Chirality control oriented molecules

Two higher homologues of the [2]-rotaxane are represented the [3]-rotaxane and the [4]-rotaxane. Whereas [3]-rotaxanes are relatively common [56-60], this is not the case for the linear [4]-rotaxane [61] and its branched or dendritic analogue [62]. Very important in the recent developments of rotaxane chemistry is the [2]-rotaxane whose dumbbell contains two separated sites for interaction of the threaded macrocycle. In this molecule, the macrocycle may be moved between two sites of the dumbbell, in a degenerate manner, if the sites are identical [63-65], or in a controlled manner, thanks to an external trigger, if the sites are different [28-31]. A chiral [2]-rotaxane, whose chirality arises from the mechanical bonding, is obtained if the macrocycle is oriented (by an appropriate substitution pattern) and the dumbbell made non-symmetrical, for example by attaching two different stoppers [66],... 

It was proposed that this reaction relies on thermodynamic control of the addition of an in 5 // -generated chiral thiazolium anion, incorporating ketone 12, to another molecule of 12. It is not clear what role, if any, the bulky 5,6-0-isopropylidene group, which sits on the exo-face of the bicycle, plays in directing e/ido-addition to ketone 12. However, the relative orientation of this bulky... 

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