As nicotinamide coenzyme models the 2.2pyridinophanes 3 -6 were prepared which consist of two interacting nicotinic ester units in the four different orientations possible. 3 -6 were obtained by photolytic sulfur extrusion from the corresponding dithia-3.3pyridinophanes 18-21 the syntheses of which are described. -The molecular structures for all four isomers 3-6 were determined by X-ray analysis. The sterical interactions and some spectroscopic properties of these compounds are discussed on the basis of the structure determinations.
Nikotinamid
-Coenzym-Modelle, I. -Synthese und Molekiilstruktur von vier isomeren Bis(methoxy carbon y I) [ 2.21 (2,5)pyridinophanen Als Nikotinamid-Coenzym-Modelle wurden die 2.2Pyridinophane 3-6 dargestellt, die jeweils zwei in Wechselwirkung stehende Nikotinester-Einheiten in den vier verschiedenen moglichen Orientierungen enthalten. 3 -6 wurden durch photolytische Schwefel-Extrusion aus den entsprechenden Dithia3.3pyridinophanen 18 -21 erhalten, deren Synthesen beschrieben werden. -Die Molekulstruktur wurde fur alle vier Isomeren 3-6 durch Rontgen-Strukturanalyse bestimmt. Die sterischen Wechselwirkungen und einige spektroskopische Eigenschaften werden auf der Grundlage der Strukturbestimmungen diskutiert.
Since, 50 years ago, the classical work of Warburg and Karrer') established the structure and coenzyme function of nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+) the biological importance of these coenzymes has been stressed by the discovery of several hundreds of NAD + -or NADP+-depending enzymatic reactions2). Concerning the mechanism of redox-equivalent transfer in these reactions early experiments led to the conclusion that 1,Cdihydropyridine structures are involved3), and that hydrogen transfer between coenzymes and substrates occurs directly4' and stereo-~pecifically~.~). Although a hydride migration is quite generally postulated for this hydrogen transfer, alternative mechanisms like stepwise electron-proton-electron or electron-hydrogen atom transfers have not been ruled out definitely6). Recently,