Oxygenated clerosterols isolated from the marine alga Codium arabicum

Clerosterol [1], (24S)-2-ethyl-3-oxocholesta-4,25-dien-6 beta-ol [2], (24S)-24-ethyl-5 alpha-hydroperoxycholesta-6,25-dien-3 beta-ol [3], (24S)-24-ethyl-7-oxocholesta-5,25-dien-3 beta-ol [4], (24S)-24-ethyl-7 alpha-hydroperoxycholesta-5,25-dien-3 beta-ol [5], and (24S)-24-ethylcholesta-5,25-dien-3 beta,7 alpha-diol [6], were isolated from the marine green alga Codium arabicum. A portion of steroid 5 was epimerized to (24S)-24-ethyl-7 beta-hydroperoxycholesta-5,25-dien-3 beta-ol [8]. LAH reduction of an inseparable mixture of 5 and 8 yielded diol 6 and (24S)-24-ethylcholesta-5,25-dien-3 beta,7 beta-diol [7]. Among these compounds, sterols 2, 3, and 5 were isolated for the first time from a natural source. Metabolites 2-6 showed significant cytotoxicity toward various cancer cell lines.