Enantioseparation and absolute configuration of the atropisomers of a naturally produced hexahalogenated 1,1′-dimethyl-2,2′-bipyrrole

Hexahalogenated 1,1′-dimethyl-2,2′-bipyrroles (HDBPs) are a group of marine halogenated natural products (HNPs) that have been detected in environmental samples from all over the world. The most frequently described congener is the 5,5′-dichloro-1,1′-dimethyl-3,3′,4,4′-tetrabromo-2,2′-bipyrrole (DBP-Br4Cl2, BC-10). This compound is axially chiral, by virtue of hindered rotation about the interannular pyrrole–pyrrole bond forming stable atropisomers. This effect was proven by the separation of synthesized racemic DBP-Br4Cl2 by enantioselective high performance liquid chromatography (HPLC). Pure enantiomers were isolated using enantioselective HPLC. Crystallization led to white crystals studied by X-ray analyses to determine the absolute configuration. Subsequent polarimetric measurements verified the first eluting enantiomer on HPLC as Ra-(+)-DBP-Br4Cl2 and the second as Sa-(−)-DBP-Br4Cl2. We also investigated the gas chromatography (GC) enantioseparation of DBP-Br4Cl2. However, too high temperatures in the injector port led to partial racemization at temperatures >150 °C. GC coupled to mass spectrometry was used to study DBP-Br4Cl2 in marine mammal samples. All samples contained both atropisomers of the natural product DBP-Br4Cl2 with enrichment of the levo (−) enantiomer. This led to the assumption that both enantiomers of DBP-Br4Cl2 were already produced in the environment.