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Addition of an enantiopure sulfoxide auxiliary is the key to the resolution of highly stable [4]helicenium dyes. This transformation allows a facile chromatographic separation of the diastereomers (the structure of one such diastereomer is shown; S yellow, O red, N blue). Then an unprecedented Pummerer-like CC bond fragmentation takes place to release the enantiopure cations.
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Laleu et al. (2005) studied this question.
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