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-Br(4)Cl(2), 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-Br(4)Cl(2) 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 R(a)-(+)-DBP-Br(4)Cl(2) and the second as S(a)-(-)-DBP-Br(4)Cl(2). We also investigated the gas chromatography (GC) enantioseparation of DBP-Br(4)Cl(2). However, too high temperatures in the injector port led to partial racemization at temperatures >150 degrees C. GC coupled to mass spectrometry was used to study DBP-Br(4)Cl(2) in marine mammal samples. All samples contained both atropisomers of the natural product DBP-Br(4)Cl(2) with enrichment of the levo (-) enantiomer. This led to the assumption that both enantiomers of DBP-Br(4)Cl(2) were already produced in the environment.