Reaction Types General Considerations

A major advantage of enzymes as catalysts is that they are capable of inducing very high degrees of enantioselectivity and, consequently, they are particularly useful in the synthesis of enanhomerically pure compounds. In cases where the enanh-oselectivity is less than optimum it can generally be improved using protein engineering techniques such as in vitro evolution [10]. Hence, in this chapter we shall be mainly concerned with the application of enzymatic cascade processes to the 

---

Thus solvolysis of (+)C6HsCHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (+)C6H13CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of ( + )C6H5CHMeCl in 80 % acetone/20 % water leads to 98 % racemisation (above), but in the more nucleophilic water alone to only 80% racemisation. The same general considerations apply to nucleophilic displacement reactions by Nu as to solvolysis, except that R may persist a little further along the sequence because part at least of the solvent envelope has to be stripped away before Nu can get at R . It is important to notice that racemisation is clearly very much less of a stereochemical requirement for S l reactions than inversion was for SN2. 

Most publications dealing with chromatographic reactors focus on theoretical issues of this very complex system. Models of different complexity were derived and used to predict the behavior of chromatographic reactors. Such models typically take into consideration different types of mass transfer, adsorption isotherms, flow profiles, and reactions. A general scheme of these models, not including the reaction, is presented in Fig. 4. There are also several review papers... 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

A + B - A + B Considerable literature is available on this type of reaction. The general agreement of vibrational-energy transfer associated with lower vibrational levels is encouraging. Flames, flash photolysis, shocks, theory 84, 86, 93... 

The reactions of organoboranes are in general not discussed here, but are distributed throughout Comprehensive Organic Synthesis wherever appropriate reaction types are under consideration. However, protonolysis of organoboranes is treated in Section 3.10.7 because of its necessary role in the reduction of carbon-carbon multiple bonds via hydroboration (Scheme 1). Reviews of other synthetically useful organoborane reactions appear elsewhere. - ... 

Although metoclopramide has lost some ground to newer congeners, it has been the most widely used of the neuroleptic-type antiemetic drugs and is therefore the one for which the clearest picture of adverse effect, typically neuroleptic and endocrine effects, has emerged. Reactions are generally short-lived provided treatment is withdrawn the duration of reactions does not always seem to be explained by simple pharmacokinetic considerations. 

With the exception of the Fischer reaction, the general methods employed for the formation of the glycoside bond result in the formation of glycoside acetates (or benzoates). Deacylation is accomplished by treatment with alkali, to which the glycoside bond is stable, unless the bond approximates to the ester type. The original method of deacetylation was by quantitative saponification with sodium hydroxide, potassium hydroxide, or barium hydroxide. It was noted, however, that, in anhydrous media, much less than the theoretical amount of alkali was required, and the reaction has since been made entirely catalyt.ic, with considerable gain in efficiency and convenience. The same methods have been employed for debenzoylation. 

Organic electrosynthesis is not a new technique and there is considerable knowledge of the types of reactions that take place at cathodes and anodes. It is also true, however, that there have been few attempts to develop electrochemical reactions as general synthetic tools and very few attempts to use electrochemistry to synthesize complex molecules. Therefore, electrosynthesis is at present a promising, but seldom tested technique for the synthesis of medicinals. 

---