(14aS,1aR)-4,8,12,14a-Tetramethyl-2,3,6,7,11,13,14,14a,1a,9a-decahydrofurano[2',3'-1,2]cyclotetradeca[5,6-b]oxirane AIDS-094973 AIDS094973 Isosarcophytoxide Mainly marine derived natural product