Stereoisomerization of Permethric Acid

Resolution of racemic permethric acid also yields the unwanted IS-isomers. Transformation of one enantiomer into its mirror image demands the fission of the C —C -bond of the cyclopropane-ring. The complete isomerization to the equilibrated racemic cis/trans-mixture occurs under influence of fight and sensitizer in the case of ester and salts [346] or in an ionic manner [347] for the acid, involving the intermittent formation of the anhydride 205 and the acyl cation 206 (Reaction scheme 131). 

On heating above 110 °C, the 1-S-trans-permethric acid chloride epimerizes only in the a-position [348] (Reaction scheme 132) via a ketene to give the IRcis- and the IStrans-system in equilibrium  

6 Synthesis of Individual Enantiomers of Chrysanthemic Add, Permethric Add and Other Pyrethroid Adds from Optically Active Naturally Occurring a-Pinene and Carene 

Only a few reagents are needed to convert a-pinene or carene to chrysanthemic acid (Reaction scheme 133) ozone, permanganate, peradd or bromine and methyl-magnesium chloride. 

How to turn a-pinene 207 into chrysanthemic acid, is shown in Reaction scheme 133 [920]  

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