Three novel tetraprenylated alkaloids, malonganenones A–C (1–3), were isolated from the gorgonian Leptogorgia gilchristi collected near Ponto Malongane, Mozambique. Compound 1 is the first 3,7-disubstituted hypoxanthine to be discovered from a marine source, while 2 and 3 represent the first formamides to be isolated from gorgonians. The unexpected facile exchange of the formamide proton of 2 with a deuteron originating from the NMR solvent used for spectroscopic analysis hampered initial attempts at the structural elucidation of this compound. The anti-oesophageal cancer activities of 1–3 are compared with that of rietone (4), a related triprenylated compound previously isolated from a South African soft coral.