literature showed that 1 had a structure including the same relative configuration of three asymmetric centers as discorhabdin A from the New Zealand sponge Latrunculia sp. [3] or prianosin A from the Okinawan sponge Prianos melanos [22], which had the same structure. However, 1 isolated by us from L. oparinae had a specific optical rotation index that was opposite in sign to that in the literature. Thus, for 1, [D] D -449° (c 0.01, MeOH); for discorhabdin A, [D] D +440° (c 0.05, MeOH) [3], for prianosin A, [D] D +248° (c 0.19, MeOH) [22]. Furthermore, 1 was obtained by us as reddish crystals whereas discorhabdin A and prianosin A that were isolated previously were reported to be green [3,22]. We hypothesized that these differences were related to the fact that the crystals of 1 that we obtained from EtOH extracts, in contrast with the related compounds, which were obtained from MeOH solutions. An x-ray structure analysis of 1 [23] showed that discorhabdin A was solvated by EtOH, a molecule of which was placed into the unique cavity formed by the N-containing and spirocycles of alkaloid. Obviously, the solvate formed in this manner was rather strong and had a color and optical rotation angle different from the free compound. This hypothesis was confirmed experimentally. We found that storing 1 in MeOH solution at room temperature for 3 d destroyed the strong solvate. The compound acquired an optical rotation close to that reported for discorhabdin A and prianosin A.