Epimerisation of trans-chrysanthemic acid alkyl ester(s) - to form cis-ester(s) in high yields, uses strong base to form intermediate enolate

摘要

Epimerisation of alkyl esters of 1S or 1R trans chrysanthemic acid of formula (II) comprises reacting (II) at low temp. under anhydrous conditions, in a suitable solvent with a strong alkaline base to form the enolate of formula (III); (b) reacting (III) with the reagent Y-A to form a ketal ketene of formula (IV); (c) reaction of (IV), at low temp., under anhydrous conditions, in a suitable solvent with a proton-donator to obtain a mixt. of alkyl esters of cis-chrysanthemic acid and trans-chrysanthemic acid and (d) sepn. of the cis-chrysanthemic acid alkyl ester (Ia) from the mixt. and conversion of it to the cis-chrysanthemic acid (Ib) by known methods. In the formulae R is 1-6C alkyl; M is an alkali metal; A is Cl, Br, I or another hydracid anion; and Y is an organic radical such that Y-A form a ketal ketene. IR, cis, 2,2-dimethyl-3- (2,2-dihalovinyl)- cyclopropane-1-carboxylic acid esters, which are readily prepd. from (Ia) or (Ib) are known insecticides. The cis cpd. (Ib) can be prepd. in yields approaching 70%.