Enantiomeric pair

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

FIGURE 7.4 D-Fructose and L-fructose, an enantiomeric pair. Note that changing the configuration only at C5 would change D-fructose to L-sorbose. 

In this context, the enantiomeric pair containing the eutomer of cyclothiazide can be resolved by HPLC on cellulose-derived coated CSPs. Nevertheless, the poor solubility of this compound in solvents compatible with this type of support makes this separation difficult at preparative scale. This operation was achieved with a cellulose carbamate fixed on allylsilica gel using a mixture of toluene/acetone as a mobile phase [59]. 

Let us assume that a given compound has a purity of 98 % ee, and that this compound is reacted with a derivatizing agent which has also a purity of 98 % ee. The two major compounds plus the minor impurities in the compound to be analyzed and the derivatizing agent will create a set of four diastereomers. Two pairs of diastereomers (-i-)-A(-i-)B and (-)-A(-)-B as well as (- )-A(-i-)-B and (-i-)-A(-)-B are enantiomeric pairs, and thus elute together on an achiral column. Therefore, a peak area of 98.011 % will be detected for (-i-)-A(-i-)-B, which leads to a purity of 96.03 % ee for (-i-)-A. This is a quite significant deviation from the true value for (-i-)-A. 

Reaction of racemic a-alkyl-a-(l H-1,2,4-triazol-l-yl)acetophenones with Grignard reagents in boiling diethyl ether affords exclusively the (RS/S -enantiomeric pairs. On the other hand, reaction of racemic a-alkoxy-x-(17/-l,2,4-triazol-l-yl)acetophenonc with Grignard reagents leads, under chelation control, to the (R/ /5S)-enantiomeric pair82. 

Many anaesthetics exist as enantiomeric pairs which when separated show selectivity in terms of anaesthetic potency. Studies on the action of anaesthetics at GABAa receptors have mimicked this selectivity, again... 

The reactions of trans- and m-Co(en)2Cl7 (recall that the cis isomer can be resolved into an enantiomeric pair) can be considered.18 The question is whether a common intermediate prevails along the pathway for the reaction in which a single Cl is lost ... 

Which of the following complexes are chiral, and which form enantiomeric pairs ... 

The varieties of isomerism are summarized in Fig. 16.18. Enantiomeric pairs of optical isomers rotate the plane of polarization of light in opposite directions. 

Enantiomers have identical chemical properties, except when they react with other chiral compounds. Because many biochemical substances are chiral, one consequence of this difference in reactivity is that enantiomers may have different odors and pharmacological activities. The molecule has to fit into a cavity, or slot, of a certain shape, either in an odor receptor in the nose or in an enzyme. Only one member of the enantiomeric pair may be able to fit. 

If 2,6-pyridinedimethanol is condensed with arylboronic acids in non-polar solvents, the tetrameric boron complexes 74 and 75 are formed rapidly (within 15-30 min) in yields of 80 and 93% (Fig. 20). In both cases only the RSRSI SRSR enantiomeric pair with approximate S4-symmetry is obtained, so that the reaction is diastereoselective. 

Quirion, R. Rice, K.C. Skolnick P. Paul, S. and Pert, C.B. Stereospecific displacement of 3H phencyclidine (PCP) receptor binding by an enantiomeric pair of analogs. Eur J. 

Still another way to characterize metal surface sites by a chemical reaction is with the unique molecules (+)— and (—)—apopinene (Fig. 1.5).25-28 The apopinenes are an enantiomeric pair of molecules with a double bond steri-cally hindered on one side by a gem-dimethyl group. During hydrogenation, each enantiomer may hydrogenate to the saturated symmetrical apopinane or isomerize to its enantiomer, which will have the same reactivity on a symmetrical surface (Scheme 1.1). 

So much for one chiral centre. The problems really begin when you come up against molecules which have two or more chiral centres With two chiral centres, we can construct four possible stereoisomers. These can be separated into two enantiomeric pairs (indistinguishable by NMR). But, (key sentence coming up) if we compare one member of each of these enantiomeric pairs, we will find that they may be distinguished from each other by NMR, because they are diastereoisomers. Diastereoisomers are stereoisomers which are not mirror images of each other - they are different compounds with distinct physical and chemical properties. See Figure 6.1 if this isn t clear. 

For the enantiomeric pair, (2R, 3R) 19a and (2,S , 35) 19b, the odor thresholds are very much higher (> 12 to > 30 pg L 1 in H20, not determined in air). These two materials both were contaminated with small amounts of the highly potent (2R, 35) enantiomer so these values are not too reliable. The (2R, 35) enantiomer is one of the most powerful flavor components to be discovered. 

Differential. thermal analysis can also be used to construct binary phase diagrams on the basis of observed melting points. This information is of importance since the nature of the phase diagram as would exist for an enantiomeric pair can be instrumental in choosing a resolution strategy [23,24]. When a drug candidate contains one or more chiral centers, it is frequently... 

The stereochemical outcome can be rationalized by the mechanism illustrated in Scheme 14.22. The formation of an enantiomeric pair of allylpalladium complexes (Sp)/(RP)-99 offers two possibilities for the attack of the nucleophile in the subsequent addition leading to the formation of the stereoisomers (R)- and (S)-101. It should be mentioned that the structure of intermediate 102, prepared from a-allenic phosphate 91, could be proved by both NMR spectroscopy and single-crystal X-ray analysis and therefore serves as evidence for the formation of intermediate 100 (Scheme 14.22 and Eq. 14.12) [49]. 

While cyclopentanes have also been produced by antibody-catalysed cyclization (Appendix entry 15.2) (Li et al., 1996), much the most striking example of cationic cyclization by antibodies is the formation of the decalins [79], [80] and [81] (Fig. 29). The rra/w-decalin epoxide [82] (tu2 100 h at 37°C) was employed as a mixture of two enantiomeric pairs of diastereoisomers as a TSA to raise antibodies, among which HA5-19A4 emerged as the best catalyst for cyclization of substrate [83] (Appendix entry 15.4) (Hasserodt et al., 1997). 

The first indication of the chirality of sulfuranes was provided by the X-ray analysis of spirosulfurane 176 (192). This work clearly demonstrated the presence of enantiomeric pairs of 176 in the crystal lattice. In 1975, the optically active chlorosulfurane 177, the first example of an optically active tetracoordinate sulfurane, was synthesized by Martin and Balthazor (194,195) by the route indicated in Scheme 15. Reaction of (-)-(5)-menthyl benzenesulfinate 178 with the protected Grignard reagent 179 gave the corresponding sulfoxide alcohol (-)-(5>180 which was cyclized to the chlorosul-... 

Another factor that remarkably affects the enantioresolution of given enantiomeric pairs has been shown to be the binding chemistry used for the silica immobilization of glycopeptides (see Section 2.2). This was illustrated in the case of ristocetin A, which was covalently bonded to silica microparticles by immobilization onto epoxy-activated silica under mild conditions [22], and compared with the corresponding commercially available CSP, where the macrocycle was immobilized as previously reported for vancomycin, rifamycin B, thiostrepton [7], and teicoplanin [30]. The comparison proved that immobilization of ristocetin A onto epoxy-activated silica could significantly improve the enantioselectivity and the resolution of the corresponding CSP in the separation of a-amino acids under RP conditions [22]. 

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