R/S, configuration

What are the R S configurations of the three chirality centers in D nbose" (A molecular model will be helpful here )... 

One further point needs to be mentioned—the matter of absolute configuration. How do we know that our assignments of R,S configuration are correct in an absolute, rather than a relative, sense Since we can t see the molecules themselves, how do we know that the R configuration belongs to the dextrorotatory enantiomer of lactic acid This difficult question was finally solved in 1951, when J. M. Bijvoet of the University of Utrecht reported an X-ray spectroscopic method for determining the absolute spatial arrangement of atoms in a molecule. Based on his results, we can say with certainty that the R,S conventions are correct. 

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

Chloramphenicol, a powerful antibiotic isolated in 1949 from the Streptomyces venezuelae bacterium, is active against a broad spectrum of bacterial infections and is particularly valuable against typhoid fever. Assign R,S configurations to the chirality centers in chloramphenicol. 

Assign R,S configuration to each chirality center in the following molecular model of the amino acid isoleucine (blue = N) ... 

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... 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

Diels-Alder reaction and. 494-495 El reaction and, 392 E2 reaction and, 387-388 R.S configuration and, 297-300 S 1 reaction and, 374-375 S -2 reactions and, 363-364 Stereogenic center, 292 Stereoisomers, 111 kinds of, 310-311 number of, 302 properties of, 306 Stereospecilic, 228, 494 Stereospecific numbering, sn-glycerol 3-phosphate and, 1132 Steric hindrance, Sjvj2 reaction and, 365-366 Steric strain, 96... 

Fig. 12. Salient features of the GC-MS data for a hydrothermal petroleum (Guaymas Basin, Gulf of California, Mexico) (a) TIC trace of total oil, (b) mjz 191 key ion for hopanes, and ctmjz 217 key ion for steranes. Numbers refer to carbon chain (/ -alkanes) or skeleton, UCM = unresolved complex mixture, Pr = pristane, Ph = phytane, asterisks = other isoprenoids, ot, ot, R, S = configurations of biomarkers.

With n dissimilar chiral atoms the number of stereoisomers is 2" and the number of racemic forms is 2" as illustrated below for 2-chloro-3-bromobutan.e (n = 2). The R,S configuration is shown next to... 

Problem 5.34 For the following reactions give the number of stereoisomers that are isolated, their R.S configurations and their optical activities. Use Fischer projections. 

Aldol reactions of sugar enolates have provided a good entry to C-C double bond branched-chain sugars [178]. As already mentioned, condensation of aldehydes, such as acetaldehyde or propionaldehyde, gives a mixture of aldol 120 of R,S configuration at the... 

Some exercises follow in which you will work out R,S configurations from projection or stereo formulas and vice versa. If you have difficulty with these, we recommend you use the procedure of Figures 5-12 and 5-13 to translate projection formulas to or from ball-and-stick models, which then can be oriented, as in Figure 19-6, to determine, or to produce, particular R or S configurations. 

All three topological forms (cis-a., cis-j8 and trans) have been isolated769 for CoCl2(trien)+ and the formation of the optically active trans isomer from the chiral cis-fi establishes the (RR,SS) configuration for the racemic form of the latter. The alternative R,S configuration for the chiral cis- S would lead to an inactive R,S-meso) trans form. 

These rules and applications are summarized completely in most introductory organic texts. It is important to be able to assign R,S configurations to stereogenic centers in molecules and to construct chiral molecules given the R or S configuration. In this way it is very easy to determine the stereochemical relationships between stereoisomers. 

TABLE 4 Effect of the Structure of S,S and R,S Configurations of N,N-2-hydroxy-propyl-phenylalaninamide CMPA on the Chiral Resolution of Some Dansyl Amino Acids... 

In order to separate the preferred, S,S-isomer, a suspension of 10.0 g of the mixture in 200 ml of methylene chloride was stirred at room temperature for five min and filtered the solid was washed with additional methylene chloride and finally ether. The solid material, melting point 202°-208°C (dec.) was the less preferred diastereoisomer having the R,S-configuration (S referring to the portion derived from L-alanine). The preferred S,S-diastereoisomer was recovered from the filtrate after concentration and trituration of the residue with ether melting point 137°-139°C. The free amino acid (S,S-form) was prepared by treatment of an aqueous solution of the hydrochloride with saturated sodium acetate. The ethyl a-[(l-... 

In Ama-L-Phe-OMe (47) (14, 15), it is also not known whether the sweet-tasting isomer has the L-L(or S-S) or the D-L(or R-S) configuration. In the case of aspartyl dipeptide esters, the L-L isomer was sweet. By analogy, other researchers deduced that the L-L(or S-S) isomer ((47b) in Figure 4) would be sweet. However, it seemed to us that the D(or i )-configuration would be preferred for the aminomalonic acid because the D-L(or R-S) isomer ((47a) in Figure 4) was compatible with the sweet formula and could also fit the spatial barrier model (13), whereas the L-L(or S-S) isomer could neither fit the receptor model nor meet the sweet formula. 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (R,S configuration may be readily assigned at the appropriate chiral carbons. 

Figure 12 shows the crystal structures of pyrr-1 and pyrr-2 viewed along the c axis before irradiation. There are four crystallographically independent molecules, A, B, C, and D, in a PI cell. In the crystal of pyrr-1, the A, B, C, and D molecules have R, R, 5, and S configuration, respectively therefore the crystal is racemic. In the crystal of pyrr-2 with the R S ratio of 75 25, the A, B, C, and D molecules have / ,/ ,/ + 5, and R + S configuration, respectively. The R S ratips of the C and D molecules are 10 15 and 16 9, respectively. As a whole, the pyrr-2 crystal has an R S ratio of 76 24, which is identical to the R S ratio of the solution, 75 25, within experimental error. The pyrr-3 and pyrr-4 crystals were also analyzed. The R S ratios of the A and B molecules are 25 0, whereas the R S ratios of the C and D molecules are 15 10 and 19 6 for pyrr-3 and 17 8 and 23 2 for pyrr-4, respectively. These values indicate that the R S ratios of the pyrr-3 and pyrr-4 crystals are 84 16 and 90 10, respectively, which are very similar to the R S ratios in the respective solutions. Since the crystals of pyrr-1 to pyrr-4 are all chiral, the chirality of the crystals was selected so that the A molecules have R-1 -cyanoethyl groups. 

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