Oxirane-opening reactions of some 6,19-oxygenated 4α,18- epoxy- neo-clerodanes isolated from Teucrium. Biogenesis and antifeedant activity of their derivatives

Abstract

Some 4α, 18-epoxy- neo-clerodane diterpenoids possessing oxygenated functions at the C-6 and C-19 carbons have been subjected to oxirane-opening reactions with several reagents obtaining chlorohydrins, 4-hydroxy-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. These results revealed that the functionality at C-6 and its stereochemistry determine the course of the reaction and affect the retention or inversion of the configuration at the C-4 asymmetric centre. In the light 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 stereochemistry at C-4 in these derivatives, as well as the unambiguous assignment of the structure of montanin E by an X-ray diffraction analysis and the results achieved in the biological assay as insect antifeedants of several non-natural neo-clerodane derivatives are also reported.