Reaction transesterification

A proposed mechanism of PET glycolysis would be (1) glycol diffusion into die polymer (2) swelling of the polymer, which increases the rate of diffusion and (3) reaction (transesterification) of the glycol hydroxy group at an ester group in die polyester chain.24 Since it is a transesterification process, metal acetate salts are effective catalysts. 

B. Electrophilic Reactions.—Transesterification followed by rearrangement is a common route from simple phosphites to more complex phos-phonates. This has now been applied to the preparation of cyclic phos-phonates (85). Both phosphites (86) and phosphoranes (87) containing phosphorus-hydrogen bonds are obtained from the cyclic biphosphite (88) and butanol. ... 

Another type of important selectivity is that between hydrolysis and transferase reactions (transesterification, transglycosylation, etc.) catalyzed by hydrolases. In this case, water can act both as a reactant and as a substance that modifies the properties of the enzyme. Effects of water as a reactant can be expected to be governed by the concentration or activity of water, as with other substrates. The effects of water as an enzyme modifier are considerably more difficult to predict. 

The study of posttranscriptional processing of RNA molecules led to one of the most exciting discoveries in modern biochemistry—the existence of RNA enzymes. The best-characterized ribozymes are the self-splicing group I introns, RNase P, and the hammerhead ribozyme (discussed below). Most of the activities of these ribozymes are based on two fundamental reactions transesterification (Fig. 26-13) and phosphodiester bond hydrolysis (cleavage). The substrate for ribozymes is often an RNA molecule, and it may even be part of the ribozyme itself. When its substrate is RNA, an RNA cat-... 

Polycondensation Reaction. Transesterification was carried out in the conventional manner (4) with 97 grams (0.50 mole) of dimethyl terephthalate (DMT) and 69 grams (1.10 moles) of ethylene glycol in the presence of 0.088 gram of Ca(OAc)2 H20 and 0.044 gram of Sb203 at 160°-230°C. After completing the transesterification, 0.080... 

When switching from water to an organic solvent, or switching between organic solvents, the substrate specificity can change. In the example of the standard reaction, transesterification of N-acetyl-i-phenylalanine ethyl ester with n-propanol by Subtilisin Carlsberg, which has been mentioned several times in this chapter already, the relative specificity between the rather hydrophobic phenylalanine compound and its more hydrophilic analog N-acetyl-L-serine ethyl ester varies with the solvent (Table 12.8) (Wescott, 1993). 

Cleavage of nucleic acids refers to a reaction that results in the breakage of bonds in the phosphodiester backbone of a polynucleotide chain. There are two types of reactions resulting in the cleavage of either P-O or C-O bonds in the nucleic acid backboue (see Figure Id). Cleavage of the P-O bond occurs as a result of uucleophilic attack ou the phosphorus atom via either an intermolecular reaction with a H2O molecule hydrolysis) or an intramolecular reactiou involving the ribose 2 -OH transesterification). Of these two reactions, transesterification is specific to RNA (since DNA lacks a 2 -OH moiety) and can be catalyzed by many M- +... 

Classical reactions involving nucleophiles such as saponification ("OH as the nucleophile), aminolysis (with amines also ammonia in ammonolysis reactions), transesterification (alkoxides, "OR) and others (hydrazinolysis, hydroxamic acid synthesis, etc.) have been adapted to solid phane and used to obtain, for instance, carboxylic acids, amides and esters. Internal or intramolecular nucleophilic attack has been employed to obtain cyclic products such as lactones, lactams (including cyclic peptides) and a great variety of heterocycles (hydantoins, diketopiperazines, benzodiazepinones, etc.). 

In 1973, Bonatz et al. [101] finally showed by cai efudy performed experiments diat die first reaction model proposed by Hoftyzer and van Krevelen [100] is correct. Thus, die polycondensation reaction (transesterification of bEG) takes place in die entire melt phase and the removal of EG is the rate-determining step for the overall polycondensation process. 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

Regioselective Reactions. Transesterification has often been used for the modification of natural products, such as steroids, glycerides, carbohydrates, and nucleosides (41,42). Examples of enzymatic regioselectivity have already been shown in Figures 3, 11, and 18, presenting glycerides and an amino acid derivative as a substrate. 

The ROP of six-membered cyclic carbonates with nucleophilic initiators is a chain reaction in which, besides initiation and propagation reactions, transesterification reactions can also take place (Scheme 32).Intramolecular nucleophile (e.g., alkoxide) attacks on carbonyl carbon atom (backbiting) lead to cyclic oligomers, while intermolecular transesterifica-tion leads to a change of the macromolecule length with the consequence that, at equilibrium, the most probable distribution of the molecular weight is obtained (Scheme 32). 

Ester hydrolysis, esterification or acylation, and transesterification reactions play an important role in nature and organic synthesis including the synthesis of natural products [1, 2], Applications of hydrolytic or esterification reactions range from laboratory syntheses (e.g. with the acyl moiety as key intermediate or as protecting group) to industrial scale production of bulk chemicals, biodiesel or food (e.g. ester units as building blocks for polymerisation and polycondensation reactions, transesterification of triglycerides) [3, 4]. Transesterification reactions in nature include for example enzymatic reactions of lipases, esterases and other hydrolases that rely on the catalytic triad of serine proteases, which will be discussed in the first part of this section [5-8]. 

The ROP of cyclic carbonates with nucleophihc initiators is a chain reaction in which, besides initiation and propagation reactions, transesterification reactions must also be considered, with termination and transfer reactions being absent (Scheme 12.1) [3]. 

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