Short side chain sterols from the tunicate Polizoa opuntia.

The sterol composition of the Patagonian tunicate Polizoa opuntia was examined, and twenty-four compounds were identified as having either a delta 5, delta 7, or delta 0 nucleus. Two of them, the rare 20-methylpregn-5-en-3 beta-ol and 20-ethylpregn-5-en-3 beta-ol, were previously unreported as natural products from tunicates. These compounds were present only as trace components; therefore, their structures were confirmed by synthesis from suitable precursors and by comparison of the spectral and chromatographic constants of the synthetic and natural compounds.