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 Δ 5, Δ 7, or Δ 0 nucleus. Two of them, the rare 20-methylpregn-5-en-3β-ol and 20-ethylpregn-5-en-3β-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.