Abstract: The genus of Laurencia has been well known for its production of terpenes and C₁₅acetogenins, which are usually halogenated. Anew iodolactone (1) was isolated from the ethanol extract of the alga Laurencia majuscula, collected from the Xisha Islands in the South China Sea. Compound 1 is a kind of white solid, m. p. 63-64℃. The molecular for-mula was established by HREIMSas C₂₆H₄₇O₄I. Generally, chlorinated or brominated metabolites are common in marine organisms. In contrast, iodosubstituted compounds are quite rare and interesting. The structure of 1 was elucidated by the spectral analysis, having a 3-iodo-δ-lactone structure moiety. Besides, two known compounds, cholesterol and n-e-icosanoic acid, were obtained as well.

Key words: Alga, Laurencia majuscula, Iodolactone