Staudinger and Ruzicka's Altered Pyrethrolone: the Cyclopentadienone Dimers Derived from Pyrethrin I

The reaction of pyrethrin I (1) with sodium hydroxide in ethanol produced the cyclopentadienone 29 which rapidly underwent Diels-Alder dimerisation to give two cyclopentadienone dimers which were isolated in 30 and 38% yield, respectively. A combination of one-dimensional and two-dimensional NMR spectroscopic studies allowed determination of the structure and stereochemistry of the higher and lower R-f dimers as being 26 and 27, respectively. The dimers are formed by cycloaddition reactions of the least substituted alkene of the cyclopentadienone, and via both possible regioisomeric endo transition states.