Chrysanthemo-lactone

The isomer 31 of chrysanthemic acid is cychzed to the chrysanthemo-lactone 32, which after transesterification and saponification [46] affords the desired cis-chrysanthemic acid. 

Strong bases epimerize at the very weakly acidic a-position of the carboxy group of add, ester or salts. Therefore not only the transformation of the insecticidally uninteresting 1-S-trans isomer to 1-R-cis is possible via the thermally enforced formation of chrysanthemo-lactone 32 (Reaction scheme 39), but also the transition from 1-S-cis to 1-R-trans (see also Reaction scheme 142) [81, 94, 95, 96]. 

---