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Complex Polypropionates from a South China Sea Photosynthetic Mollusk: Isolation and Biomimetic Synthesis Highlighting Novel Rearrangements.

Placobranchus ocellatus is well known to produce diverse and complex γ -pyrone polypropionates. In this study, the chemical investigation of P. ocellatus from the South China Sea led to the discovery of ocellatusones A-D, a series of racemic non- γ -pyrone polyketides with novel skeletons, characterized by a bicyclo[3.2.1]octane ( 1 , 2 ), a bicyclo[3.3.1]nonane ( 3 ) or a mesitylene-substituted dimethylfuran-3(2 H )-one core ( 4 ). Extensive spectroscopic analysis, quantum chemical computation, chemical synthesis, and/or X-ray diffraction analysis were used to determine the structure and absolute configuration of the new compounds, including each enantiomer of racemic compounds 1 - 4 after chiral HPLC resolution. An array of new and diversity-generating rearrangements is proposed to explain the biosynthesis of these unusual compounds based on careful structural analysis and comparison with six known co-occurring γ -pyrones ( 5 - 10 ). Furthermore, the successful biomimetic synthesis of ocellatusone A ( 1 ) from its γ -pyrone precursor ( 5 ) confirmed the biosynthetic relationship of these two compounds and the proposed rearrangement through an unprecedented acid induced cascade reaction. Besides, an initially untargeted semisynthetic product, polycyclic lactone 11 with another novel carbocyclic skeleton was proved to be naturally occurring in the mollusk by comparative LC-MS/MS analysis, which provides a strategy to fish a vast number of novel trace components in Nature.