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 R_a-(+)-DBP-Br₄Cl₂ and the second as S_a-(−)-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.     

Through-space hydrogen-fluorine and carbon-fluorine spin-spin coupling in 5-fluoro-3,3-dimetryl-1,2,3,4-tetrahydrophenanthrene

The ¹R and ¹³C NMR spectra of the title compound ($\text{1}$) reveal through-space couplings between the fluorine and the C-4 methylene group ¹H and ¹³C), as well as coupling between the fluorine and the C-3 methine carbon and the C-2 methylene carbon.     

The anti‐inflammatory triterpenoid methyl 2‐cyano‐3,12‐dioxooleana‐1,9(11)‐dien‐28‐oate methanol solvate hydrate

The title compound, C₃₂H₄₃NO₄·CH₄O·H₂O, has a nearly planar cyano‐enone A ring in an otherwise normal oleanane triterpenoid. Rings A, B and C are non‐chairs, but rings D and E adopt essentially cyclo­hexane chair conformations. The structure clearly establishes the C‐D‐E ring stereochemistry as trans‐syn‐cis, as predicted from a nuclear Overhauser effect (NOE) NMR measurement.     

A concise total synthesis of bruceolline E

An efficient synthesis of bruceolline E was completed in three steps from the known ethyl indole-1-carboxylate (9d) in 60% overall yield, via a tandem acylation/Nazarov cyclization with 3,3-dimethyl acrylic acid followed by selenium dioxide oxidation to install the α-diketone functionality.