Special Topic Biological Oxidation

FIGURE 16.84 (a) The dehydration approach fails to give very much of the desired product, (b) The Wittig reaction is the preferred alternative. 

PROBLEM 16.29 Starting with cycloheptanone, cyclopentanone, triphenylphos-phine, propyl iodide, butyllithium, and inorganic reagents of your choice, devise synthesis of the following molecules  

In this reaction, the NAD behaves as a Lewis acid, and accepts a hydride ion. Ethanol is a Lewis base in this setting, because it donates the H .The enzyme serves to bring the NAD and the ethyl alcohol together, but this bringing together is of immense importance to the reaction, as it allows the redox process to take place without the requirements that molecules in solution have for finding each other and orienting properly for reaction. 

This hydride reduction may seem strange, but it is not. There is a close relative in the intramolecular hydride shifts in carbocation chemistry. These, too, are redox 

FIGURE 16.86 The NAD is reduced by transfer of hydride (H ) from ethanol. 

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Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

The biological aging of wines has aroused increasing interest in recent years, as reflected in the large number of papers on this topic over the last decade. Biological aging in wine is carried out by flor yeasts. Once alcoholic fermentation has finished, some Saccharomyces cerevisiae yeast races present in wine switch from a fermentative metabolism to an oxidative (respiratory metabolism) and spontaneously form a biofllm called flor on the wine surface. Wine under flor is subject to special conditions by effect of oxidative metabolism by yeasts and of the reductive medium established as they consume oxygen present in the wine. These conditions facilitate... 

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