Ring systems, interconversion

The oxidative and reductive interconversions of different oxidation states of the carboline ring system are systematically discussed in Section III. 

Colquhoum, H. M. Lewis, D. E Ben-Haida, A. Hodge, P. Ring-chain interconversion in high-performance polymer system. 2. Ring-opening polmerization-copolyetherification in the synthesis of aromatic poly(ether sulfones). Macromolecules 2003, 36, 3775-3778. 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

The steric bulk of the substituents on B and N in an iminoborane RB=NR has a major influence on the outcome of the oligomerisation process. Small substituents favour the formation of borazines. For example, only the borazine Me3B3 N3Me3 is formed for the combination R = R = Me. An increase in the steric bulk of R and/or R may lead to the formation of a Dewar borazine C, a four-mem-bered ring D or an eight-membered ring E. Speciflc examples of each of these ring systems and the mechanisms of their interconversions are discussed below. 

The chroman ring system is stable to organometallic reagents, for example in the formation of the tertiary alcohol (690) in high yield (63HCA650), and to the usual interconversion of carboxylic acid, acyl chloride, carboxamide and nitrile. 

Theoretical treatment of the conformational properties of the related l,4-benzodiazepin-2,5-dione ring system has received less attention but HF and DFT methods have been applied to examine the relative conformational energies and activation energy for the 4/-to-P-isomer interconversion <2004JST37>. The DFT method provided superior prediction of the bond distances and angles that were comparable to those observed by X-ray crystallography. 

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. 

Mw of 73 000 (Scheme 55) [229]. Colquhoun and coworkers also reported a ring-chain interconversion system in poly(m-phenylene isophthalamide) (Nomex) [230]. Cyclodepolymerization was best achieved by treating a solution of the polymer in DMSO containing CaCl2 or IiCl as a solubilizing... 

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