New myrsinol-related polyfunctional pentacyclic diterpene esters from roots of Euphorbia prolifera

Five Euphorbia substances, SPr1-SPr5, were isolated from the roots of Euphorbia prolifera. They were found to have similar structures but were inactive in a mouse ear inflammation assay. By nmr analysis and after single-crystal X-ray crystallography the structure of SPr5 was established as a hexaester (tetraacetate-benzoate-propionate) of a hitherto unknown polyfunctional pentacyclic diterpene parent alcohol, structurally related to myrsinol. As judged from its nmr spectra, SPr4 is an analogue of SPr5, carrying an isobutyrate substituent in place of a benzoate ester functionality. SPr1-SPr3 were partially characterized by their mass spectra as esters of diterpene parent alcohols possibly related to the myrsinol structure. SPr1-SPr5 may represent one of the product lines branching off the proposed main route of biogenesis of the oligocyclic diterpenoid skin irritants and tumor promoters occurring in many, but not all, of the species in the plant families Thymelaeaceae and Euphorbiaceae.