Separation, Isolation, Chromatography

The development and adaptation of procedures for the separation, isolation, purification, identification, and analysis of the components of the pyrethrum mixture have been studied and evaluated. Results of studies to determine the molar extinction coefficient of pyrethrin I as well as a gas chromatographic procedure for the determination of pyrethrins are reported. The use of chromatographic separation procedures (including partition, adsorption, gas, and thin-layer chromatography), colorimetry, and infrared spectrophotometry are discussed. 

Interestingly, both invertomers of the obtained M-chloroaziridines 16 were clearly observable in the H-NMR spectrum and they could even be separated by chromatography. The dehydrochlorination was investigated with a variety of bases however, the resulting yields were disappointingly low. Only for R = Ph, a yield of 39% of azirine 17 was obtained using DBU as the base, in all other cases the yields were lower [22]. Davis et al. [23] successfully applied the -elimination of the sulfinyl group in chiral non-racemic N-sulfinylaziridines (Scheme 9), whereby the eliminated sulfenate was trapped by an excess of methyl iodide, which facilitated the isolation of the desired product (18). 

The concentration of neomycin in urine has been determined with a fluorimetric method 15. Interfering amines were separated by chromatography on SE Sephadex C-25 before reacting the isolated neomycin with fluorescamine. 

Thin-layer chromatography (TLC) is one of the most popular and widely used separation techniques because it is easy to use and offers adequate sensitivity and speed of separations. Furthermore, multiple samples can be run simultaneously. It can be used for separation, isolation, identification, and quantification of components in a sample. The equipment used for performance of TLC, including applications of TLC in drug discovery process, is covered in Chapter 13. This technique has been successfully used in biochemical, environmental, food, pharmacological, and toxicological analyses. 

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. ... 

The carbanion was made with lithium diethylamide in THF cooled to ice bath temperature. The epoxide opening was conducted in refluxing dimethoxyethane giving a diastereomeric mixture of y-hydroxy sulfoxides (1.5-2 1 d.r.) in 63% yield. Each diastereomer was separated by chromatography followed by crystallization from methanol/water. The pure diastereomers were each isolated in 13% yield,... 

Many of the colors associated with higher plants (green leaves in the spring and summer, yellow or red leaves in the fall, the orange color of carrots, some colors in flower petals) are due to the presence of pigment molecules such as chlorophylls and carotenoids. In this experiment a mixture of these pigments will be isolated by solvent extraction of plant tissue, separated by chromatography, and the components identified by visible spectrophotometry. 

The addition of an acetylenic ester to the anion derived from the pyrrolidine-2,4-dione 96 has been used by Lowe, Ridley, and Yeung115-118 in an elegant synthesis of /3-lactams related to the cephalosporins. Dibenzyl acetylenedicarboxylate gave a mixture of the fumarate (major product) and maleate (97), which could be separated by chromatography. Photolysis of the mixture in the presence of /-butylcarbazate gave the lactam 98, which was isolated only as the E-isomer. 

Four of the six possible isomers of [Co(acac)(iV-methyl-S-alaninato)2] have been separated by chromatography, and identified as AtransN, AtransN, AcisN, and Acis-N—Ct, the AcisN—C2 and AcisN—Cx were not isolated.379 In the [Co(acac)-(S-aminoacidate)2] systems, six isomers have been separated for S-alanine, and five for S-valine,380 the AcisN—C2 being obtained in very small yield for S-alanine and being absent in the S-valine system, probably due to steric effects. 

This represents the isolated yield of both isomers, either as a mixture or separated, following chromatography. 

Compound 58a is able to react with furan (unpublished results) to yield a mixture of three adducts easily separated by chromatography, in the isolated yields indicated in Scheme 33 (13-23% of the starting dienophile is always recovered). Different conditions have been used, but the selectivity is very little dependent upon them. Furan must be used as solvent (2 days, room temperature), because in other cases (8 equiv.) the reaction does not work unless high pressures (4 kbar, 6 days) are used. The addition of ZnBr2 or Me2AlCl as catalysts reduces the reaction time (1 day, furan as solvent at room temperature). The stereochemistry of the major adduct endo-61a (51% isolated yield) was unequivo-... 

As mentioned earlier, the primary adducts 198 are isolable with good yields in exceptional cases only. To determine the stereoselectivity of the hydroxyalkylation step, the enolate of the more stable /m-butyldimethylsiloxy derivative 211 has been combined with acetone. The two diastereomeric adducts 212 and 213 could be isolated and separated by chromatography 96 b). Interestingly the product 213 formed by the contrasterical approach of the electrophile predominates although the effect is much less pronounced than with alkyl halides (see above). 

Insulin was reacted with performic acid to cleave the disulfide bridges. Two polypeptides were isolated from this reaction, polypeptide A with 21 amino acids and polypeptide B with 30 amino acids. Polypeptide B was reacted with Sanger s reagent and then partially hydrolyzed. This produced a very complex mixture containing individual amino acids, dipeptides, tripeptides, and so on. All of the peptides that had the dinitro-phenyl group on their N-terminal amino acid were then isolated from this mixture. (This was relatively easy to accomplish because these labeled peptides have very different solubility properties from the unlabeled polypeptides in acidic solution.) The labeled peptides were separated by chromatography, and each component was hydrolyzed. The following results were obtained ... 

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