Stereoselective Synthesis of Empenthrin, a Novel Insecticide against Fabric Pests, in a Regioselectively ¹⁴C-Labeled Form

Abstract

Two stereoisomers of empenthrin, (E) -1-ethynyl-2-methyl-2-pentenyl (1 R) -cis, trans-chrysanthemates, were regioselectively labeled with carbon-14 for use in metabolic studies on the characteristic ethynyl alcohol moiety. Action of ethyl [¹⁴C] formate (2) on ethylidenetri-phenylphosphorane gave the ¹⁴C-formylated phosphorane (3) . Wittig reaction of 3 with propanal stereoselectively yielded the E isomer of 2-methyl-2- [1-¹⁴C] pentenal (4), which was derivatized to a crystalline imidazolidine (5) for purification. Regeneration of 4 from purified 5 followed by reaction with ethynylmagensium bromide gave (E) -1-ethynyl-2-methyl-2- [1-¹⁴C] -pentenol (6), the a-ethynyl alcohol of ¹⁴C-labeled empenthrin, in 36% overall radiochemical yield from 2. Esterification of 6 with (1R) -cis- and (1R) -traps-chrysanthemoyl chlorides afforded the corresponding optically active [pentenyl-1-¹⁴C] empenthrins (1a and 1b) in good yields after chromatographic purification.