Development of Optically Pure Pyrethroid-Like Fluorescent Substrates for Carboxylesterases

Pyrethroids are now the world's most extensively used insecticides. One of the common
metabolic routes of pyrethroid insecticides in living systems is hydrolysis by carboxylesterases,
and this hydrolysis may be stereospecific since most pyrethroid insecticides have chiral centers.
In previous studies, pyrethroid-like fluorescent substrates have been shown to be hydrolyzed
in a fashion similar to actual pyrethroids. It is important to synthesize the stereoisomers of
pyrethroid-like fluorescent substrates to study the stereointeraction between carboxylesterases
and these substrates. In this study, an effective synthetic method for preparing optically
enriched (R)- and (S)-α-2-hydroxy-2-(6-methoxy-2-naphthyl)acetonitrile was developed. With
this alcohol, an efficient synthetic route for preparation of optically pure cypermethrin and
fenvalerate analogues was provided. Identification of these stereoisomers was determined based
on GC, HPLC, 1H NMR, and X-ray crystallography. In addition, stereointeraction between
carboxylesterases and chiral fluorescent substrates indicated that (i) stereospecificity of
recombinant mouse liver carboxylesterases (NCBI accession nos. BAC36707 and NM_133960)
varied significantly (up to 300-fold difference) with different stereoisomers of cypermethrin
and fenvalerate analogues; (ii) on the basis of Vmax, the sensitivity of this analytical method,
using a single stereoisomer of cypermethrin analogues instead of a mixture of eight
stereosiomers, could be enhanced by 4−6 times for detection of these carboxylesterases; and
(iii) possible usage of these carboxylesterases for chiral synthesis is discussed.