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Surprisingly difficult resolution of N-methylated cationic [4]helicenes
Nathalie Mehanna1, Stéphane Grass, Jérôme Lacour
1Department of Organic Chemistry, University of Geneva, Geneva, Switzerland.
Chirality
|May 9, 2012
Summary
Separating enantiomers of N-methylated quinacridinium helicenes is challenging. Introducing N-methyl groups hinders separation, requiring advanced chromatography techniques instead of simple flash chromatography.
Area of Science:
- Organic Chemistry
- Stereochemistry
- Chiral Separations
Background:
- 1,13-Dimethoxyquinacridinium ions are configurationally stable [4]helicenes.
- Racemic mixtures are synthesized from tris(2,6-dimethoxyphenyl)methylium ion.
- Enantiopure helicenes were previously obtained via Pummerer fragmentation of sulfoxide adducts.
Purpose of the Study:
- To investigate the impact of N-methylation on the chromatographic separation of quinacridinium helicene enantiomers.
- To evaluate the effectiveness of existing resolution protocols for N-methylated derivatives.
Main Methods:
- Synthesis of N-methylated quinacridinium helicene derivatives.
- Attempted separation of diastereomeric sulfoxide adducts using flash chromatography.
- Utilized semi-preparative high-performance liquid chromatography (HPLC) for separation.
Main Results:
- N-methylation significantly impedes the chromatographic separation of quinacridinium helicene enantiomers.
- The expected large retention difference over silica gel is diminished.
- Extensive semi-preparative HPLC is necessary, replacing simpler flash chromatography methods.
Conclusions:
- The introduction of N-methyl side chains negatively affects the resolution of quinacridinium helicene enantiomers.
- Established chromatographic separation methods are insufficient for N-methylated derivatives.
- Advanced HPLC techniques are required for obtaining enantiopure N-methylated quinacridinium helicenes.
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