Cyclophanes template effect

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

Control experiment indicated that template effect (in the presence of different metal salts) [108] was not operating for this transformation. The presence of NH function in 86 that could potentially form a H-bond with oxazole ring, thus preorganizing the cyclization precursor [109], was not an obligation. Indeed, compound 86 (R = H) and 87 (R = Et, Fig. 2) was obtained in essentially identical yield. Aliphatic diamines are suitable substrates, as cyclophane 88 and 89 can be prepared in reasonable yield. It is interesting to note that a 47% yield of 89 meant 88% yield per chemical bond created, including the macrocyclization step. 

It was subsequently discovered that incorporation of a 1,5-dioxynaphthalene residue in the template in place of the 1,4-dioxy benzene moiety causes a dramatic increase in the yield of the cyclophane, an effect that can be further increased using high pressure (12 kbars). The latest improvement in the synthesis of the tetracationic cyclophane—three decades after the first using a template— involved the use of l,5-bis[2-(2-hydroxyethoxy)ethylamino] naphthalene as a replacement for the template mentioned previously because the amine can be protonated and... 

The first approach has been pioneered by Hopf and coworkers by covalently attaching polyene chains onto a rigid molecular fragment, a [2.2]-paracyclophane. " The paracyclophane scaffold serves as a template to hold the two polyenes in the vicinity for a sequential [2 + 2] photodimerization. The covalent attachment of the polyene chains onto the cyclophane effectively makes the dimerization an intramolecular process (Figure 15). Upon UV-irradiation... 

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