Pestalotiopens A and B: Stereochemically Challenging Flexible Sesquiterpene-Cyclopaldic Acid Hybrids from Pestalotiopsis sp.

From the endophytic fungus Pestalotiopsis sp. isolated from the leaves of the Chinese mangrove, Rhizophora mucronata, two novel hybrid sesquiterpene-cyclopaldic acid metabolites with an unusual carbon skeleton, named pestalotiopensA and B, were obtained, together with the already known phytotoxin altiloxinB. PestalotiopenB even contains a third, triketide-derived module. The constitutions and the absolute configurations of the new metabolites and of altiloxinB were unambiguously determined by a combination of spectroscopic methods and quantum-chemical optical-rotatory dispersion (ORD) and circular dichroism (CD) calculations. A biosynthetic pathway to pestalotiopensA and B is proposed with altiloxinB as one of the suggested precursors. PestalotiopenA shows moderate antimicrobial activity against Enterococcus faecalis.