OXIRANE-OPENING REACTIONS OF SOME 6,19-OXYGENATED 4-ALPHA,18-EPOXY-NEO-CLERODANES ISOLATED FROM TEUCRIUM - BIOGENESIS AND ANTIFEEDANT ACTIVITY OF THEIR DERIVATIVES

Some 4 alpha, 18-epoxy-neo-clerodane diterpenoids possessing oxygenate d functions at the C-6 and C-19 carbons have been subjected to oxirane -opening reactions with several reagents obtaining chlorohydrins, 4-hy droxy-6,18-ethers, 4-hydroxy-18-O-methyl- and 18-O-acetyl derivatives, allylic C-18 primary alcohols and 4,6,18 and 4,6,19-orthoacetates. Th ese results revealed that the functionality at C-6 and its stereochemi stry determine the course of the reaction and affect the retention or inversion of the configuration at the C-4 asymmetric centre. In the li ght of these reactions, a hypothesis on the biogenetic pathway of some neo-clerodane diterpenoids isolated from teucrium species is proposed . Moreover, useful criteria for establishing the stereochemsitry at C- 4 in these derivatives, as well as the unambiguous assignment of the s tructure of montanin E by an X-ray diffraction analysis and the result s achieved in the biological assay as insect antifeedants of several n on-natural neo-clerodane derivatives are also reported.