Alkene addition reactions biological molecules

Carbon-carbon double bonds are present in most organic and biological molecules, so a good understanding of their behavior is needed. In this chapter, we ll look at some consequences of alkene stereoisomerism and then focus on the broadest and most general class of alkene reactions, the electrophilic addition reaction. 

The most important functional groups with double bonds are the C=C of alkenes and the C=0 of aldehydes and ketones. Both appear in many organic and biological molecules. Their most common reaction type is addition. 

Chapter 13 discusses the substitution reactions of alkanes— hydrocarbons that contain only single bonds. In previous chapters, we have seen that when a compound reacts, the weakest bond in the molecule breaks first. Alkanes, however, have only strong bonds. Therefore, conditions vigorous enough to generate radicals are required for alkanes to react. Chapter 13 also looks at radical substitution reactions and radical addition reactions of alkenes. The chapter concludes with a discussion of some radical reactions that occur in the biological world. 

Alkene polymers—large molecules resulting from repetitive bonding together of many hundreds or thousands of small monomer units—are formed by chain-reaction polymerization of simple alkenes. Polyethylene, polypropylene, and polystyrene are examples. As a general mle, radical addition reactions are not common in the laboratory but occur much more frequently in biological pathways. 

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

One final difference between laboratory and biological reactions is in their specificity. A catalyst such as sulfuric acid might be used in the laboratory to catalyze the addition of water to thousands of different alkenes (Section 6.5), but an enzyme, because it binds a specific substrate molecule having a very specific shape, will catalyze only a very specific reaction. It s this exquisite specificity that makes biological chemistry so remarkable and that makes life possible. Table 6.4 summarizes some of the differences between laboratory and biological reactions. 

Nitrones can undergo a variety of synthetically useful reactions 1,3-dipolar cycloaddition with alkenes and allqmes to isoxazolidines, nucleophilic addition which provide hydrojgrlamines, or Sm mediated cross couplings with carhonyl compounds affording vicinal amino alcohols.Recently, special attention is focused on the preparation and application of the optically pure cyclic nitrones, being used in the synthesis of biologically important molecules. ... 

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