Chiral centers labelling

Wnte the organic products of each of the following reactions If two stereoisomers are formed show both Label all chirality centers / or 5 as appropriate... 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

Several more examples of chiral molecules are shown below. Check for yourself that the labeled carbons are chirality centers. You might note that carbons in — CK2—, -CJH3, C—O, C=C, and C=C groups can t be chirality centers. (Why )... 

Which of the following compounds are chiral Draw them, and label the chirality centers. 

The BINAP-Ru(II)-catalyzed enantioselective hydrogenation of f>-keto esters is used for the synthesis of a wide range of important natural and man-made compounds [1-4, 48] some examples of these are listed in Figure 32.10, wherein chiral centers created by the enantioselective reaction are labeled with R or S. 

This procedure illustrates a general method for the reduction of aldehyde and ketone functions to methylene groups under very mild conditions. Since strong acids and bases are not employed, this procedure is of particular importance for the reduction of ketones possessing an adjacent chiral center. Moreover, the use of deuterated metal hydrides permits the preparation of labelled compounds. ... 

While most polymers contain only one chiral or asymmetrical center in each repeat unit, it is possible to have diisotacticity where two different substituents are present at chiral centers. These isomers are labeled erythro- and threodiisotactic and erythro- and threosyndiotactic isomers (Figure 2.6). This topic is further described in Appendix J. 

Fig. 8. Comparison of NMR-derived and computed structural data for [Ga(coelichelin)]. Top ROESY-derived H-H distances (normal text) and LFMM distances (italic). Middle HMBC-derived H-X-Y-H torsion angles (normal text) and LFMM torsions (italic). Bottom The LFMM calculated structure which best reproduced the NMR constraints. Nonpolar hydrogens omitted for clarity and chiral centers are labeled R or S as appropriate. (Distances in A and angles in degrees.)...

In molecules, the regions that can be R or S are called stereocenters (or chiral centers—chiral is Greek for hand, and we can understand the symbolism there). In this chapter, we will learn how to locate stereocenters, how to draw them properly, how to label them as R or S. and what happens wdien you have more than one stereo-center in a compound. 

Just as chiral centers can be labeled if or S not only in enantiomers but also in many diastereomers, so the designations pro-R and pro-S are not confined to enantiotopic ligands but may also be used for a number of diastereotopic ones (for exceptions, see below). Thus, for example, the labeling in Fig. 13 is such that HA (compounds 30, 32, 34, 36) or Me1 (compound 38) is the pro-R group the reader should verify this proposition. The same is true for compounds 46 and 5(5 in Fig. 18. Compounds 48, 50, 52 and 54 in Fig. 18 cannot be labeled in this manner since replacement of the diastereotopic ligands does not produce chiral products. In 54 (pro-pseudoasymmetric center) the substitution gives rise to a pseudoasymmetric center which, in the compound of the left is s, in the compound on the right r. Hence HA is called pro-r and HB pro-s 6>. 

The synthesis of CHDTC02H is, in principle, straightforward and was first accomplished in two laboratories in 1969 150). A modified version of the Comforth synthesis151) is shown in Fig. 67. Most of the steps are self-evident. The doubly labeled phenyl methyl carbinol is, of course, a dl pair in the first instance the absolute stereochemistry at the CHDTX chiral center is RS but the relative stereochemistry of the two chiral centers is defined as IRS,2RS, i.e. only one of the two possible diastereomers is obtained. The resolution at the methyl group occurs automatically when the carbinol is resolved by classical methods. 

Label each chirality center in these compounds with an asterisk and calculate the maximum number of stereoisomers for each ... 

Racemization of chiral centers during the isolation or synthesis of a particular peptide will lead to diastereoisomeric mixtures. In addition, in peptide synthesis it is often more efficient to use DL-amino acids, particularly if they are specifically labeled with C, H, S, etc. Rapid... 

I>) Draw a Fischer projection of a threonine diaslereoiner, and label its chirality centers as R or 5. 

MSA does not contain any chiral carbon centers. Before the aromatization of the six-membered ring occurs, two prochiral carbons (C-2 and C-4 in the six-carbon intermediate) evolve, each of which loses a hydrogen in the process of the dehydratization/aromatization steps. In addition, C-3 of the six-carbon intermediate forms a chiral center when the ketone is reduced to a hydroxyl by a ketoreductase activity (Fig. 5). The chirality of this hydroxyl carbon is unclear since the intermediate has not been isolated. It is also unknown if this carbon retains its chirality in an eight-carbon intermediate or whether the hydroxyl is eliminated by dehydration prior to the third condensation reaction. The stereospecificity at the prochiral C-2 and C-4 carbons in the reaction intermediates was addressed using chemically synthesized (] )- and (S)-[1- C, 2- H]malonate precursors which were enzymatically converted into CoA derivatives via succinyl CoA transferase [127,128]. Thus, the prochiral methylene in malonyl CoA was replaced by chiral, double-labeled (S)- or (J )-[1- C, 2- H]malonyl CoA substrates in the reaction mixture with 6-MSAS. The condensation is expected to occur with inversion of configuration and the intact methylene... 

When a molecule contains only one chiral center, the two stereoisomers are known as enantiomers. These may be referred to or labeled using the configurational descriptors as either R rectus meaning righthanded) or S (sinister meaning left handed), or alternatively, D (dextrorotatory) or l (levorotatory). The D and l configurational descriptors are... 

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