Preferential Crystallization of a Helicene-Viologen Hybrid - An Efficient Method to Resolve [5]Helquat Enantiomers on a 20 g Scale

The bistriflate salt of racemic [5]helquat 1 was found to form a conglomerate. Based on this finding, the preferential crystallization of this chiral helicene-viologen hybrid was developed to deliver pure enantiomers on a 20 g scale (10.5 g of each enantiomer). To the best of our knowledge, this is the largest amount of nonracemic helicene-like compound obtained by preferential crystallization to date. The absolute configuration of the P enantiomer was confirmed by X-ray crystal structure analysis. The chromatography-free synthesis of racemic 1 on a multigram scale (30 g) is also presented as an entry point to this resolution study.