Marine sterols. 18. Isolation and structure of four novel oxygeneted sterols from a gorgonian coral Melithaea ocracea

Abstract

Four new marine polyhydroxysterols, melithasterols A–D (1a–1d), were isolated from a gorgonian coral Melithaea ocracea of the Okinawa Islands. The ¹H and ¹³C NMR spectral analyses indicated them to be cholestane and 24-methylcholestane derivatives, having an unprecedented 3β,7α-dihydroxy-5α,6α-epoxy-Δ⁸ steroid nucleus. PCC (pyridinium chlorochromate) oxidation of the 3-monoacetate mixture (3a–d) afforded the corresponding α,β-unsaturated ketone mixture (4a–d). The predominant constituent melithasterol A (1a) was identified as its diacetate (2a) by direct comparison with the authentic compound, prepared by lead tetraacetate oxidation of cholest-6-ene-3β,5α,8α-triol 3-monoacetate (5).