13C NMR Analyses on Conjugated Dienic Pheromones of Lepidoptera

Abstract

13C NMR spectra of all the geometrical isomers of 5,7- and 8,10-dodecadien-1-ols and 9,11-dodecadienyl acetate were studied, and a new method for assigning 13C signals of a conjugated system is proposed. The method is based on two empirical rules. The first involves the 13C shift differences of olefinic carbons induced by a substitutional change at an end of the conjugated diene system, namely the change from a methylene to a methyl at Cf in the Ca-Cb = Cc-Cd = Ce-Cf diene system. The second rule concerns the chemical shift changes of allylic and olefinic carbons by converting the geometry of conjugated diene systems. Namely, when the (E)-configuration of a Cb = Cc double bond in the diene system is changed into a (Z)-configuration without the conversion of the Cd = Ce double bond, the signals of Ca, Cb, Cc and Cd shift upfield by 5.1 ± 0.3, 2.6 ± 0.3, 1.9 ± 0.3 and 4.9 ± 0.2 ppm, respectively, and the signal of Ce downfield by 2.2 ± 0.2 ppm, regardless of the geometry of the Cd = Ce double bond.