Sweritranslactones A–C: Unusual Skeleton Secoiridoid Dimers via [4 + 2] Cycloaddition from Swertiamarin

Skeleton-diversity-oriented
chemical conversion from pure natural
products is a valuable method to obtain natural product-like compounds,
especially those with novel architecture. The application of phytochemical
methods to iridoids yielded three novel secoiridoid dimers: sweritranslactones
A–C (1–3). These molecules
possess a 6/6/6/6/6/6-fused hexacyclic
skeleton and were obtained from swertiamarin, one of the major constituents
of the genus Swertia, via a [4 + 2] cycloaddition
and intramolecular nucleophilic addition under aqueous conditions.
The structures were established based on extensive spectroscopic characterization
and X-ray crystallographic diffraction analysis.