Carbon atom, 3-dimensionality configuration

In this context, one may also pay attention to the so-called "betaine-ylides" that act as key intermediates in stereocontrolled Wittig olefination reactions. They are generated from the ordinary adducts obtained by the combination of a phosphine ylide and an aldehyde in the presence of lithium bromide (or another soluble lithium salt). When the P-betaines are a-deprotonated with phenyllithium, the stereocenter at the phosphorus-adjacent carbon atom becomes configurationally mobile. In this way, erythro/threo mixtures can spontaneously convert into virtually pure /Areo-betaine ylides (p-lithiooxy ylides, P-oxido ylides). Reprotonation and subsequent elimination of triphenylphosphine oxide affords trans olefins, whereas a-substitution by electrophiles other than acids leads to branched alkenes exhibiting a well-defined stereochemistry "("three-dimensional"" Wittig reaction or SCOOPY method). ... 

Fig. 15. The stereoisomeric thiourea-platinum complexes shown in (a) display large 4/ptjC couplings for the a-carbon atom in that branch of the X.X-dialkyl substituent which is in a favourable W configuration with respect to the metal atom (thick bonds). The assignment of the (fortuitously overlapping) H and 195Pt NMR signals of all three stereoisomers was feasible by means of indirect detection of these couplings from a XH detected H/(13C)/195Pt correlation spectrum which is shown in (b) together with the one-dimensional and 195Pt NMR projections. Reproduced from Ref. 44 by permission of John Wiley Sons.

It is called an a-amino acid because the amino group is attached to the a (or number 2) carbon atom. To indicate its three-dimensional structure on a flat piece of paper, the bonds that project out of the plane of the paper and up toward the reader are often drawn as elongated triangles, while bonds that lie behind the plane of the paper are shown as dashed lines. The isomer of alanine having the configuration about the a-carbon atom shown in the following structural formulas is called S-alanine or L-alanine. The isomer which is a mirror image of S-alanine is R-alanine or D-alanine. Pairs of R and S compounds (see Section B for definitions) are known as enantiomorphic forms or enantiomers. 

Until recently, the phenomenon of chirality has been better known as optical isomerism, and configurational isomers that are enantiomers were referred to as optical antipodes. The reasons for this are mainly historical. It was discovered early in the nineteenth century that many compounds, whether solid, liquid, or gas, have the property of rotating the plane of polarization of polarized light and can be said to be optically active. A satisfactory explanation of the origin of optical activity came much later and developed in its modern form from the classic researches of Louis Pasteur, and from the concept of the three-dimensional carbon atom expressed independently by J. H. van t Hoff and J. A. Le Bel.2... 

Recognizing that the substitutents residing on a tetra-substituted carbon atom are spherically spaced equidistant from one another, if all substituents are connected with lines, a tetrahedron is formed. Furthermore, as shown in Figure 4.1, if all four substituents are unique, they can be arranged in two configurations where the two molecules are mirror images and not superimposable. Because these two molecules are identical in composition but not configuration in three-dimensional space, they are referred to as... 

Two-dimensional techniques (HETCOR) also give information about triad and tetrad configuration [42]. The H multiplets at 1.89 and 1.5 ppm couple to three secondary carbon atoms at 34.94,34.23, and 33.42 ppm in the C spectrum. These methylene carbons do not show crosspeaks with any other region in the proton spectra. Thus, these results show that the major isomer isolated from the ACT reaction of 24 is all isotactic. Although this characterization does not allow determination of the absolute stereochemistry of 27, the tentative assignment shown is consistent with expectations based upon previous work done with chiral oxazolidine auxiliaries. 

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