A new cleistanthane diterpenoid from the cultures of a mutant of Calcarisporium arbuscula NRRL 3705

Calcarispol A (1), a new cleistanthane diterpenoid bearing a rare oxetan-2-one moiety, was isolated from a culture of an aurovertins-null mutant of Calcarisporium arbuscula NRRL 3705, along with two known diterpenoids, 3α,5α,8β-trihydroxycleistanth-13(17),15-dien-18-oic acid (2) and calcarisporic acid J (3). The structure of 1 was elucidated by detailed interpretation of NMR and HR-ESI-MS data. Its absolute configuration was established by single-crystal X-ray diffraction analysis. Biological evaluation showed that 1 had marked cytotoxicity against MCF-7 and HCT-116 cancer cells.