Esters, active s. Carboxylic

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,... 

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. 

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... 

Such condensation reactions are also promoted by certain trTvalent phosphorus compounds, e.g. triphenyl phosphite (2) or diphenyl ethylphosphonite (3), or to a lesser extent by pFosphonate esters, e.g. diphenyl n-butylphosphonate (3). "Bates reagent," p-oxobi s[tri s(cTi methyl ami no)phosphoni urn] bi s-tetra-f1uoroborate (2) may also be used to activate the carboxyl function towards amide bond formation during peptide synthesis (4) and to bring about the Beckmann rearrangement of ketoximes (F). 

The above stereospecific tiirane polymerisations have generally been run in heterogeneous systems. Such conditions essentially make it impossible to determine the detailed structure of active species involved in these polymerisations. Thus, enantiosymmetric and enantioasymmetric polymerisations of propylene sulphide have also been studied in a homogeneous phase by using chiral cadmium thiolates of cysteine esters and chiral cadmium carboxylates of cysteine and methionine [157,160-164]. The most studied is living polymerisation using the cadmium derivative of the isopropyl ester of (.S)-cysteine [160] ... 

Mild alkaline hydrolysis of S. sobrinus GTase-S glucosyl-enzyme releases glucose exclusively as the /3-anomer. This contrasts with a-D-glucose liberated during sucrose hydrolysis by native enzyme. The difference can be attributed to the character of acylals, which can hydrolyze at the acetal carbon or the ester carbon with respective anomeric retention or inversion (229, 230). The native enzyme hydrolyzes sucrose as an acetal so that released a-o-glucose translates to an active site carboxyl linked as the )3-anomer or ion paired to a carbonium ion from the re face. The denatured covalent complex, without the active site structure to cleave at the acetal carbon, hydrolyzes the /3-linked D-glucose at the ester carbon, with release of the retained jS-anomer. 

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