Interconversion between ring systems

There arc no methods for the preparation of azepines by ring transformation of other seven-membered systems however, interconversion between the four tautomeric azepines, either by [1,5]-H sigmatropic shifts or under base catalysis, is common and almost always results in formation of a 3//-azepine. Such transformations are dealt with in Section 3.1.1.5.7. 

Figure 21. The equilibrium between the helical interlaced system precursor of the trefoil knot and its face-to-face analogous complex leading to the face-to-face complexes. Interconversion between the two isomeric cyclic products is, of course, not possible. For the cyclic compounds, the total number of atoms x connecting two phenolic oxygen atoms is 16 if n=4 (pentakis(ethyleneoxy) fragment) or 19 if n = 5 (hexakis(ethyleneoxy) linker). Each knot is represented by the letter k accompanied by the overall number of atoms included in the cycle. The face-to-face complexes contain two monocycles (letter m), the number of atoms in each ring also being indicated. It can be noted that each knot has a face-to-face counterpart. For instance [Cu2(k-90)]2+ and [Cu2(m-45)2]2+ are constitutional isomers. They are by no means topological stereoisomers [34, 35].

Compound 156 adopted the 3H tautomer exclusively, even in hydrogen bond acceptor solvents, such as acetone-46. It was proposed by analogy with the 1,5-benzodiazepine system which adopts an analogous tautomer, the seven-membered ring of 156 is likely to be nonplanar and, regarding NMR spectra, interconversion between two equivalent forms may be present. 

As seen in the structure, moxalactam disodium has three asymmetric centers. Two centers in the ring system, C6 and C7, are stereospecifically defined during the biosynthesis of the penicillin used to produce the compound. A third asymmetric center exists adjacent to the amide carbon on the side chain of the antibiotic. The configuration of this center is free to equilibrate and thus a pair of diasterioisomers is possible for moxalactam disodium. The rate of interconversion between the isomers increases with increasing acidity, with the maximum rate occurring at a pH of about 2.5. For solutions with pH lower than 2.5, the rate of interconversion is decreased only slightly from the maximum rate. 

No example of carbene or nitrene interconversion between five- and six-membered rings is known (Eq. 104). Reasons have been given,10 but such rearrangements could still be expected in favorable systems. 

This interconversion was examined using as a model for the tricyclic ring system via MM2 calculations, albeit low level calculations, in an attempt to evaluate the feasibility of such a proposal. Enone 2S was chosen as the model for the ring system in an effort to simplify the calculations. The dihedral angle between the C-4 carbonyl and the sp carbon of the bicyclo[2.2.2] system was incremented by 5 from 0 to 105 and the individual... 

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