Penipyranicins A-C: Antibacterial Methylpyran Polyketides from a Hydrothermal Spring Sediment Penicillium sp.

Four new aromatic polyketides (1-4) were isolated from Penicillium sp. RO-11, obtained from the sediment of a hydrothermal spring in the southwestern region of Saudi Arabia. The new compounds are penipyranicins A-C (1-3), characterized by a 4-methyl-4H-pyran moiety, a structural motif unprecedented among fungal polyketides, and the naphthopyrone derivative isopyrenulin (4). The structures of the new compounds were elucidated on the basis of data from mass spectrometry, 1D and 2D NMR analysis, and comparison between experimental and time-dependent density functional theory-calculated electronic circular dichroism spectra. A plausible biosynthetic pathway connecting penipyranicins and isopyrenulin is proposed. The isolated compounds were active against Gram-positive and Gram-negative bacteria.