And diastereomers

Note carefully the difference between enantiomers and diastereomers. Enantiomers have opposite configurations at all chirality centers, whereas diastereomers have opposite configurations at some (one or more) chirality centers but the same configuration at others. A full description of the four stereoisomers of threonine is given in Table 9.2. Of the four, only the 2S,3R isomer, [o]D= -28.3, occurs naturally in plants and animals and is an essential human nutrient. This result is typical most biological molecules are chiral, and usually only one stereoisomer is found in nature. 

Stereoisomers (Section 4.2) Isomers that have their atoms connected in the same order but have different three-dimensional arrangements. The term stereoisomer includes both enantiomers and diastereomers. 

For compounds that contain a limited number of fluorine atoms, heteronuclear correlation spectroscopy experiments such as F H HETCOR and 2H-19F heteronuclear Overhauser enhancement spectroscopy (HOESY) can provide considerable assistance distinguishing structural isomers and diastereomers as well as for conformational analysis. HOESY experiments have been frequently used for conformational analysis of biomolecules containing fluorine labels.18... 

This chapter has reported the only extensive and coordinated investigation of the effects of chirality on the properties of monolayer films spread at the air-water interface. Twenty compounds of varied headgroup and chain length have been examined carrying one and two chiral centers. In every case, all of the optical isomers—enantiomers and diastereomers—were made and their properties measured both as pure compounds and as mixed monolayers in order to compare phase changes in the films with mixed melting points of the crystals. 

The corresponding syn-compound can also be synthesized by simply inverting the stereochemistry of the hydroxyl group of the epoxy alcohol by the Mitsunobu reaction [54], Therefore, this method provides a simple and reliable method for the synthesis of any enantiomers and diastereomers of straight-chain 1,2-polyols. 

Biological activities such as enzyme reactions and metabolic changes are highly stereospecific, hence enantiomers and diastereomers may have entirely different... 

We refer to any measurable difference between diastereomeric pairs as a diastereomer discrimination. We use the term chiral discrimination to include both enantiomer and diastereomer discrimination for the general case. 

Because this area is not too well known, the authors have taken pains to describe, in some detail, experimental monolayer chemistry. The centerpiece of the chapter is enantiomer and diastereomer discrimination in monolayers. It concludes with a discussion of surface properties, in particular energetics, which are quite sensitive to stereochemistry. We call attention to the fact that this chapter is of potential interest to biochemists, notably those concerned with lipids and with cell membrane organization. 

In a series of papers, the methodologies applied for amino acids were extended to peptide stereoisomer (enantiomer and diastereomer) separations [59,116,121-123],... 

Stereoisomers are chemical componnds having the same elemental composition bnt differing in strnctnre. We have seen three snbtypes of stereoisomers constitutional isomers, enantiomers, and diastereomers. 

Unsymmetrically 2,5-disubstituted furan derivatives 61 reacted with 1-Me in good to very good yields, but gave mixtures of regioisomers and diastereomers endo/exo-62y endo/exo-65,the bicyclic acetals 62c-d showed a distinct tendency to be cleaved upon chromatographic separation. Dimethylfuran 61b, surprisingly, is less reactive towards 1-Me than furan (57) which is probably due to steric reasons (Scheme 16) [46]. 

Since first proposed by Bax et al. [4, 5] and Kessler et al. [6], the ID TOCSY technique has been gaining popularity among NMR spectroscopists as well as practicing chemists. The ID TOCSY technique has been applied to the assignment of peptides [6, 12, 19-21], carbohydrates [22-34], steroids and alkaloids [35-43], carotenoids [44] and mixtures (of reaction products, isomers and diastereomers) [22b, 45 9]. The references cited here may represent only a small fraction of the work in the literature reporting the use of the ID TOCSY technique. 

The pyramidal inversion of phosphorus was studied by dynamic NMR spectroscopy using the H signals of N—Me diastereotopic groups in (60) and P signals in Z- and -diastereomers of (61). Kinetics at coalescence temperature revealed the value of AG to increase in the order of increasing electronegativity Si < Sn < Ge <82JOM(224)247>. 

Jones KA, Wilding TJ, Huettner JE, Costa AM (1997) Desensitization of kainate receptors by kainate, glutamate and diastereomers of 4-methylglutamate. Nemopharmacology 36 853-863... 

The use of chiral azomethine imines in asymmetric 1,3-dipolar cycloadditions with alkenes is limited. In the first example of this reaction, chiral azomethine imines were applied for the stereoselective synthesis of C-nucleosides (100-102). Recent work by Hus son and co-workers (103) showed the application of the chiral template 66 for the formation of a new enantiopure azomethine imine (Scheme 12.23). This template is very similar to the azomethine ylide precursor 52 described in Scheme 12.19. In the presence of benzaldehyde at elevated temperature, the azomethine imine 67 is formed. 1,3-Dipole 67 was subjected to reactions with a series of electron-deficient alkenes and alkynes and the reactions proceeded in several cases with very high selectivities. Most interestingly, it was also demonstrated that the azomethine imine underwent reaction with the electronically neutral 1-octene as shown in Scheme 12.23. Although a long reaction time was required, compound 68 was obtained as the only detectable regio- and diastereomer in 50% yield. This pioneering work demonstrates that there are several opportunities for the development of new highly selective reactions of azomethine imines (103). 

Its extension to enantio- and diastereo/opos-differentiating reactions is straightforward. Enan-tio- and diastereomer-differentiating reactions do not require special rules the CIP system itself suffices for their description. 

Figure 13. Rapid simultaneous separation of eight stereoisomers (enantiomers and diastereomers) of 2-mcthyl-3-(l -methylpropyl)oxiranc on 0.125 M nickel(ll) bis[3-(heptafluorobutanoyl)-(l / )-camphorate] in SB-30 [20 m x 0.25 mm (i.d.) glass capillary column. 90 C. 1 bar nitrogen].

Diastereodifferentiating Reactions (Diastereoface-, Diastereotopos-, and Diastereomer-Differentiating Reactions)... 

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