Chirality center detection

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Detecting chirality centers in a complex molecule takes practice because it s not always immediately apparent that four different groups are bonded to a given carbon. The differences don t necessarily appear right next to the chirality center. For example, 5-bromodecane is a chiral molecule because four different groups are bonded to C5, the chirality center (marked with an asterisk). A butyl substituent is similar to a pentyl substituent but it isn t identical. The difference isn t apparent until four carbon atoms away from the chirality center, but there s still a difference. 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

Enantiomers of the same compound may have differing threshold values. This property was observed for a new, strong and high impact, flavor compound, 3-thio-2-methylpentan-l-ol, first detected in thermally processed materials but that is actually present in raw onions.54 First of all, there is a marked concentration effect at 1 ppm in 5% saltwater, the odor is described as sulfuric, burnt gum, sweaty, onion and at 0.5 ppb as meat broth, sweaty, onion, leek . Since there are two chiral centers (both dependent on carbon) there are four stereoisomers, all of which have been prepared 18a, 18b, 19a, 19b (Scheme 7). For this compound, enantiomers have the following marked differences in odor thresholds (but apparently all have the same general odor). 

Muscarine is a tetrahydrofuran derivative with the structure shown in Figure 3.7a. Because of the three chirality centers present in the molecule, muscarine exists in eight isomers, of which only one, L-(- -)-muscarine, is active. The remaining isomers also have been detected in toxic fungi, but because of their low biological activity and low concentration they do not contribute to toxicity. 

The extracts of three species of male North American decorator wasps, Eucerceris rubripes, E. conata and E. tricolor, were analyzed to reveal the presence of one major component in large quantities. This component detected in the head extract of males was identified as (Z )-3-hexenyl-3-hydroxybutanoate. The structure was confirmed by synthesis and the absolute configuration of the chiral center was determined to be R for the three species. In addition, 2- and 3-hexenoic acid and a few other aromatic compounds were also detected in varying quantities in males and females, along with hydrocarbons and fatty acids. 

On the other hand, azomethine (113) was synthesized by condensation of (S)-valinol with phenylacetaldehyde in good yield. The reaction of this chiral azomethine (113) with aryllithium afforded again chiral ot-substituted phenethylamines (114). The relationship between (112) and (114) was diastereomeric, due to the different configurations at the newly created chiral center at the 1-position. The diastereo-selectivity is more than 98 %, because the other diastereomer was not detectable in any case 125>. 

C. Detection of intermediates from retention or inversion of configuration at chiral centers... 

The corresponding Wittig reagent, CHj PPhj, reacts smoothly with both aldehydes and ketones to give methylenated products In high yield but with one subtle limitation. The problem cannot be detected with aldehydes because they react rapidly even at temperatures as low as -78°C, but ketones react more slowly, and an adjacent enolizable chiral center can be epimerized as a result of competitive reversible enolization. This limitation of the Wittig... 

The molecular structure of a compound is very important. For example, one can usually deduce from the structure whether or not the compound will absorb UV radiation and be detectable with a UV detector. The molecular structure also reveals if the compound has ionizable functional groups and will require a mobile phase modifier if HPLC analysis is used. Examination of the molecular structure may also tell something about the chemical reactivity of the molecule. The molecular structure indicates whether the molecule contains any chiral centers. If the molecule is chiral and non-racemic, then an assay to determine chiral stability may be required. 

Examination of the structure of the example compound clearly indicates that it possesses both a phenyl and an indole moiety and should therefore be amenable to UV detection. The compound has a tertiary amine, which is an ionizable functional group with a pAa of approximately 9, therefore, a buffered mobile phase will likely be required if HPLC is chosen as the analytical method. Since the compound does not contain a chiral center, there is... 

Selectivity enhancement is in general a result of any of a number of circumstances, for example (i) not all of the sub-bands in either the absorption or fluorescence spectrum of the derivative are necessarily CD-active, (ii) the chromophore must be located adjacent to an intact chiral center or the functional group is not CD-active, so any molecule, other than the analyte, that contains this group but fails to meet the requirement, is eliminated as a potential CD detectable interference (although it may still absorb or fluoresce strongly), (iii) if a chiral center in the molecule of a potential interfering compound is lost in the derivatization process that molecule will also be removed from the list of possible interferences. 

Fig. 12. The four diastereoisomeric 1 1 adducts which can be formed on reaction of the bisdiene 37 with the syn-bisdienophile 38. The stereoelectronic control in operation during the reaction is such that only the syn/endo-H isomer 40 has been detected [21, 38, 110, 112]. The remote stereochemical descriptors, syn and anti, refer to the relative configurations of the endoxide bridges across each newly-formed cyclohexene ring. The close stereochemical descriptors, exo-H and endo-H, refer to the relative configuration adopted by the hydrogen atoms at the ring junctions associated with the newly created chiral centers...

Several absolute asymmetric Norrish type II cyclizations have been reported in the solid state. Achiral a-(3-methyladamantyl)-/ -chloroacetophenone 114 formed a chiral crystal in space group P2i2i2i. Irradiation of these crystals caused Norrish/Yang cyclization to afford the cyclobutenol-type photoproduct 115 (six chiral centers) with respectable enantiomeric excess at low conversion (Scheme 27) [101]. Photolysis in solution phase led to mixtures of four of the six possible cyclobutanols, and no trace of optical activity could be detected in the mixtures. 

The overall changes in the chemical shift of the methine protons can be directly correlated with the amounts of ions if ionization is significant (>1%). However, even minute amounts of intermediate cations can be detected in some systems by dynamic NMR. Because ionization leads to the formation of a planar carbenium ion, the chirality at the carbon atom is lost. In the particular case of the isobutyl vinyl ether derivatives, the isobutoxy group has a built-in probe (CH2-CHMe2) separated from the chiral center by oxygen atom. The methylene protons on the ether group are magnetically nonequivalent due to the presence of four different substituents at the electrophilic carbon center. They become equivalent only... 

Mixtures of isomers of (MeS)(Me2S)Bi2H1 " were alkylated by Soloway et al. using alkylhalides and tosylates. We obtained and characterized several products starting from isomerically pure anions (6-7) and (10-12). Monoalkylation yields a racemic product. Since two chiral centers appear upon alkylation of (10-12), a mixture of meso- and (d,l)-diastereomers is obtained. There is no evidence for diastereomers in the H NMR spectra of 1,7- and 1,12-derivatives, but in the case of l,2-(HCCCH2SMe)2B,2H10 two diasteromers are detected probably, because of the proximity of the chiral centers. 

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