We report here on the discovery and structure determination of three new diastereomeric pairs of cyclic ether acetogenins, laurefurenynes A–F, isolated from the aqueous extract of the alga Laurencia sp. collected in the Philippines. Extensive use was made of NMR spectroscopic data and high resolution MS to determine the structures of the pure compounds. The most stable and the lowest energy conformation was determined using molecular modelling, and their cytotoxic activity was tested against different tumour cells, a significant indication that laurefurenyne C and F are moderately cytotoxic, but non selective whilst the others are inactive.