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One, two, three, four: twist! Four consecutive (one-pot) electrophilic sulfur insertions on triarylamines with phthalimidesulfenyl chloride (see scheme), offer an easy access to chiral hetero[4]- and -[6]helicenes that are remarkably stable against a reversal of the helicity. These heterohelicenes were resolved by HPLC on an amylose-based chiral stationary phase, and the absolute configuration was established by ab initio calculations and experimental vibrational circular dichroism spectra.
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Lamanna et al. (2008) studied this question.
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