New polyhydroxy sterols from the marine sponge Callyspongia fibrosa (Ridley & Dendly)

Four new polyhydroxylated sterols are isolated from Marine sponge Callyspongia fibrosa collected from the Gulf of Mannar, western Bay of Bengal (India). The structural assignment is based on ¹H and ¹³C NMR spectra. All sterols are based on the known 24S-24-methyl cholesterol 1 which is also isolated, and contain 3β,6β-dihydroxy system and 25-O-acetate as common features (except in the case of sterol 6 that has a Δ²⁵ in the place of 25-OAc). Additional OH substitution is also present at 5α in 4a and at 8β in 5. A further 12β-OH is present in 6 and 7. The hydroxylation pattern is so far known only in coral sterols but is without a precedent in sponge sterols. The major steroid 4a showed antimalarial activity against Plasmodium falciparum on the chloroquine-resistant stain better than on the chloroquine-sensitive strain.