Revisiting Staudinger and Ruzicka’s altered pyrethrolone: the cyclopentadienone dimers derived from pyrethrin I, cinerin I and jasmolin I

The reactions of pyrethrin I, cinerin I and jasmolin I with sodium hydroxide in ethanol afforded an approximately 1:1 ratio of two respective cyclopentadienone dimers, isolated in good yield. A combination of one-dimensional and two-dimensional NMR spectroscopic studies allowed determination of the structure and stereochemistry of the dimers. The dimers are formed by cycloaddition reactions of the less substituted alkene of the cyclopentadienone, and by regioisomeric endo transition states. Density functional theory calculations were in accord with the experimental findings showing the products formed by ambimodal transition states. One such transition state led to two initial products with the less stable product undergoing facile conversion to the other more stable, experimentally observed product. These studies clarify the structures of the altered pyrethrolone reported by Staudinger and Ruzicka in 1924.