Five Pairs of Meroterpenoid Enantiomers from Rhododendron capitatum.

Chemical investigation on the aerial parts of Rhododendron capitatum resulted in the discovery of five enantiomeric pairs of new meroterpenoids, (+)-/(-)-rhodonoids C (1a and 1b), E (3a and 3b), F (4a and 4b), and (-)-/(+)-rhodonoids D (2a and 2b) and G (5a and 5b). These enantiomeric pairs existed as partial racemates in a plant and were obtained by chiral HPLC separation. Their structures with absolute configurations were assigned by spectroscopic data, single-crystal X-ray diffraction, and ECD analysis. Compounds 1a and 1b are the first pair of meromonoterpenes incorporating an unprecedented 6/6/6/5 ring system, and 1a showed antiviral activity against herpes simplex virus type 1 (HSV-1) in vitro. Compounds 2a and 2b are the first examples of meromonoterpenes featuring a unique 6/6/5/5 ring system.