Biological Carbonyl Condensation Reactions

What products would result after hydrolysis from reaction of the enamine prepared from cyclopentanone and pyrrolidine with the following o , 8-unsat-urated acceptors  

Show how you might use an enamine reaction to prepare each of the following compounds  

Aldol reactions occur in many biological pathways but are particularly important in carbohydrate metabolism, where enzymes called aldolases catalyze the addition of a ketone enolate ion to an aldehyde. Aldolases occur in all organisms and are of two t5rpes. Type I aldolases occur primarily in animals and higher plants type II aldolases occur primarily in fungi and bacteria. Both types catalyze the same kind of reaction, but type I aldolases operate through an enamine, while type II aldolases require a metal ion (usually Zn +l as Lewis acid and operate through an enolate ion. 

CHAPTER 17 CARBONYL ALPHA-SUBSTITUTION AND CONDENSATION REACTIONS 

FIGURE 17.16 Mechanisms oftype I and type II aldolase reactions in glucose biosynthesis. 

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Biological Carbonyl Condensation Reactions 962 CHEMISTRY WORK A Prologue to Metabolism 963... 

Living organisms use carbonyl condensation reactions for the biological synthesis of a great many different molecules. Fats, amino acids, steroid hormones, and many other kinds of compounds are synthesized by plants and animals using carbonyl condensation reactions as the key step. 

Biochemistry is carbonyl chemistiy. Almost all metabolic pathways used by living organisms involve one or more of the four fundamental carbonvl-group reactions we ve seen in Chapters 19 through 23. The digestion and metabolic breakdown of all the major classes of food molecules—fats, carbohydrates, and proteins—take place by nucleophilic addition reactions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Similarly, hormones and other crucial biological molecules are built up from smaller precursors by these same carbonyl-group reactions. 

Other indices measure a secondary stage of oxidation, such as the anisidine value (ANV), pointing to formation of carbonyl compounds, capable of undergoing condensation reactions with p-anisidine, and the thiobarbituric acid reactive substance (TBARS) pointing to the presence of malondialdehyde (MDA) in particular. In biological systems, TBARS is of widespread use as a measure for the extent of oxidation damage. Another test for stability of oils to oxidation is based on the development of acidity as secondary product, for example, standards using the Rancimat equipment or a similar setup. 

Carbonyl condensations are among the most widely used reactions in the biological world for the assembly of new carbon-carbon bonds in such important biomolecules as fatty acids, cholesterol, and steroid hormones. One source of carbon atoms for the synthesis of these biomolecules is acetji-CoA, a thioester of acetic acid and the thiol group of coenzyme A. The function of the coenzyme A group of acetyl-CoA is to anchor the acetyl group on the surface of the enzyme systems that catalyze the reactions we examine in this section. In the discussions that follow, we will not be concerned with the mechanism by which each enzyme-catalyzed reaction occurs. Rather, our concern is with recognizing the type of reaction that takes place in each step. 

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