Configuration of compound

FIGURE 1.19 Viewing angle as a means of designating the absolute configuration of compounds with a chiral axis, (a) (R )-2-Butanol (sequence clockwise) (b) (fi)-2-butanol (sequence counterclockwise). 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

For a molecule without a heavy atom, the absolute configuration can also be determined by attaching another chiral moiety of known configuration to the sample. The absolute configuration can then be determined by comparison with this known configuration. For example, the absolute configuration of compound 37 or 38 cannot be determined by X-ray diffraction because of the lack of heavy atoms in the molecules. But the configuration can be determined by... 

Figure 1-25. Mosher s method for determining the absolute configuration of compound...

In B the straight line is inclined and so is different from A. Since according to a general rule, the configurations of compounds forming a quasi-racemate must be opposite, in the above example, the acids with the same sign of rotation have the same configuration. 

Falk and Schlogl 42> as late as 1968 established the absolute configuration of compound 13a as (+) —(S)-4-carboxy[2.2]paracyclophane. They did this by kinetic resolution of the racemic carboxylic anhydride with ( —)-a-phenylethylamine this kinetically controlled amidation afforded the dextrorotatory compound (13) in 3.8% optical yield 42C The similar topology of the carboxyl-group environment in 13 and in a-substituted metallocene carboxylic acids (14), configurations con-... 

The structure of 58 was elucidated as 1-hydroxy-2-oxoeremophil-1(10),7(1 l),8(9)-trien-12(8)-olide. This proposal was corroborated by an X-ray analysis. Finally, the absolute configuration of compound 58 was proposed as depicted on the basis of the CD spectrum which displayed a strong negative Cotton effect on 378 nm. Furthermore, as (-) ligu-larenoHde, whose absolute stereochemistry was determined by chemical correlation, compound 58 possesses a negative optical rotation. 

A-D (101-104), and xuulanins (105 and 106). (25)-5,7,4 -Trihydroxyflavan-4-ol (107) was claimed to have been identified from natural sources also. This may be unlikely in view of the high reactivity of 5-oxyflavan-3,4-diols as electrophiles in weakly acidic conditions. The absolute configuration of compound 100 was established as 25, 45 by reference to the ORD data of its likely flavanone precursor and using the relative configuration as established by NMR coupling constants of the heterocyclic protons. 

Like the cWo-derivative, acetals may be formed using a similarly structured exo-annulated MBF-OH43. Conformational analysis can be carried out to determine the absolute configuration of compounds of a similar structure as with the endo-annulated reagent. Differences in chemical shift of the same hydrogens and carbons are similar and reveal that other acetals may be equally useful for enantioselectivity studies. 

Rotatory dispersion curves often are helpful in establishing configurations thus the relative configurations of compounds 18 and 20a must be the same because, if they were not, the two curves would resemble mirror images of one another. Therefore, if the absolute configuration of 18 corresponds to the formula shown, then compound 20a has the configuration shown and not 20b. 

Compound 38 is isolated as a pure substance, m.p. 37-39 °C. A mass-spectrometric investigation confirms the formula Si4CioH240 calc. M.W. for the fragment CH3 259,082598 meas. 259,083646. Besides the structure illustrated here, another configuration of compound 38 could be considered in which one six-membered ring is in the chair form and the other in the boat form. However, transmutation have not yet been observed. A structure with both rings in the chair form can be excluded for steric reasons. 

Route B Transfer of hydride from the endo side of the molecule to the tertiary carbon with concurrent displacement of the oxygen bridge. This operation would account for the configuration of compound III. 

The configuration of compound 23 was characterized by analyses of H NMR data <2006JOM604>. The spectrum showed two singlets at 1.64 and 3.62 ppm for two protons of a methylene and for two benzylic protons, respectively, indicating a /ra r-relationship between the two phenyl substituents on the five-membered ring, that is, the two protons of the methylene and the two benzylic protons are in chemically equivalent environments. 

In addition to the dynamic study of compound (120), Freyer (77JPR895) has used N-labelled compounds ( H- N coupling constants) to assign the configuration of compounds (123) and (124). For the latter, a /( H- N) coupling constant of 92 Hz establishes the tautomeric structure as (124) (78JPR508). 

Now, how do we specify the configuration of compounds which, like these, contain more than one chiral center They present no special problem we simply specify the configuration about each of the chiral centers, and by use of numbers tell which specification refers to which carbon. 

Designation of optical isomers can be by the symbols D and L, which are used to indicate the relationship between configurations based on d (+)-glycer-aldehyde as an arbitrary standard. If such relationship is unknown, the symbols (-I-) and (-) are used to indicate the sign of rotation of plane polarized light (i.e., dextrorotatory and levorotatory). In 1956, Cahn et al. [1] presented a new system, the (R) and (S) absolute configurations of compounds. 

---