Cleavage acid esters

The chemical inertness of the three-membered ring permitted many conversions of functional groups in diazirines. Esterifications, cleavage of esters and acetals, synthesis of acid chlorides, oxidation of hydroxy groups to carboxyl groups as well as Hofmann alkenation all left the three-membered ring intact (79AHC(24)63). 

Esterification. The esterification of rosin provides important commercial products for the adhesive industry. Rosin esters are formed by the reaction of rosins with alcohols at elevated temperatures. Because the carboxyl group of the resin acids is hindered by attachment to a tertiary carbon, esterification with an alcohol can only be accomplished at elevated temperatures. This hindrance is in turn responsible for the high resistance of the resin acid ester linkage to cleavage by water, acid and alkali. 

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

Sealants obtained by curing polysulfide liquid polymers with aryl bis(nitrile oxides) possess stmctural feature of thiohydroximic acid ester. These materials exhibit poor thermal stability when heated at 60°C they soften within days and liquefy in 3 weeks. Products obtained with excess nitrile oxide degrade faster than those produced with equimolar amounts of reagents. Spectroscopic studies demonstrate that, after an initial rapid addition between nitrile oxide and thiol, a second slower reaction occurs which consumes additional nitrile oxide. Thiohydroximic acid derivatives have been shown to react with nitrile oxides at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol. In the case of a polysulfide sealant, the rupture of a C-S bond to form the thiol involves cleavage of the polymer backbone. Continuation of the process leads to degradation of the sealant. These observations have been supported by thermal analysis studies on the poly sulfide sealants and model polymers (511). 

Arylalkanoic acid esters, 72 166 Arylaldehydes, microwave-accelerated transformation to nitriles, 76 580-581 Aryl alkyl ethers, acid cleavage of, 70 569 Arylalkylsulfones, 9 273 Arylamine dye intermediates, 9 272t... 

Guengerich FR Peterson LA, Bocker RH. Cytochrome P-450-catalyzed hydroxylation and carboxylic acid ester cleavage of Hantzsch pyridine esters. J Biol Chem 1988 263(17) 8176-8183. 

The a-, (3- and y-carotenes, which are found in most plants, are vitamin A provitamins and are converted to vitamin A alcohol (all- ran,v-retinol), which is usually called vitamin Aj (Figure 12.8) by oxidative mid-point cleavage. Retinol and its fatty acid esters are the main forms in which vitamin A is stored in animals and humans, and its oxidation product, 1 1-c/s-retinal (vitamin A, aldehyde), is required for the visual process. 

Before discussmg the mechanism of cleavage of carboxylic acid esters and amides by hydrolases, some chemical principles are worth recalling. The chemical hydrolysis of carboxylic acid derivatives can be catalyzed by acid or base, and, in both cases, the mechanisms involve addition-elimination via a tetrahedral intermediate. A general scheme of ester and amide hydrolysis is presented in Fig. 3. / the chemical mechanisms of ester hydrolysis will be... 

Fig. 7.1. a) Specific acid catalysis (proton catalysis) with acyl cleavage in ester hydrolysis. Pathway a is the common mechanism involving a tetrahedral intermediate. Pathway b is SN1 mechanism observed in the presence of concentrated inorganic acids. Not shown here is a mechanism of alkyl cleavage, which can also be observed in the presence of concentrated inorganic acids, b) Schematic mechanism of general acid catalysis in ester hydrolysis. 

The above enzymes are not the only ones able to cleave phosphoric acid esters. Indeed, the cleavage of some organophosphates can be catalyzed by monooxygenases, as illustrated below. Furthermore, glutathione S-transfe-rases can also act on some methyl and ethyl phosphates [73], but these reactions will not be covered here. 

Fig. 9.11. Postulated mechanism of cytochrome P450 catalyzed phosphoric acid ester cleavage of the bisphophonate U-91502 (9.51) to produce a triester metabolite 9.52 [118]...

Furthermore, the mediator has been used for the bond cleavage of benzyl ethers, the oxidation of benzyl alcohol to benzaldehyde, the oxidation of toluene derivatives to benzoic acid esters, and the oxidation of aliphatic ethers [47]. 

Esterases and amidases E. coli, P. vulgaris, B.. subtilis, B. mycoides Cleavage of esters or amidases of carboxylic acids... 

The polymer-bound p-nitrobenzophenone oxime (71d) has been found to be a suitable support for stepwise peptide synthesis. Protected peptides can be assembled on 70d by coupling and deprotection steps similar to those employed in the usual Merrifield solid-phase procedures (Scheme 39). Cleavage of peptides from 71d can be accomplished with hydrazine and amino acid esters under mild conditions, which do not affect benzyl ester side-chain protecting groups. 

Ozonolysis has been applied to the oxidative functionalization of triglycerides. Papers and reviews, and a few patents, have been issued on the ozonolysis of seed oils for the production of plastics [95, 96]. Theoretically, the oxidation products that are accessible via ozonolysis of fatty acid esters are the same as those that are isolated from the photo-induced singlet oxygen cleavage of the same substrates, via Hock cleavage [28-33]. The mixture of products that come from the ozonolysis of... 

Cleavage of esters 6 (p 407) to give hydroxy acids 7 of known configuration96. 

At the alkyl group of the 0-alkyl substituent (cleavage of an alkyl cation and formation of an unsaturated phosphinic acid ester). 

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