Dihydronicotinamide cofactors

A different approach for utilization of the photoproducts in chemical routes involves the introduction of natural enzymes as catalysts in the photochemical system. In nature, dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide dinucleotide phosphate (NADPH) participate as reducing cofactors in a variety of enzymatic reduction processes. Thus, the development of photochemical NADH and NADPH regeneration cycles is anticipated to allow a variety of reduction processes by inclusion of substrate specific NAD(P)H dependent enzymes. 

A different redox system model - the model for NADH - was also described by our group. [16] As electron transfer mediators, FMN and FAD accept two electrons from NAD(P)H and transfer one electron to metal centres in heme-containing proteins, nonheme iron, or molybdenum sites. However, the transfer of electrons between reduced pyridine - dinucleotide cofactors is slow under physiological conditions and must be catalysed by enzymes. Function of these enzymes was mimicked by a modification of the cofactor by a recognition site for its counterpart and, thus, efficient electron transfer was enabled directly. Functionalised 1,4-dihydronicotinamides bearing a recognition unit for flavins were synthesised (Scheme 18). 

The catalytic, asymmetric hydrogenations of alkenes, ketones and imines are important transformations for the synthesis of chiral substrates. Organic dihydropyridine cofactors such as dihydronicotinamide adenine dinucleotide (NADH) are responsible for the enzyme-mediated asymmetric reductions of imines in living systems [86]. A biomimetic alternative to NADH is the Hantzsch dihydropyridine, 97. This simple compound has been an effective hydrogen source for the reductions of ketones and alkenes. A suitable catalyst is required to activate the substrate to hydride addition [87-89]. Recently, two groups have reported, independently, the use of 97 in the presence of a chiral phosphoric acid (68 or 98) catalyst for the asymmetric transfer hydrogenation of imines. 

When a series of dihydronicotinamides with variable substituents R is oxidized either by flavin or by the le"-only nitroxide radical, the Nernst equation can be verified using the forward rate as well as the equilibrium constants, since the back reaction rate is independent of the nature of R. In the case of the flavin acceptor, the Nernst number n turns out to be 2, as required for 2e"-transfer. Hence, the transformase cofactor flavin will react with nicotinamide in the 2e"-mode, showing that the latter is—for the biological acceptor—a 2e"-only agent and requires a very potent le"-only oxidant such as nitroxide for artificial le"-behavior. 

Several enzymes such as reductases and dehydrogenases utilize nicotinamide derivatives as reversible carriers of redox equivalents. The reduced dihydronicotinamide moiety NAD(P)H acts by donating a hydride equivalent to other molecules. In the corresponding two-electron oxidized NAD(P) form, the cofactor formally accepts a hydride ion from the substrate. Functional models of such reversible hydride transfer processes are of considerable interest for biomimetic chemistry, and the strategies to regenerate nicotinamide-type cofactors are crucial for the performance of many organic transformations involving biocatalytic key steps 139,140). 

All biological C=X CHXH reductions are dependent on the nicotinamide coenzymes NAD(P)H, as illustrated for ketone reduction in equation (1), which shows only the reactive dihydronicotinamide moiety of the NAD(P)H cofactor. 

Dehydrogenases, reductases and a number of other enzymes, such as UDP-glucose epimerase, utilize NAD or NADP as an enzymatic cofactor and catalyze the oxidation/reduction of various substrates, facilitating the usually reversible stereospecific hydride transfer from the C4 position of the 1,4 dihydronicotinamide ring of NAD(P)H to substrate. The reaction catalyzed by lactate dehydrogenase and a schematic drawing of the putative hydride transfer reaction that takes place are shown in Fig. 15.1. 

Nakata T, Kuwabara T, Tani T, Oishi T (1982) Total synthesis of (+)-oudemansin. Tetrahedron Lett 23 1015-1016 Nambiar KP, Stauffer DM, Kolodziej PA, Benner SA (1983) A mechanistic basis for the stereoselectivity of enzymatic transfer of hydrogen from nicotinamide cofactors. J Am Chem Soc 105 5886-5890 Ng GY, Yuan L-C, Jakovac IJ, Jones JB (1984) Enzymes in organic synthesis. 29. Preparations of enantiomerically pure cis-2,3- and 2,4-dimethyl lactones via horse liver alcohol dehydrogenase-catalyzed oxidations. Tetrahedron 40 1235-1243 Oae S, Nagata T, Yoshimura T, Fujimori K (1982) Reduction of diaryl disulfides with 1-benzyl-1,4-dihydronicotinamide. Tetrahedron Lett 3189-3192... 

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