Protecting enzymatically introduced

Two new sections on the protection of phosphates and the alkyne-CH are included. All other sections of the book have been expanded, some more than others. The section on the protection of alcohols has increased substantially, reflecting the trend of the nineties to synthesize acetate- and propionate-derived natural products. An effort was made to include many more enzymatic methods of protection and deprotection. Most of these are associated with the protection of alcohols as esters and the protection of carboxylic acids. Here we have not attempted to be exhaustive, but hopefully, a sufficient number of cases are provided that illustrate the true power of this technology, so that the reader will examine some of the excellent monographs and review articles cited in the references. The Reactivity Charts in Chapter 10 are identical to those in the first edition. The chart number appears beside the name of each protective group when it is first introduced. No attempt was made to update these Charts, not only because of the sheer magnitude of the task, but because it is nearly impossible in... 

Enzyme electrodes. Guilbault52 was the first to introduce enzyme electrodes. The bulb of a glass electrode was covered with a homogeneous enzyme-containing gel-like layer (e.g., urease in polyacrylamide) and the layer was protected with nylon gauze or Cellophane foil when placed in a substrate solution (e.g., urea) an enzymatic conversion took place via diffusion of substrate into the enzymatic layer. 

Oligonucleotides containing amine groups introduced by enzymatic or chemical means may be modified with SATA (Chapter 1, Section 4.1) to produce protected sulfhydryl derivatives. The NHS ester end of SATA reacts with a primary amine to form a stable amide bond. After modification, the acetyl protecting group can be removed as needed by treatment with hydroxylamine under mildly alkaline conditions (Fig. 401). The result is terminal sulfhydryl groups that can be used for subsequent labeling with thiol-reactive probes or activated-enzyme derivatives (Kumar and Malhotra, 1992). 

The stereogenic center at C20 is introduced by enantioselective enzymatic hydrolysis of MOM-protected malonic acid dimethyl ester derivative 60 (Scheme 10) with pig liver esterase (PLE). The asymmetric compound 61 is obtained in 90 % yield and 98 % ee. Amide formation with Mu-... 

The application of the non-urethane PhAc blocking group results in ca. 6% racemization during the construction of the phenylacetamido-protected dipeptides by chemical activation of the phenylacetamido amino acids. This disadvantage can be overcome by forming the peptide bonds enzymatically, e.g. with trypsin [26,27], chymotrypsin [26,28], or carboxypeptidase Y [26,29]. An interesting example is the biocatalyzed synthesis of leucine-enkephalin tert-butyl ester [25e], in which phenylacetamides are introduced and cleaved by means of penicillin G acylase, and the elongation of the peptide chain is carried out with papain or a-chymotrypsin. 

Structural modifications and formulation are two additional approaches proposed for overcoming the enzymatic barrier (and references therein). Using recombinant or synthetic techniques, selective modifications to the protein or peptide sequence can be introduced effectively reducing proteolytic susceptibility, but these changes must not have significant impact on the pharmacological properties of the molecule (e.g., reduced potency or altered selectivity). Moreover, modifications to address a specific enzymatic action will not eliminate vulnerability to others. The formulation approach essentially involves encapsulation systems to protect the protein or peptide from reactions with enzymes, and selected examples include emulsions, liposomes, or enteric-coated capsules (and references therein). 

Recently, phenylhydrazide has been introduced as an enzyme-labile carboxy protecting group1122, 123]. This protecting group can be removed by mild enzymatic oxidation using a peroxidase1122, 1231 or mushroom tyrosinase11241 (Fig. 18-14). 

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