Insect pest control agents: Novel chiral butanoate esters (juvenogens)

During the investigation of ester derivatives (juvenogens, biochemically activated insect hormonogenic compounds) of biologically active alcohols with potential application in insect pest control, a need for availability of all existing stereoisomers of ethyl N-{2-[4-(2-butanoyloxycyclohexyl)methyl]phenoxy}ethyl carbamate occurred. They were synthesized from their chiral precursors, the corresponding stereoisomers of 2-(4-methoxybenzyl)cyclohexyl butanoate, by removing their protecting group (methyl), and by subsequent condensation of the aromatic hydroxyl moiety with ethyl N-(2-bromoethyl) carbamate. The requested enantiomers of 2-(4-methoxybenzyl)cyclohexyl butanoate were obtained by a Candida antarctica lipase-mediated transesterification and chiral resolution of the respective racemic cis- and trans-isomers of 2-(4-methoxybenzyl)cyclohexanol either directly or after a subsequent chemical esterification of the chiral precursor. In this synthesis, two convenient butanoic acid activating esters, vinyl butanoate and 2,2,2-trifluoroethyl butanoate, were employed, and the chiral precursors in the synthesis of the target molecules were obtained in 41–48% yields (i.e., 82–96% conversion), and with enantiomeric purity ee = 96–98%, respectively. The enantiomeric purity of the products was determined by chiral HPLC analysis, and their absolute configuration was assigned on the basis of analyzing the 1H and 19F NMR spectra of their diastereoisomeric Mosher acid (3,3,3-trifluoromethyl-2-methoxy-2-phenylpropanoic acid) esters.