Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Diastereoisomer determination

LC-MS or LC-MS-MS using ESI or APCI are versatile tools for the isomer-specific determination of trace levels of HBCDs, monitoring the specific transitions mlz 640.6 to mlz 78.9 and 80.9. Budakowsky and Tomy [117] showed that APCI has lower intensities than in a similar experiment with ESI. Consequently, the ESI mode was preferred for determining diastereoisomers in several studies. Different methods for the analysis of diastereoisomeric HBCD using LC-ESI-MS-MS and SRM were developed, obtaining LODs of 0.5-6 pg on-column [117, 118]. [Pg.56]

From a variety of differently substituted compounds, best results were obtained with the catalysts 195a-c in combination with /-methyl diazoacetate and monoolefins, cyclopropanes were obtained with a relatively high trans/cis ratio and enantiomeric excesses of 44-89% (Table 12). The absolute configuration at the catalyst s chiral center determines the enantioselectivity for both diastereoisomers. [Pg.161]

NU(C) base atoms (5) The stereoselectivity of the BPDEs during intercalative covalent binding in kinked DNA and (6) The possible reorientation of the complex to yield an externally bound adduct. The energetics for each of these processes will be presented to identify the important steps that influence the binding of specific isomers. It will be shown that the orientation of each diastereoisomer of BPDE about specific base atoms in kinked receptor sites in the duplex DNA during covalent bond formation is the determining factor in stereoselectivity. [Pg.255]

Based on the results in this paper, the following experimental data should be obtained for each of the diastereoisomers. (l) The relative yields of trans and cis addition products should be determined for adduct formation to each base atom. (2) Alternating and non-alternating homopolymers should be used to evaluate the base sequence specificity. (3) Binding to sites IQ and IIX should be correlated to trans and cis adducts and to stereoselectivity. [Pg.287]

In theory, the two diastereomeric complexes will have different association constants. The evaluation of any chiral discrimination will depend upon measurement of the different proportions of the diastereoisomers formed. For example, nmr experiments have been successful in determining the degree of complex formation by each enantiomer. Alternatively, an extraction procedure has been employed this involves the interaction... [Pg.142]

The enantiomeric purity of protected amino acids used in peptide synthesis can be determined by pre-column partial deprotection followed by derivatization with Marfey s reagent (116). The Marfey diastereoisomers can be easily resolved and determined by RP-HPLC using an ODS-Hypersil column288. Fifteen amino acids collected from mammalian tissues were derivatized with Marfey s reagent and subjected to two-dimensional TLC. Each individual spot (enantiomeric mixture of a diasteroisomer) was then resolved by RP-HPLC. Except for tyrosine (46) and histidine (117), subnanomole quantities of enantiomers could be analyzed289,290. [Pg.1089]

Ermer, for example, utilized LCQ to monitor impurity profiles of various batches of ramorelix used in toxicological studies, clinical stndies and scale-up. Ramorelix is a synthetic glycosylated decapeptide with monoisotopic of 1530.7. The toxicological batch served as the benchmark against which all other batches were compared. Molecnlar weights of impurities were determined by ESI mass spectrometry, and nsed in conjunction with UV peak area % to gauge impurities in batches nsed in clinical trials. These impurity profiles were compared to those of batches used in the toxicologically qualified batch. Eour impurities were detected with the same value. They were believed to be diastereoisomers of ramorelix, i.e., a peptide sequence with one of the amino acids in the opposite enantiomeric form. [Pg.544]

Driffield M, Harmer N, Bradley E, Fernandes AR, Rose M, Mortimer D, Dicks P (2008) Determination of brominated flame retardants in food by LC-MS/MS diastereoisomer-specific hexabromocyclododecane and tetrabromobisphenol A. Food Addit Contamin... [Pg.289]

In order to determine whether the partial suppression of the free-rotor effect was required for the success of the ODPM rearrangement process, the study was extended to the aldehyde 33 [51], When 33 is irradiated (15 min), under similar conditions to those used for 29, the cyclopropyl aldehyde 34, resulting from an ODPM rearrangement, was obtained, as the tran -diastereoisomer, in 90% isolated yield. This result demonstrated clearly that the ODPM reactivity of (3,y-unsaturated aldehydes is not restricted to cyclic compounds, such as 29, but can also be extended to acyclic derivatives. Therefore, the suppression of the free-rotor effect is not essential for the success of the rearrangement and the reaction is probably controlled by both the excitation of the molecule to the TiCtt, -it )... [Pg.13]

Upon hydrogenation of 24 a 1,2-rearrangement of the epoxide occurred generating aldehyde 25 as a mixture of diastereoisomers. After reaction with methyl lithium, the diastereomeric alcohols 26 and 27 were separated and isolated in yields of 23% and 71%. While alcohol 26 as the minor diastereo-isomer could be oxidized with pyridinium dichromate (PDC) and methyle-nated to give the enantiomer of kelsoene (cnM), its diastereoisomer 27 with the inverse configuration at C-7 required a supplementary epimerization step with sodium methanolate. The enantiomerically pure ent- allowed for the determination of the absolute configuration of natural kelsoene (1) [9, 10]. The previously reported assignment based on NMR-correlation experiments [5] was corrected. [Pg.9]

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. [Pg.20]


See other pages where Diastereoisomer determination is mentioned: [Pg.84]    [Pg.84]    [Pg.211]    [Pg.211]    [Pg.563]    [Pg.186]    [Pg.187]    [Pg.389]    [Pg.390]    [Pg.53]    [Pg.563]    [Pg.245]    [Pg.12]    [Pg.131]    [Pg.25]    [Pg.128]    [Pg.128]    [Pg.248]    [Pg.255]    [Pg.26]    [Pg.296]    [Pg.203]    [Pg.122]    [Pg.123]    [Pg.237]    [Pg.61]    [Pg.823]    [Pg.453]    [Pg.454]    [Pg.283]    [Pg.293]    [Pg.121]    [Pg.66]    [Pg.334]    [Pg.219]    [Pg.540]    [Pg.19]    [Pg.237]    [Pg.71]   
See also in sourсe #XX -- [ Pg.237 ]




SEARCH



Diastereoisomers

© 2024 chempedia.info