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Only in apolar media is the calix[6]arene wheel threaded from its upper rim by asymmetric bipyridinium axles. This leads to the formation of oriented pseudorotaxanes. Their stoppering with a different stopper yields rotaxanes characterized by the unequivocal orientation of the wheel rims with respect to the two different stoppers (see picture, R1=CHPh2/CHCy2, R2=CHCy2/CHPh2, Cy=cyclohexyl, Ts=toluene-4-sulfonyl).
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Arduini et al. (2004) studied this question.
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