Limbetazulone: a new 'decahydro-8-oxanaphtho[2,1-f]azulen-7-one' diterpenoid from Ballota limbeta, and occurrence of two conformational isomers in the crystal

A new diterpenoid, limbetazulone (= (3S,4S,4aR,12bS)-1,2,3,4,4a,5,6,11,12,12b-decahydro-3-hydroxy-4-(hydroxymethyl)naphtho1',2': 5,6]cyclohepta1,2-b]furan-7(7H)-one; 1), with the very rare 'naphtho2,1-f]azulene-7-one' skeleton, was isolated from the aerial parts of the Asian medicinal plant Ballota limbeta. Its structure was established by extensive spectroscopic investigations, especially 1D and 2D NMR. X-Ray diffraction studies showed the presence of two conformational isomers (1a and 1b) in the crystal.