Absolute structure of shoreaketone: a rotational isomeric resveratrol tetramer in Dipterocarpaceaeous plants

A rotational isomeric shoreaketone (1), identified as a skeletal member of resveratrol tetramers, was isolated from three species of Dipterocarpaceaeous plants: Shorea uliginosa, Shorea hemsleyana, and Vateria indica. The structure was elucidated by spectroscopic analysis including NMR experiments and their absolute configurations determined based on circular dichroism data. Shoreaketone has 10 asymmetric carbons and a framework of fused heptacyclic ring system including a spiro ring and an α,β-unsaturated carbonyl group that has not been reported in any other natural product. NMR experiments using shoreaketone indicate the presence of two conformers due to restricted rotation of a C–C bond in solution. The complex stereochemistry is due to its skeleton, 10 asymmetric carbons, and a chiral axis. The conformations of rotational isomeric stilbenoid were studied by variable-temperature NMR, ROESY, a skeletal conversion. The coexistence of two conformers for shoreaketone (1) was confirmed to be 1a and 1b, in which the diaryl-dihydrobenzofuran moiety (unit 1B) is extended below or above the plane of the cyclopentane ring (unit 1A), respectively.