Catenanes dimer-tetramer

Figure 9. Incorporation of 2,6-pyridyl moieties to macrocyclization affords the folded te-tramer 21 and the dimer-tetramer catenane 22.

Two-dimensional NMR studies and mass spectrometry identified the isomers as being the cyclic tetramer 6 and the [2]catenane 7 consisting of two interlocked cyclic dimers. Later X-ray crystal structures confirmed these results [16, 17],... 

The tetramer exhibits an interesting folded conformation (in contrast to the open conformation of 6) which enables the binding of two p-benzoquinone molecules. Intramolecular hydrogen bonds between the nitrogen lone pair and the amide protons force the 2,6-pyridyl moiety into a NH cis conformation which consequently leads to the observed folded structure (Figure 10). In contrast to this, the open conformation of 6 enables the isophthaloyl unit to adopt the usually energetically favored NH trans conformation to optimize the amide-amide interactions. Because of the folded structure of tetramer 21 a tetramer-dimer catenane 22 could be isolated in 29% yield and even traces of the corresponding [3]cate-nane were detected. 

The analogous cyclization of the nitro substituted 18 (R = NO2) with 17 yielded the dimer 21 (11 % yield) as the main product and a tetramer (7 % yield). The FAB-MS spectrum of the latter hints at the macromonocycle 22 and the isomeric catenane 16 (R = NO2) has not been detected so far. In the template interaction here, the N02-groups... 

---