Esters, active s. Carboxylic acid

Esterification s. Carboxylic acid esters from carboxylic acids, Redoxesterification Esters, active s. Carboxylic acid esters, active Etherates s. Alane etherates Ethers (s. a. Alkoxy...,... 

Esters, active s. Carboxylic acid esters, active,... 

Mayer and co-workers improved the amide bond formation of TenBrink s method using unprotected amino alcohols and active esters of bromoalkyl carboxylic acids.[5] More recently, Anthony et al.,[6] and Norman and Kroin[7l reported stereospecific alkylation of acylmorpholinone (Schemes 3 and 4) using sodium hexamethyldisilazanide and lithium diisopropylamide, respectively. 

Efforts have also been made to utilize ketone acetals bearing a trichlorosilyl group as an enolate donor (Scheme 6.12) [63a]. This reaction led to optically active / -hydroxy carboxylic acid esters (S)-23 in good yields, although enantioselectivity remained modest only (ee values up to 50% ee when phosphoramide is used in... 

Carbanions derived from optically active sulfoxides react with esters, affording generally optically active )S-ketoesters ° . Kunieda and coworkers revealed that treatment of (-t-)-(R)-methyl p-tolyl sulfoxide 107 with n-butyllithium or dimethy-lamine afforded the corresponding carbanion, which upon further reaction with ethyl benzoate gave (-l-)-(R)-a-(p-tolylsulfinyl)acetophenone 108. They also found that the reaction between chiral esters of carboxylic acids (R COOR ) and a-lithio aryl methyl sulfoxides gave optically active 3-ketosulfoxides The stereoselectivity was found to be markedly influenced by the size of the R group of the esters and the optical purity reached to 70.3% when R was a t-butyl group. 

Since the imidazolide method proceeds almost quantitatively, it has been used for the synthesis of isotopically labeled esters (see also Section 3.2), and it is always useful for the esterification of sensitive carboxylic acids, alcohols, and phenols under mild conditions. This advantage has been utilized in biochemistry for the study of transacylating enzymes. A number of enzymatic transacylations (e.g., those catalyzed by oc-chymo-trypsin) have been shown to proceed in two steps an acyl group is first transferred from the substrate to the enzyme to form an acyl enzyme, which is then deacylated in a second step. In this context it has been shown[21] that oc-chymotrypsin is rapidly and quantitatively acylated by Af-fraw.s-cinnamoylimidazole to give /ra/w-cinnamoyl-a-chymotrypsin, which can be isolated in preparative quantities and retains its enzymatic activity (see also Chapter 6). 

Solutions of Ru3(CO)i2 in carboxylic acids are active catalysts for hydrogenation of carbon monoxide at low pressures (below 340 atm). Methanol is the major product (obtained as its ester), and smaller amounts of ethylene glycol diester are also formed. At 340 atm and 260°C a combined rate to these products of 8.3 x 10 3 turnovers s-1 was observed in acetic acid solvent. Similar rates to methanol are obtainable in other polar solvents, but ethylene glycol is not observed under these conditions except in the presence of carboxylic acids. Studies of this reaction, including infrared measurements under reaction conditions, were carried out to determine the nature of the catalyst and the mechanism of glycol formation. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor. 

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