Dolabellane diterpenoids from the liverworts Barbilophozia floerkei, B. lycopodioides, and B. attenuata: spectroscopic and X-ray studies of structure, stereochemistry, and conformation. X-Ray molecular structure of 3S,4S;7S,8S-diepoxy-10R,18-dihydroxydollabellane, 18-acetoxy-3S,4S;7S,8S-diepoxydolabellane, and 10R,18-diacetoxy-3S,4S-epoxydolabell-7E-ene

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Abstract
The dolabellane diterpenoids 10R,18-diacetoxy-3S,4S;7S,8S-diepoxydolabellane (barbilycopodin)(1), 10R,18-diacetoxy-3S,4S-epoxydolabell-7E-ene (12), 18-acetoxy-3S,4S;7S,8S-diepoxydolabellane (4), and 18-hydroxydolabell-7E-en-3-one (14) have been isolated from the liverworts Barbilophozia floerkei, B. lycopodioides, and B. attenuata. The 1H and 13C n.m.r. spectroscopic properties of the compounds and some of their transformation products are described. The structures, relative stereochemistries, and conformations of compounds (4), (12), and 3S,4S,7S,8S-diepoxy-10R,18-dihydroxydolabellane (2) were defined by X-ray crystallographic studies. The eleven-membered ring has essentially the same conformation in compounds (2), (4), and (12), a conformation which has been characterised as a low-energy form of cycloundeca-1,5-diene by molecular-mechanics calculations.