Practical, general, and systematic method for optical resolution of gem-dihalo- and monohalocyclopropanecarboxylic acids utilizing chiral 1,1′-binaphtholmonomethyl ethers: Application to the synthesis of three chiral pesticides

Abstract

We performed an efficient practical and systematic optical resolution method for gem-dihalo- and monohalocyclopropanecarboxylic acids 1 and 5 utilizing chiral 1,1′-binaphthol monomethyl ether (R)-2 as the key auxiliary. Direct esterification of 1 with (R)-2 gave two 1R- and 1S-diastereomeric esters3 with marked different R_fvalues, both of which were easily separated using simple column chromatography. Monodehalogenation of separated chiral esters3 using t-BuMgCl and cat. Co(dppe)₂Cl₂ gave two 1,2-trans- and 1,2-cis-diastereomers4 with markedly different R_f values, both of which were similarly separated using simple column chromatography. The obtained diastereomers 3 and 4 were easily hydrolyzed to the desired enantiopure acids 1 (>99%) and 5 (>99%), respectively, with recovery of (R)-2, both in good to excellent yields. Utilizing the present method, important chiral agrochemicals, carpropamid6 and fencyclate7, were readily synthesized. Pyrethroid 9 with three asymmetric centers was efficiently synthesized in a much better yield compared with the reported method.