Acetyl ester from

Contrary to PAE, RGAE removed at random the acetyl esters from apple pectic hairy ramified regions (MHR), to a maximum of 70 %. RGAE was essential for the activity of rhamnogalacturonase (RG), and as such comparable with A. aculeatus RGAE. 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

The anomeric forms derived from equilibration of aldoses give rise to multiple peaks when trimethylsilylated and gas chromatographed [311]. A method of overcoming this problem, assuming that mutarotation itself is not under study, is to modify the aldose. It can be oxidised and lactonised to the aldonolactone, for example, and characterised as its TMS derivative [322]. Alternatively for the identification of aldoses and alditols, more use may be made in the future of the separations achievable on open tubular columns of the poly-0-acetylaldonic nitriles (18) produced from aldoses and the poly-acetyl esters from alditols [323]. Figure 1.18 shows the separation of 32 assorted polyols and aldoses. 

Intramolecular Cyclopropanation. The resultant a-diazo-acetyl ester from the reaction of (1) and an unsaturated alcohol undergoes cyclization in the presence of transition metals to give cyclopropyl derivatives (eq 3, 4) the reaction proceeds via an intermediary carhene species. Owing to the geometric constraints of the intramolecular cyclopropanation, the substituents and the product acquire all-c/5 configurations. This is in contrast to the bimolecular cyclopropanation, which is unable to achieve stero-chemical control, resulting in mixtures of products. 

Hydrolysis. The primary functions of hydrolysis are to remove some of the acetyl groups from the cellulose triester and to reduce or remove the combined acid sulfate ester to improve the thermal stabiUty of the acetate. 

Candida cylindracea, phosphate buffer pH 7, Bu20. The 6-0-acetyl of Q -methyl peracetylglucose was selectively removed. Porcine pancreatic lipase will also hydrolyze acetyl groups from carbohydrates. These lipases are not specific for acetate since they hydrolyze other esters as well. In general selectivity is dependent on the ester and the substrate. ... 

Hydrolysis removes the acetyl group from nitrogen and converts the two ester functions to carboxyl groups. Decarboxylation gives the desired product. 

The reaction of hydroxyethyl-TPP with the oxidized form of lipoic acid yields the energy-rich thiol ester of reduced lipoic acid and results in oxidation of the hydroxyl-carbon of the two-carbon substrate unit (c). This is followed by nucleophilic attack by coenzyme A on the carbonyl-carbon (a characteristic feature of CoA chemistry). The result is transfer of the acetyl group from lipoic acid to CoA. The subsequent oxidation of lipoic acid is catalyzed by the FAD-dependent dihydrolipoyl dehydrogenase and NAD is reduced. 

The acid value of this on it alwavs below 1 and the ester value does not exceed 2 5.. A large amount of free alcohols, Uow.wcr, is present, the ester value after acetylation vaning from -16 m 129. 

C) Preparation of N-(p-Acetylphenoxy-Acetyl)-Piperidine The ester from (B) is refluxed for 8 hours with 2.5 mols of thoroughly dried piperidine. Then 1 volume of water is added and the product Is left to crystallize in the cold. The desired amide is obtained in an 80% yield. 

Conversion of Acid Anhydrides into Esters Acetic anhydride is often used to prepare acetate esters from alcohols. For example, aspirin (acetylsalicylic acid) is prepared commercially by the acetylation of o-hydroxybenzoic acid (salicylic acid) with acetic anhydride. 

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

Enzymes can be used to specifically modify the pectins. Pectin methyl esterase is already widely used to adjust the gelling properties of commercially available pectins. The acetyl esters also strongly affect the gelation [2,3] and removal is important for the upgrading of sugar beet pectin, extractable from a by-product of the sugar industry. 

The A. niger preparation investigated in this study contains at least three different acetyl esterases, each with its own specificity. The activities of RGAE and FAE are comparable to those of similar enzymes isolated previously from A. aculeatus and a different A. niger preparation. PAE appears to be a new enzyme, with an activity specific towards one type of acetyl ester in the homogalacturonan chain of beet pectin. 

The enantiomeric excess (ee) of the hydrogenated products was determined either by polarimetry, GLC equipped with a chiral column or H-NMR with a chiral shift reagent. Methyl lactate and methyl 3-hydroxybutanoate, obtained from 1 and 2, respectively, were analized polarimetry using a Perkin-Elmer 243B instrument. The reference values of [a]o(neat) were +8.4° for (R)-methyl pyruvate and -22.95° for methyl 3-hydroxybutcinoate. Before GLC analysis, i-butyl 5-hydroxyhexanoate, methyl 5-hydroxyhexanoate, and n-butyl 5-hydroxyhexanoate, obtained from 1, 5, and 6, respectively, were converted to the pentanoyl esters, methyl 3-hydroxybutanoate was converted to the acetyl ester, and methyl 4-methyl-3-hydroxybutanoate obtained from 2 was converted the ester of (+)-a-methyl-a-(trifluoromethyl)phenyl acetic acid (MTPA). 

Hydrolysis involves nucleophilic attack by the serine hydroxyl onto the ester carbonyl (see Box 7.26). This leads to transfer of the acetyl group from acetylcholine to the enzyme s serine hydroxyl, i.e. formation of a transient acetylated enzyme, and release of choline. We have met this type of reaction before under transesterification (see Section 7.9.1). Hydrolysis of the acetylated enzyme then occurs rapidly, releasing acetate and regenerating the free enzyme. 

A methylene group between the oxadiazole ring and a carboxyl group acts like a malonic acid with respect to ease of loss of the carboxyl group. Likewise the acetyl group from an acetonyl side chain at C-5 is lost in basic solution, analogous to the ketone split in a fi-keto ester or / -diketone. This was recognized in the early work on oxadiazoles by Tiemann,48 Wiese,49 Richter,47 and Schubart.50... 

An attempted synthesis of 9-methylnaphtho[crf]oxepine-2-one 494 by heterocyclization of 8-acetyl-1-naphthoic acid 491 (R = Me, R = H) has failed. In acidic medium or on heating acid 491 to 150°C, as well as under formation conditions for the acid chloride or ester from acid 491, 2-acetylacenaphthene-l-one 495 is obtained (79ZOR1562). A synthesis of tribenzo[c]oxepine derivatives 499 and 501 has been described as resulting from heterocyclization of the products of reduction (498) or oxidation (500) of 4-formyl-5-carboxyphenanthrene 497. The latter compound was obtained on ozonolysis of pyrene 496 [71JCS(C)729]. 

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