Rubensteroid A, a new steroid with antibacterial activity from Penicillium rubens AS-130

A new steroid with strong antibacterial activity, rubensteroid A (1), along with its decarboxylic analogue, solitumergosterol A (2), were isolated and identified from the Magellan Seamount-derived fungus Penicillium rubens AS-130. The structure and absolute configuration of compound 1 were established by detailed interpretation of NMR spectroscopic analysis, mass spectrometry data, and TDDFT-ECD calculations. Compound 1 had a rare 6/6/6/6/5 pentacyclic system, which might be the [4 + 2] Diels-Alder adduct of 14,15-didehydroergosterol (14-DHE) cycloaddition with maleic acid or maleimide, followed by decarboxylation. Rubensteroid A (1) exhibited potent antibacterial activity against Escherichia coli and Vibrio parahaemolyticus, both with MIC value of 0.5 μg/mL.