Conformation of Nonconjugated Compounds

Some conformational studies of piperazic acids (hexahydropyridazine-3-carboxylic acids) have been reported 1998CSR437 . Piperazic acids are important compounds as they appear as subunit in many natural products (see Sections 8.01.6 and 8.01.12.2). They can be considered as rigid proline equivalents 1998JA80 . The conformation of derivatives of 3,4-dihydrophthalazine-2(l//)-carboxylic acid, a new conformationally restricted analog of phenylalanine, was also studied 1998T165 . 

Conformational studies using C NMR show that the 1,2-dihydropyridazine (24) is planar, while the 1,2,3,6-tetrahydropyridazines (25) and (26) have a half chair or boat conformation, respectively, as a consequence of the position of ring substituents. The hexahydropyridazine (27) probably exists as a mixture of three conformers due either to restricted rotation or inversion of the N-substituents with the bromines d -axial. In the absence of the 4,5-methyl groups the situation is made more complex by the additional presence of di-equatorial bromines 8772443). 

Cyclic voltammetry has been used to study the reversible oxidation of 5,6-diphenyl- and 5,6-dimethyl-5,6-dihydrobenzo[c]cinnoline in liquid sulfur dioxide. Both substrates start with an axial, axial conformation, but the conformation is changed on formation of the radical cation to equatorial, equatorial because of the sp hydridization. This conformational change has a dramatic effect on the redox potential for the formation of the dication, most marked in the case of the diphenyl compound where the second redox potential is lower than that of the first 86AG(E)1021 . 

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