Racemic acid

Racemic acid, ( )-tartaric acid, is a compound of the two active forms. M.p. 273 C (with IHjO), m.p. 205°C (anhydrous). Less soluble in water than (-t-)-tartaric acid. Formed, together with mesotartaric acid, by boiling (4-)-tartaric acid with 30% NaOH solution, or by oxidation of fumaric acid. Potassium hydrogen racemate is very insoluble.  [c.385]

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive.  [c.525]

The racemic acid is not a primary product of plant processes but is formed readily from the dextrorotatory acid by heating alone or with strong alkaU or strong acid. The methods by which such racemic compounds can be separated into the optically active modifications were devised by Pasteur and were apphed first to the racemic acid. Racemic acid crystallizes as the dihydrate triclinic prisms. It becomes anhydrous on drying at 110°C  [c.526]

Synthesis. Racemic acid is obtained synthetically by treatment of maleic acid with hydrogen peroxide in the presence of a catalyst, eg, tungstic acid (69). Other synthetic routes that have been explored include the production of (R-R, R -tartaric acid by bacterial fermentation of glucose (70) or 5-keto-D-gluconic acid (71), catalytic oxidation of 5-keto-D-gluconic acid with gaseous oxygen (72,73), and nitric acid oxidation of carbohydrates (qv), eg, glucose (74). Production of (R, R -tartaric acid by catalytic chlorate oxidation of fumaric or maleic acid also has been described (75).  [c.526]

R, R -Tartaric (racemic) acid is obtained synthetically by epoxidation of maleic acid with hydrogen peroxide in the presence of a catalyst followed  [c.526]

Racemic Acid and Mesotartaric Acid.  [c.122]


Resolution of Racemic Acid.—The racemic acid is dissolved in water (250 c.c.) and divided into two equal volumes. Half of the solution is carefully neutralised with caustic soda and the other half with ammonia, and the two solutions then mixed.  [c.123]

The following example of an antithesis of a complicated polycyclic and chiral (although racemic) compound, which can be transformed into commercially available starting materials in a few steps, provides convincing evidence for the power of the retro-Diels-Alder transform. Its application can result simultaneously in a decrease of the number of rings and of chiral centres, in disconnection of a molecule into two stable fragments, and finally in simplification of functionality patterns. The target molecule (S. Danishefsky, 1979) is first transformed into the more symmetrical dicarboxylic acid (A) by FGA. Synthetic decarboxylation of this vinyl-ogous /3-keta acid should occur readily in mildly basic media. The cyclohexenone system must then be transformed into a substituted cyclohexene derivative to prepare for the anticipated re/ro-Diels-Alder disconnection. The choice of (B) and the corresponding disubstituted butadiene is dictated by the (commercial) availability of 4-methoxy-3-buten-2-one (see p. 176), which can be converted into the silyl enol ether. The next transforms are FGI of the cyclic an-  [c.209]

The N-to-C assembly of the peptide chain is unfavorable for the chemical synthesis of peptides on solid supports. This strategy can be dismissed already for the single reason that repeated activation of the carboxyl ends on the growing peptide chain would lead to a much higher percentage of racemization. Several other more practical disadvantages also tend to disfavor this approach, and acid activation on the polymer support is usually only used in one-step fragment condensations (p. 241).  [c.235]

Mesotartaric acid crystallizes in plates (IHjO), m.p. 140 C (anhydrous). Very soluble in water. Obtained from the mother-liquors in the preparation of racemic acid or by oxidation of maleic acid. Potassium hydrogen mesotartrale is soluble in water.  [c.385]

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers  [c.310]

Wine studies were cmcial in the development of organic chemistry as well as microbiology and biochemistry. Polarized light was known to be rotated by products from living systems, but not by apparently identical synthesized compounds. The molecular reasons for this were unknown. Tartaric acid [87-69-4] is the major acid of grapes, but is present in very few other plant sources. It occurs in the l(+) form. Racemic acid [133-37-9] was the optically inactive form produced by racemization of the natural acid in alkaU, raceme being the term for the kind of stem a grape cluster has, derived from a Latin word for grape clusters. Pasteur found that when the racemic derivative was allowed to crystallize slowly, two mirror-image crystals were produced. Separated by hand, the two forms rotated polarized light in opposite directions. The crystal stmcture mirrored the asymmetry of the molecules of tartaric acid. From such studies the whole field of stereochemistry was derived. Pasteur became famous for these studies in 1847 at the age of 26. It is said that is was a good thing Pasteur worked under less than ideal conditions, because, if his laboratory temperature had exceeded about 20°C, separate crystals would not have formed.  [c.366]

Thiomahc acid [70-49-5] (mercaptosuccinic acid), C H O S, mol wt = 150.2, is a sulfur analogue of malic acid. The properties of the crystalline, soHd thiomalic acids ate given in Table 6. The racemic acid has the following acid dissociation constants at 25°C pTf i — 3.30 pffc2 — 4.94.  [c.524]

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769.  [c.524]

Chemical Designations - Synonyms 2-Hydroxypropanoic acid alpha-Hydroxypropionic acid Milk acid Racemic acid Chemical Formula CHjCHOHCOOH-HjO.  [c.228]

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he conectly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules. The rare form of tartar-ic acid was optically inactive because it contained equal fflnounts of (+)-tartaric acid and (—)-tartaric acid. It had earlier been called racemic acid (from Latin racemtis, meaning a bunch of grapes ), a nane that subsequently gave rise to our present term for an equal mixture of enantiomers.  [c.310]

Borneol and isoboineol are respectively the endo and exo forms of the alcohol. Borneol can be prepared by reduction of camphor inactive borneol is also obtained by the acid hydration of pinene or camphene. Borneol has a smell like camphor. The m.p. of the optically active forms is 208-5 C but the racemic form has m.p. 210-5 C. Oxidized to camphor, dehydrated to camphene.  [c.64]

Walden inversion A phenomenon discovered in 1895 by Walden. When one of the atoms or groups attached to the asymmetric carbon atom in an optically active compound is replaced by a different atom, the product is sometimes a derivative of the optical isomer of the original compound. It is thus possible to pass from one isomer to the other without the formation and separation of a racemic compound. ( + )-Malic acid, when treated with PCI5 gives (— )-chlorosuccinic acid, which may be converted to ( —)-malic acid by AgjO or back to (-f)-malic acid by KOH. Similarly, ( —)-malic acid is converted to (-l-)-chloro-succinic acid which undergoes similar changes. A Walden inversion occurs at a tetrahedral carbon atom when the entry of the reagent and the departure of the leaving group are synchronous - the so-called bimolecular nucleophilic substitution mechanism. Since the reagent must approach from the side of the molecule opposite to that of the leaving group an inversion of optical configuration results.  [c.424]

It is important to notice that the united-atom simplification cannot be applied to functional hydrogens which are involved in the formation of a hydrogen hond or a salt bridge. This would destroy interactions important for the structural integrity of the protein. Removing the hydrogen at the u-carbon of the peptide backbone is also dangerous, because it prevents racemization of the amino acid.  [c.363]

Acetophenone similarly gives an oxime, CHjCCgHjlCtNOH, of m.p. 59° owing to its lower m.p. and its greater solubility in most liquids, it is not as suitable as the phenylhydrazone for characterising the ketone. Its chief use is for the preparation of 1-phenyl-ethylamine, CHjCCgHslCHNHj, which can be readily obtained by the reduction of the oxime or by the Leuckart reaction (p. 223), and which can then be resolved by d-tartaric acid and /-malic acid into optically active forms. The optically active amine is frequently used in turn for the resolution of racemic acids.  [c.258]

The most practical route to acyl azides starts with the hydrazinolysis of esters. The usually rather poorly soluble and poorly stable hydrazides are dissolved in mixtures of organic solvents (e.g. THF, DMF, AcOH) and strong acids (e.g. HQ, TFA) and then mixed with an equimolar amount of sodium nitrite or amyl nitrite at -10 °C to yield the azide almost instantaneously. The coupling step with amines at room temperature may require several days. A great advantage of the acyl azide method is the lack of a-racemization (see p. 230f.). The acyl chloride method is quicker and may also be applied for the preparation of esters and amides. Here the free acid is used as starting material, and aptotic solvents (CHCI3, DMF, pyridine) must be applied in the chlorination. Thionyl chloride and oxalyl chloride are the most common agents, and low temperatures are again advantageous. Nitiosation or chlorination of activated CH groups by nitrites or SOClj, resp., are sometimes troublesome side reactions.  [c.143]

The best preventive measure against racemization in critical synthetic steps (e.g. fragment condensation, see p. 239) is to use glycine (which is achiral) or proUne (no azlactone) as the activated carboxylic acid component. The next best choice is an aliphatic monoamino monocarboxylic acid, especially with large alkyl substituents (valine, leucine). Aromatic amino acids (phenylalanine, tyrosine, tryptophan) and those having electronegative substituents in the /3-position (serine, threonine, cysteine) are, on the other hand, most prone to racemization. Reaaion conditions that inhibit azlactone formation and racemization are non-polar solvents, a minimum amount of base, and low temperature. If all precautions are taken, one still has to reckon with an average inversion of 1% per condensation reaction. This means, for example, that a synthetic hectapeptide contains only 0.99 °° x 1(X)% = 37% of the fully correct diastereomer (see p. 233f.).  [c.232]

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976).  [c.237]

The 9 — 15 fragment was prepared by a similar route. Once again Sharpless kinetic resolution method was applied, but in the opposite sense, i.e., at 29% conversion a mixture of the racemic olefin educt with the virtually pure epoxide stereoisomer was obtained. On acid-catalysed epoxide opening and lactonization the stereocentre C-12 was inverted, and the pure dihydroxy lactone was isolated. This was methylated, protected as the acetonide, reduced to the lactol, protected by Wittig olefination and silylation, and finally ozonolysed to give the desired aldehyde.  [c.322]

The amino group of the amino acids 668, protected as allyl carbamates, can be cleaved by Pd-catalyzed treatment with formic acid[135], dimedone (669)[415,426], A-hydroxysuccimide[427], tin hydride[428.429], and silyla-mine[429] under mild conditions. These methods are applied to the protection of amino acids used in peptide synthesis without racemization. p-Nitrocinnamyloxycarbonyl is used as an acid-stable protecting group of amino groups, and is removed by cinnamyl group transfer to. V.A-dimethyl-barbituric acid (DMBA) (670). As an allyl group acceptor, DMBA is. said to be better than dimedone (669)[430], A-allyloxycarbonyl i.s a good protecting group for glucosamine derivatives[43 1].  [c.382]

Another protecting group of amines is 1-isopropylallyloxycarbonyl, which can be deprotected by decarboxylation and a /3-elimination reaction of the (tt-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-1,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437].  [c.384]

See pages that mention the term Racemic acid : [c.339]    [c.385]    [c.157]    [c.329]    [c.625]    [c.264]    [c.264]    [c.333]    [c.450]    [c.450]    [c.29]    [c.46]    [c.388]    [c.93]    [c.258]    [c.161]    [c.168]    [c.318]    [c.320]    [c.327]    [c.299]   
Practical organic chemistry (0) -- [ c.122 ]