Chromatography, separation diastereoisomers

X-Ray studies on Na[(-)5 g-Co(ox)(aa)2],2H20 [aa = trans-iV-methyl-(S)-alaninate] confirm the RS configuration of the aa ligands and show the absolute configuration of the anion to be A. Fractional crystallization and column chromatography on D-lactose has been used to separate diastereoisomers of [Co(acac)2(L-Phe)] and [Co(acac)2(L-Val)]. The method was, however, unsuccessful for the corresponding L-Ala complex. ... 

Enantiomers cannot be separated by the normal processes of purification crystallisation, distillation or chromatography. But diastereoisomers can. Resolution involves using an enantiomerically... 

S -Oxide [94923-10-1]. l-Methylene-2- phenylsulfinyl)cyclopropane. l-Methylidene-2-(phenylsulfinyl)cyclopropane, [ Methylenecyclopropy[)sulfinyl benzene 9CI CioHjoOS M 178.254 Oil. Separable by careful chromatography into diastereoisomers. 

Optical enantiomers of the methyl esters of monohydroxy fatty acids can be resolved by gas chromatography if they are first converted to the (-)-menthyl, D-phenylpropionyl or related derivatives [348,468] (reviewed briefly elsewhere [456,858]). Particularly good results were obtained with racemic methyl 2-hydroxy palmitate and stearate, after conversion to the (-)-a-methoxy-a-trifluoromethyl-phenylacetate derivatives, on a WCOT column coated with OV-1 as shown in Figure 5.17 [93] it is probable that the D-form elutes first. Some limited success has also been achieved in separating diastereoisomers of polyhydroxy fatty acids [988],... 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

The 3,5-diphenylmorpholine-2-one 161 was used to prepare bicyclic lactams 162 by reaction with the cesium fluoride/tetramethoxysilane system and different Michael acceptors. The adducts were rapidly obtained as a mixture of diastereoisomers that could be separated by column chromatography just after TFA-mediated cyclization to the 8-substituted-4,8a-diphenyltetrahydro-l//-pyrrolo[2,l-r1[l,4]oxazine-l,6(7//)-diones 62 and 163 <1997SL935>. These compounds were then reduced to give substituted prolines as described in Section 11.11.6.1. 

The proton magnetic resonance spectrum of a C8De solution of the crude product exhibits benzylic CH doublets at <5 3.58 (J = 9.4 Hz., attributable to 90-96% of the threo aldol isomer) and at 8 3.42 (J = 5.3 Hz., attributable to 4-10% of the erthro aldol isomer) downfield from internal tetramethylsilane. This mixture may be separated by chromatography on acid-washed silicic acid to permit the isolation of both the threo and the erythro diastereoisomers. ... 

Figure 3.12 FLEC reacts with alcohols and amines without any racemization of the original sample to produce highly fluorescent diastereoisomers which can be separated by reverse phase chromatography.

A sample of this material obtained in a separate experiment was purified by Chromatography (silica, dichloromethane - diethyl ether 85 5) to give the pure thiazolidine as 1 1 mixture of diastereoisomers oil 1H NMR (80 MHz, CDCI3, TMS) 8 ... 

Although the separation of the diastereomeric alcohols 55 or 64 was not possible by flash chromatography we succeeded in a separation by preparative HPLC. The enantiomeric excess of the individual diastereoisomers was determined after saponification to the diol 54 or 63 by chiral GC. It turned out that the enantiomeric excess of both acetates was only 7% ee. The value was so low that we did not make an effort to continue with only marginally enantiomerically enriched material after HPLC separation. 

Products containing two adjacent stereogenic centers were obtained as mixtures of diastereoisomers that could be separated by column chromatography (dr values. Scheme 6.12). The N,N -dimethyl acetaldehyde hydrazone appeared remarkably... 

The commercial drug is in fact administered as a mixture of all four isomers and the BP monograph for labetalol checks the ratio of the two chromatographic peaks produced by the two enantiomeric pairs of diastereoisomers. In order to separate the enantiomeric pairs a chiral chromatography column would be required and separation on a chiral column produces four peaks (Ch. 12). 

In recent work, a homochiral substituent has been incorporated into the reactant to allow the separation of enantiomerically pure products. Thus, the homochiral reactant 288, prepared from (5)-1-phenylethylamine, gave a pair of diastereoisomers (289) and (290) that were separated by chromatography and identified via X-ray crystallography (178). The nitrile imine was generated by the hydrazonyl chloride-base route. The reaction showed only modest stereoselectivity that favored 289 when silver carbonate was used as the base but it was found that this was reversed when triethylamine was used. However, this was not the case for a related reaction (179). 

The intramolecular version of this heterocyclization was applied to a short synthesis of diazasteroids (Scheme 12). Thus, imines 46 were converted in one-pot into [ 1,2-a]pyrrolopyrimidines 47 when treated with LDA, nitrile, and acid (40-95% yield) the stereoselective reduction of 47 to octahydroderivatives 48 was effected in 70-92% yield with NaBH4/ MeOH at 60°C (89S230 91MI5). Compound 48 (R1 = R2 = Ph) was successively allylated to 49 (94% yield) and cyclized in the presence of triflu-oromethanesulfonic acid to afford a 86 14 mixture of diastereoisomers (C-5 epimers) in 86% yield, from which major component 50 was separated by column chromatography [90TL(31 )2189]. 

Attempts were made in order to obtain adducts 198 in enantiomeric form by cyclo-addition of 1-alkoxy-l,3-butadienes to optically active esters of glyoxylic acid96 the enantiomeric purities of the adducts were, however, poor.96 Optically active butyl 2-alkoxy-5,6-dihydro-2H-pyran-6-carboxylates (198, R1 = Bu) were obtained when the R group in 197 was a carbohydrate moiety. The diastereoisomers resulting from cyclo-addition were separated by chromatography (see Section VII and Ref. 350). 

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