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Enantiomeric ester

Differences in chirality of substrate, and nature of solvent, have no effect on the competitive nature of the displacement of 0-alkyl and S-methyl groups in the reactions between (+)-pinacolyl alkoxide and 0-ethyl (and methyl) S-methyl methylphosphonothioates (Scheme 23). For the (R)-( + ) esters, e.g. (210), the displacements are highly stereoselective and occur with configurational inversion,but the enantiomeric esters do not display such stereoselectivity. (-)-Menthol might be considered a mirror image of (S)-pinacol, and similar reactions with the sodium salt of (-)-menthol occur highly stereoselectively... [Pg.176]

In a comparable system, (I ,S)-ibuprofen can be separated by a membrane reactor [83], see Fig. 13.10. The technique comprises a stereo-specific hydrolysis by an enzyme. Subsequently, the enantiomeric ester is extracted into the organic phase on the other side of the membrane. In the system developed by Sepracor Inc., (i )-ibuprofen is selectively hydrolyzed by proteases in a hollow-fiber unit and the (S)-ibuprofen ester can be isolated at 100% yield. This configuration also applies for enantioseparation of other acids such as naproxen and 2-chloropropionic acid. [Pg.541]

Resolution of alcohols (cf., l-(naphthyl)cthyl isocyanate, 8, 356 357). A practical synthesis of the methyl ester of (S)-5-HF,TF. (1) from arachidonic acid involves chromatographic separation of the diastereomeric urethanes preparecT from the Isocyanate derived from dehydroabietylamine (hydrogen chloride and phosgene).1 Urethanes from other chiral amines are less useful. The urethanes are cleaved with triethylamine and trichlorosilanc to give the corresponding pure enantiomeric esters, which can be hydrolyzed by base. [Pg.117]

A noteworthy synthetic application for the reaction of J with a,g-unsaturated phosphoryl compounds is represented by the addition involving hitherto unknown (-)-(Sp)-methylphenylvinylphosphine oxide J 2. The resulting tertiary phosphine oxides 13 with saturated carbon chains and known stereochemistry at phosphorus constitute attractive starting materials for the preparation of optically pure phosphines. The organophosphorus substrate 12 was obtained by decarbomenthoxylation of the enantiomeric ester... [Pg.245]

A possible explanation was that the effect was connected to the changing concentration of the enantiomeric esters. However, no effect on E was observed when enantiopure (R)-lb, the faster reacting enantiomer of the butanoate of 1-phenoxy-... [Pg.99]

The development of biotechnology is nowadays opening up new approaches for synthesis, particularly of chiral target molecules. An example is a recent patent (EP 1223223, 2002) in which Symrise claim a process for the resolution of menthol using a lipase. When presented with a mixture of enantiomeric esters, the enzyme hydrolyses only the 1-enantiomer and then the 1-menthol is easily separated from the unreacted d-ester. The reaction scheme is shown in Figure 4.29. [Pg.80]

In aqueous 90% acetone at 99.6°C, At for /r Ai.s-a,y-dimethylallyl p-nitro-benzoate is 2.6x10-6 ec" [(II), R = R = CH3 Y = p-O-.NQH.CO., SOH = HoO]. The rate is not affected by addition of small amounts of lithium p-nitrobenzoate or p-nitrobenzoic acid. In aqueous 60% acetone at 60 C, At = 1.5 X lO secBy using carbonyl- o-labeled ester, it was shown that 60 scrambling is about twice as fast as allylic rearrangement, which implies that the carboxyl group of the p-nitrobenzoate ion in the tight ion pair rotates relative to the allylic carbonium ion in some instances before the ions recombine to form enantiomeric esters . This and similar observations on other allylic esters (see below) rules out the concerted Sf i mechanism as the only mechanism for allylic ester isomerization under solvolytic conditions. [Pg.427]

Some examples of DKR based on racemization of secondary alkyl amine via Shiff base were shown in Scheme 5.9. Schiffbase formation of a-amino carboxylic esters significantly increases the acidity of the a-proton in comparison to that of the parent amino acid, thus enabling enantiomer-selective hydrolysis and ammonoly-sis through DKR [23]. For example, chymotrypsin catalyses the hydrolysis of a Schiff base of phenylalanine ethyl ester and an aromatic aldehyde (Scheme 5.9, Equation 5.6) [23b]. In this particular case, natural phenylalanine precipitates, leaving the aldehyde and unhydrolysed enantiomeric ester in solution. Addition of l,4-diazabicyclo[2.2.2]octane (DABCO) (as the Bronsted base) allows DKR to take place by promoting racemization of the Schiffbase. Similarly, Novozym 435... [Pg.170]

The graft copol3nner possesses an optically active center in the vicinity of the nucleophilic catalytic site. It was therefore of interest to determine if the poly(ethylenimine-g-L-histidine) catalyst would hydrolyze one enantiomeric ester substrate in preference to the other. A hydrophobic substrate was chosen as it was felt that the presence of a strong apolar interaction would provide for a strong substrate-catalyst complex. [Pg.275]

Special attention should be devoted to less conventional applications of the enzymatic transesterification methodology such as resolution of unstable substrates as racemic secondary hydroperoxides [291]. The development of new reactions in the presence of enzymes should pursued, as, for example, the simultaneous formation of a hemithioac-etal and die irreversible transesterification in the presence of a lipase [292]. Also, for synthetic applications, the combination of enzymatic and chemical asymmetrical methods could lead to interesting results, such as the one-pot lipase-catalyzed acylation and the Mitsonobu inversion of the configuration of the unreacted alcohol, which should lead to only one enantiomeric ester [293]. [Pg.453]


See other pages where Enantiomeric ester is mentioned: [Pg.14]    [Pg.325]    [Pg.325]    [Pg.176]    [Pg.143]    [Pg.20]    [Pg.260]    [Pg.343]    [Pg.534]    [Pg.310]    [Pg.405]    [Pg.179]    [Pg.363]   
See also in sourсe #XX -- [ Pg.343 ]




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