Resolving agents neutral

This procedure is restricted mainly to aminodicarboxyhc acids or diaminocarboxyhc acids. In the case of neutral amino acids, the amino group or carboxyl group must be protected, eg, by A/-acylation, esterification, or amidation. This protection of the racemic amino acid and deprotection of the separated enantiomers add stages to the overall process. Furthermore, this procedure requires a stoichiometric quantity of the resolving agent, which is then difficult to recover efficiendy. Practical examples of resolution by this method have been pubUshed (50,51). 

In practical point of view, use of acidic ((DBTA)2Ca) or neutral (DBTACa) calcium salts of DBTA seems to be the most advantageous choice because of the easy preparation of the resolving agents, low price and toxicity, good compatibility of the calcium salts with different ligands and usually good crystallisation abilities of the coordinative complexes formed. 

A variety of unsymmetrical optically active chelate ligands LL have been used as optically active resolving agents in square-pyramidal complexes of the type C5HsM(CO)2LL (M = Mo, W) giving partly cationic species (77-81) and partly neutral complexes (79, 82-91). The field of optically active square-pyramidal complexes has been reviewed (11, 92). All of the known examples are summarized in Table I. 

Pure d-menthyl-Z-menthoxyacetate is hydrolyzed by refluxing for an hour with 1.2 mole equivalents of 5-10% ethanolic potassium hydroxide. (More vigorous hydrolysis is inadvisable since 1-menthol may thus be formed from the resolving agent by rupture of the ether linkage.) The mixture is almost neutralized to phenolphthalein with hydrochloric acid and is distilled with steam to remove first the ethanol and then the menthol. The menthol is conveniently extracted with benzene and purified by distillation under reduced pressure. It has b.p. 97-99°/10 mm., 215-2167760 mm. m.p. 42° and [aft0 + 49.5°(c = 2, 95% ethanol). The yield in this step is 80-85%. 

Since l-amino-2-cyclohexene is basic, a chiral acid is needed. When (+)-mandelic acid is the resolving agent used, a mixture of diastereomeric salts is formed as shown. These diastereomers have different physical properties and can therefore be separated (often by recrystaUization). Treatment with base neutralizes the amine and allows it to be separated from the charged mandelate salt (typically by extraction), to give the neutral amines as pure enantiomers. 

The dramatic influence of the pH of the medium was first encountered at the separation of optical isomers of a s-permethrinic add carried out with half an equivalent amount of (S)-2-benzylaminobutanol ((S)-BAB). The resolution started in the presence of a 25 mol % excess of sodium hydroxide. In such a medium the diastereoisomeric salt containing the almost pure (S)-CPA crystallized ee %%) with a yield of 27% counted to the amount of racemic acid. After removal of the precipitate by filtration, the excess alkali was neutralized with a counted amount of hydrochloric add. That time the resolving agent remained in the filtrate crystallized with (R)-CPA enantiomer as diastereoisomeric salt and in the second mother liquor an almost racemic CPA-Na salt was found (it can be recycled into the resolution). 

When chromatographic resolution of species based on modifications located at the protein surface is desired, it may be advisable to use conditions that favor retention of native conformation.17 Here, the standard acidic conditions described in the preceding text may be inappropriate, and mobile phases buffered near neutrality may be required. Buffers based on ammonium acetate, ammonium bicarbonate, and triethylammonium phosphate may prove more useful in resolving polypeptide variants with differing posttranslational modifications, amino acid substitutions, or oxidation and deamidation products. The addition of more hydro-phobic ion-pairing agents may be needed to obtain polypeptide retention, and a variety of alkyl sulfonates and alkyl amines have been described for specific applications.17... 

Fatty acids can be derivatized with p-bromophenacyl (73, 85, 86], phenacyl [87-89], naphthacyl [90, 91] or p-nitrophenacyl [74] bromide. To prepare p-bromophenacyl esters [73], fatty acids (0.001—0.5 mM) were dissolved in methanol or water and neutralized to a phenolphthalein end-point by methanolic KOH. The solvent was removed by either a rotary evaporator or lyophilization. A 3—10-fold molar excess of alkylating agent, p-bromophenacyl bromide/18-crown-6 (20 1) in acetonitrile, was then added. The mixture was stirred continuously in a sealed Reacti-Vial at 80 °C for 15 min. After cooling, the solution containing the derivatives was directly subjected to RP-HPLC with UV detection at 260 nm. The mobile phase was usually acetonitrile/ water [87, 92, 93], methanol/water [73, 88, 94, 95] or acetonitrile/methanol/water (86, 89], The separation of phenacyl derivatives of palmitoleic (Ci, ) and arachi-donic ( 20 4) acids was not achieved with the acetonitrile/ water system [87, 89], and elution with methanol/water could not resolve linolenic (Cig.3) and myristic (C]4.q) acids [88, 89]. A ternary mobile phase [89] containing a mixture of acetonitrile, methanol and water seemed to be best for the separation of phenacyl derivatives of fatty acids. 

Commercially available silica gel plates coated with acid or basic chiral selectors [o-galacturonic acid, l-(- -)-tartaric acid, L-lactic acid, (-)-brucine] were used for the separation of racemic ephedrine, atropine, neutral amino acids, and their 3-phenyl-2-thiohydantoins (PTH) derivatives. The use of amino acids as chiral selectors involved further possibilities of enantiomer separation owing to the simultaneous presence of basic and acidic groups. In fact, L-aspartic acid, L-lysine, L-histidine, L-arginine, and L-ser-ine resolved racemic alkaloids, (3-blockers, profens, some amino acids, and their Dns derivatives. Macrocyclic antibiotics [i.e., (-)-erythromycin and (-)-vancomycin] were also used as chiral agents for the separation of enantiomeric DNs amino acids. The mechanisms of chiral recognition was investigated by Aboul-Enein, El-Awady, and Heard they hypothesized that the formation of... 

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