Nigerasterols A and B, Antiproliferative Sterols from the Mangrove‐Derived Endophytic Fungus Aspergillus niger MA‐132

Abstract Two new 6,8(14),22‐hexadehydro‐5α,9α‐epidioxy‐3,15‐dihydroxy sterols, nigerasterols A and B (1 and 2, resp.), together with two known cyclopentapeptides, malformins A1 and C (3 and 4, resp.), were identified in the culture extract of Aspergillus niger MA‐132, an endophytic fungus isolated from a fresh healthy sample of the mangrove plant Avicennia marina. The structures and relative configurations of these compounds were established on the basis of extensive spectroscopic analyses. The absolute configuration of compound 1 was determined by application of a modified version of Mosher's method. Nigerasterols A and B (1 and 2, resp.) represent the first 5,9‐epidioxy‐sterol compounds of marine origin, and they displayed potent activity against tumor cell lines HL60 and A549 in a preliminary bioassay.