Nuclear magnetic resonance spectra and mosquito repellent properties of some oxazolidine derivatives of 5-methyl-4-hexenal.

Seven novel oxazolidines from 5-methyl-4-hexenal were synthesized, characterized, and evaluated in a cloth test system as mosquito repellents. These heterocycles were a homologous series of 3-alkanoyl-2-(4'-methyl-3'-pentenyl)oxazolidines, with substituents at N-3 ranging from acetyl to n-nonanoyl groups. Carbon-13 NMR spectroscopy at 100.6 MHz revealed that these molecules existed in deuterochloroform solution as unequal mixtures of cis (syn)- and trans (anti)-rotational isomers. At 25 degrees C, this isomerism at the amide group resulted in two detectable 13C signals for all assigned carbon atoms, except for the methyl carbons of the isopentenyl side chain. Differences in 13C chemical shifts for the methyl group at N-3 were observed up to the n-butanoyl homologue. The 13C signal assignments were aided by the distortionless enhancement by polarization transfer (DEPT) spectral editing technique. Two-dimensional carbon-hydrogen chemical shift correlation experiments aided in partially interpreting the complex proton spectra of these molecules. Repellent activity of the acetyl, trideuteroacetyl, and propionyl derivatives was observed in bioassays with three mosquito species, but the long-chain homologues were practically ineffective as mosquito repellents.