Mechanistic themes

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

These important synthesis reactions have a common mechanistic theme ... 

This book is a collection of 300 problems which challenge the user to devise reasonable mechanistic interpretations for sets of experimental observations. Almost all of the problems are taken from the literature of the last twenty years. Each is a separate entity, although similar mechanistic themes occur in several quite different problems. Answers are not given, nor are references to the original literature. The user who fails to solve a particular problem and reaches an appropriate level of frustration should be able, relatively quickly, to locate the original literature from the information given in the problem. 

As of 2006 the US FDA has granted marketing approval to 22 biotechnology-derived pharmaceuticals with varying applications to oncology practice (Table 25.1). Some common mechanistic themes are as follows ... 

The thermal cycloaddition of phencyclone (85) with cycloheptatriene offers additional mechanistic insight into these reactions (Scheme 13). In this case a 57% yield of the corresponding endo [2 -i- 4] adduct was isolated. Further pyrolysis of this material at 170 C provided the decarbonylated pentacyclic material (86) along with an exo [6 h- 4] adduct (87). The course of this reaction can be contrasted with the thermolysis of compound (83). l ile neither of these examples is of great potential synthetic value, they do serve to illustrate some of the typical mechanistic themes which characterize these reactions. 

The basic plan of protein synthesis in eukaryotes and archaea is similar to that in bacteria. The major structural and mechanistic themes recur in all domains of life. However, eukaryotic protein synthesis entails more protein components than does prokaryotic protein synthesis, and some steps are more intricate. Some noteworthy similarities and differences are as follows ... 

The functions of the guanidine-modifying enzymes further subdivide the family into three distinct groups hydrolases, dihydrolases, and amidinotransferases. Hydrolases catalyze the hydrolysis of guanidine derivatives to form ureido compounds, whereas dihydrolases catalyze a hydrolysis reaction to yield a primary amine, ammonia, and bicarbonate (or carbon dioxide). The amidinotransferases transfer an amidino group from one substrate to an amine. Although these are distinct reactions, they are characterized by common structural and mechanistic themes. 

ECH or crotonase is the prototypical member of the crotonase superfamily. As noted above, sequence homology between ECH and 3,2-enoyl-CoA isomerase as well as with dibydroxynapbtboate syntbase (MenB) and 4-chlorobenzoyl-CoA dehalogenase resulted in the initial proposal for a superfamily based on the crotonase scaffold. Since then, many more members of the superfamily have been identified. Most family members utilize substrates that are CoA thioesters, and a unifying mechanistic theme throughout the superfamily concerns the use of an oxyanion hole to stabilize carbanionic transition states. Figure 40 shows the reactions catalyzed by a subset of family members. ... 

The purpose of this chapter is to summarize some recent developments in the physics of lipid bilayers that demonstrate the existence of curvature-elastic stresses in bilayers and to review mechanisms whereby the resultant forces may couple to membrane protein conformations (see also references 1-3 for reviews). A consequence of these forces is that membrane proteins may have mechanistic themes that are qualitatively different from themes operative in aqueous proteins. Moreover, because these forces are directed generally parallel to the membrane surface, the actual conformational motions to which the forces couple may ultimately be simpler to understand than the complex conformations of aqueous proteins. 

Acetals, Imines, and Enamines Common Mechanistic Themes in Their Acid-catalyzed Formation from Aldehydes and Ketones... 

Chapters on carbonyl chemistry that are organized to emphasize mechanistic themes of nucleophilic addition, acyl substitution, and reactivity at the a-carbon. 

In the presence of a strong acid, the —OH group can be protonated. Protonation of the —OH group produces the good leaving group —OH2, and this is a common mechanistic theme for alcohol reactions. 

We will take these reactions one at a time and examine their common mechanistic theme. 

This overview has been divided into four families ion channels, enzymes, receptors, and cytokines. Where appropriate each targeted family has been subdivided into members that reflect the mechanistic theme. 

Beyond the two mechanistic themes presented above, that is, autooxidation and redox-type reactions involving electron transfer, a ruthenium-substituted sandwich -type polyoxometalate was shown to be a catalyst for oxidation by a dioxygenase -type mechanism as outlined in Scheme 9.13 [125]. 

The functional groups of acyl substitution reactions all relate to carboxylic acids. They include acyl chlorides, anhydrides, esters, amides, thioesters, carboxylic acids themselves, and others that we shall study in this chapter. In Special Topic C we shall see how acyl substitution reactions are used to synthesize polymers such as nylon and Mylar. In Special Topic E we shall consider the biosynthesis of fatty acids and other biological molecules by acyl substitution reactions. Although many functional groups participate in acyl substitution reactions, their reactions are all readily understandable because of the common mechanistic theme that unites them nucleophilic addition-elimination at an acyl carbon. 

Shares mechanistic themes with other reactions denoted the same symbol. 

---