Alkali racemization

Excess alkali must be avoided if the malic acid is to be recovered, since it is slowly racemized by heating with concentrated alkali. 

Br. CHa. CHa. CHa. CH(NHa). CH(CHa). CHa. CHjBr HBr. which on treatment with dilute alkali gives di-heliotridane (II). As the latter contains two asymmetric carbon atoms, two diastereoisomeric racemates might be produced in this reaction but only one was formed. It had density and refractive index in general agreement with those recorded for Z-heliotridane, as were also the melting points of characteristic derivatives. Density Df °0-902, refractive index wf, 1-4638 (<. with Adams and Rogers,3i Df ° 0-935, iijf° 1-4641), picrate, m.p. 234-6° (literature 232-6°), picrolonate, m.p. 162-3°, aurichloride, m.p. 200-1° (Konovalova and Orekhov give for these two constants 152-3° and 199-200° respectively). 

Ethynylation of the totally synthetic racemic 18-methyl-17-ketone (63) with acetylene and potassium t-butoxide in t-butanol-toluene or with alkali metal acetylide in liquid ammonia gives a low yield of rac-18-methyl-17a-ethynyl-3-methoxyestra-l,3,5(10)-trien-17/ -ol (64). 

The mold metabolite mycelianamide, isolated from the mycelium of strains of Penicillium gritieofulvum Dierckx, was first investigated by Oxford and Raistrick. The reinterpretation and extension of this work by Birch et and the revision by Bates et of the structure first proposed for the terpenoid side-chain, have led to the formulation of mycelianamide as 11. This structure has been confirmed by further degradations and by a synthesis of racemic deoxymyceli-anamide by Gallina and co-workers. The ready decomposition of the heterocyclic ring by either acid or alkali is discussed later (Section... 

Although the ion pairs of a-substituted benzyl anions and the corresponding cations are chiral species, which, in addition, often bear a pyramidal and hence stereogenic carbon atom, in most cases rapid racemization of the alkali and alkaline earth metal derivatives occurs in solution... 

By extraction of Solanacean drugs, especially Atropa belladonna, Hyoscyamus niger or other species. On careful extraction L-hyoscyamine is obtained first, which can be racemized to atropine by addition of alkali in ethanolic solution. 

Chiral recognition of racemic mandelic acid in the presence of alkali metal cations by [309] ... 

With the larger racemic cr-hydroxy-l-naphthaleneacetic acid too, extraction takes place in the presence of 1309] and the appropriate cations but enantiomeric differentiation is not observed. These results were confirmed in transport experiments in which the alkali mandelate is carried through a liquid membrane of [309] dissolved in chloroform. Lehn et al. (1978) explain these observations in terms of an ion pair included in the cavity of the crown ether. The reversal of chiral recognitions between potassium and cesium mandelate of 25% indicates that the structures of the two complexes are different. 

The resolution was then based on the enzymatic propanolysis of this derivative in dioxane as solvent. Lip Novozyme 435 selectively cleaves the L-form of the oxazolone producing an L-enriched (81-87% ee) 2-acetamido-3-(heteroaryl)propionic acid propyl ester, the dynamic aspect of the process being based on the continual racemization of the residual oxazolone. The propyl group was then removed with alkali and a second selective enzymatic step to remove the acetyl protecting group with Fluka Acylase 1 produced the L-amino acid at better than 99% ee (Scheme 13). 

In asymmetric hydrogenation of olefins, the overwhelming majority of the papers and patents deal with hydrogenation of enamides or other appropriately substituted prochiral olefins. The reason is very simple hydrogenation of olefins with no coordination ability other than provided by the C=C double bond, usually gives racemic products. This is a common observation both in non-aqueous and aqueous systems. The most frequently used substrates are shown in Scheme 3.6. These are the same compounds which are used for similar studies in organic solvents salts and esters of Z-a-acetamido-cinnamic, a-acetamidoacrylic and itaconic (methylenesuccinic) acids, and related prochiral substrates. The free acids and the methyl esters usually show appreciable solubility in water only at higher temperatures, while in most cases the alkali metal salts are well soluble. 

C12H22O2, Mr 198.30, Z PlOl.SkPa 240 °C, d 0.8901, Wp 1.4515, occurs in many essential oils either as one of its optical isomers or as the racemate. The odor of racemic citronellyl acetate differs little from that of the optical isomers. ( )-Citronellyl acetate is a liquid with a fresh-fruity rose odor. It is often used as a fragrance, for example, for rose, lavender, and geranium notes as well as for eau de cologne with citrus nuances. Since it is relatively stable to alkali, it can be used in soaps and detergents. Citrus flavors acquire speciflc character through the addition of citronellyl acetate it is also used to round off other fruit flavors. 

The corresponding ammonium salts are obtained from the quinine salts by treating them with ammonia in an alcoholic chloroform solution. They are viscous substances with rotation of an 425. On mixing equivalent quantities of the two active salts in solution a crystalline racemate is produced. Alkali or alkali carbonates decompose the salts, so that the sodium or potassium salt cannot be produced by decomposing the quinine salts with these reagents. 

O-Acetyl-P-methylcholine chloride [Methacholine chloride, Amechol, Provocholine, 2-acetoxypropyl-ammonium chloride] [62-51-1 ]M. 195.7, m 170-173°, 172-173°. It forms white hygroscopic needles from Et2O and is soluble in H2O, EtOH and CHCI3. It decomposes readily in alkalies and slowly in H2O. It should be handled and stored in a dry atmosphere. The bromide is less hygroscopic and the picrate has m 129.5-131° (from EtOH). [racemate Annis and Ely BJ 53 34 1953 IR of iodide Hansen Acta Chem Scand 13 155 7959]. 

Occurreace. IR-R. R (-Tartaric acid occurs in the juice of the grape and in a few other fruits and plants. It is not as widely distributed as citric acid or St- (-malic acid. The only commercial source is Ihe residues from the wine industry. The racemic acid is nut a primary product of plant processes but is formed readily from the dextrorotatory acid by heating alone or with strong alkali or strong acid. triewi-Tanaric acid is not found in nauire It is obtained from the other isomers hy prolonged boiling with caustic alkali. 

In this type of problem, one should work backward from the structures of the final products, analyzing each reaction for the structural information it gives. The key questions to be inferred in the preceding problem are (a) What kind of chiral compound or compounds could give 2-methyl-2-butanol and a chiral alcohol with nitrous acid (b) What kinds of compounds could give 8 on reduction (c) What does the solubility behavior of A indicate about the type of compound that it is (d) Why does chiral A racemize when dissolved in alkali ... 

The enantiomeric separation of the D- from the L-stereoisomers of amino acids is an area of growing interest. It is generally recognized that heat- and alkali-treatment of proteins can result in the racemization of L-isomers of amino acid residues to the D-analogs. Almost without exception, humans cannot utilize the D-isomers of amino acids, and some are thought to be toxic (although... 

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

M Friedman, R Liardon. Racemization kinetics of amino acid residues in alkali-treated soybean proteins. J Agric Food Chem 33 666-672, 1985. 

An improved procedure for condensation of dithioesters with a-amino groups of amino acids has been used to prepare several peptide thioamidesJ8 The N-protected amino acid dithioesters condense with alkali salts of a-amino acids, catalyzed by alkali fluoride in EtOH, to give the endothiopeptides within 60 minutes. Under these conditions, the thioamides are obtained without racemization. 

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