Sterically hindered N-aryl pyrroles: chromatographic separation of enantiomers and barriers to racemization

Abstract

The novel N-aryl-2,5-dimethylpyrrole-3-carbaldehydes (1)–(7) have been synthesized by condensation of hexane-2,5-dione with the appropriate aniline and subsequent Vilsmeier–Haack formylation of the pyrrole ring. Diastereoisomeric association complexes of these racemic pyrroles were studied by ¹H n.m.r. spectroscopy chemical shifts and the splittings induced by the optically active auxiliary compound (+)-Eu(hfbc)₃. Separation of the enantiomers of (6) was achieved by liquid chromatography on triacetylcellulose. The barrier to partial rotation about the C–N bond in (6) was determined and their lower limits in (2) and (4) were estimated by variable temperature ¹H n.m.r. spectroscopy.