Acetopterine (1) is a new tetracyclic pseudopterane alkaloid isolated from the purple sea plume Pseudopterogorgia acerosa collected in Puerto Rico. Its structure has been determined by spectroscopic methods. The unique transposed nature of the lactone function in 1 was established by two-dimensional 1 H-1-H-1 and H-1-C-13 NMR correlation experiments and confirmed by mass spectral analysis. Copyright (C) 1996 Elsevier Science Ltd.