Chemistry 30A3 Fall 2011 Labs Dr. Troendle 25

Chemistry 30A3 Fall 2011 Labs Dr. Troendle 25 - 2 and 3...

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Chemical Biology 3OA3, September 2009 Page 25 E XPERIMENT 2 1-B ENZYLDIHYDRONICOTINAMIDE FROM 1-B ENZYLNICOTINAMIDE C HLORIDE N + CH 2 NH 2 O H Cl - N CH 2 NH 2 O H H dithionite Introduction to Experiments 2 and 4 As we have seen in sugar chemistry, many natural compounds are adorned with oxygen– containing functionality, e.g. alcohols, aldehydes, ketones, and carboxylic acids. Nature has evolved enzymes to inter-convert these functional groups with exquisite regio-, diastereo- and enantio-selectivity. These oxidations and reductions require a redox reagent, and the redox couple between nicotinamide adenine dinucleotide (phosphate) (NAD(P) + ) and its reduced form, NAD(P)H, provides the “co-factor” for many of these redox reactions, while the enzyme provides catalysis. Consider the reduction of a ketose to an alditol: OH O OH OH OH OH OH OH O H + "[H-]" 1 2 3 Sodium borohydride (NaBH 4 ) would be the Chemist’s first choice of reagent. It is cheap, but would give a mixture of isomers (
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Unformatted text preview: 2 and 3 ). What type of isomers are these? NAD(P)H and an enzyme (alcohol dehydrogenase – the name describes the reverse reaction) would be selective, but hugely expensive for reactions on the gram scale. In experiment 2, we study the simpler and more readily available analog of NADH, benzyldihydronicotinamide. Just like NAD(P)H, this compound is formed by reduction of the oxidized compound benzylnicotinamide (cf. NAD ⊕ ). Although the reduction destroys the aromaticity of the pyridine ring, the positive charge on the nitrogen cation is neutralized. These two effects counter-balance each other making both forms reasonably stable, with the redox potential between the two suited to performing both oxidation of alcohols and reduction of aldehydes and ketones. Thus, in the second part of the experiment, the reduced form gives up a “hydride ion”...
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