Photo-isomerization of azadirachtin studied by high performance liquid chromatography coupled to high field proton NMR spectroscopy

Abstract

Azadirachtin was irradiated with UV light, in both oxygen and nitrogen atmospheres. The reactions were followed by supercritical fluid chromatography (SFC) and the chromatograms showed that one photo-product was formed in both cases. High performance liquid chromatography coupled to high field proton NMR spectroscopy (HPLC–NMR) was used to identify the photo-product. The HPLC–NMR data indicated that the (E)-2-methylbut-2-enoate ester group had been converted into a (Z)-2-methylbut-2-enoate ester group. Confirmation was obtained by isomerizing methyl (E)-2-methylbut-2-enoate with UV light to form a mixture of geometric isomers and then recording the ¹H NMR shift for the alkene proton. Additional structural information was obtained from the isolated product, confirmed to be the (Z)-2-methylbut-2-enoate geometric isomer of azadirachtin.