Methods of determining configuration
In all the methods, it is necessary to relate the compound of unknown configuration to another whose configuration is known. The most important methods of doing this are:
1. Conversion of the unknown to, or formation of the unknown from, a compound of known configuration without disturbing the chiral center. Since the chiral
center was not disturbed, the unknown obviously has the same configuration as the known. This does not necessarily mean that if the known is (R), the unknown is also (R). For example, when (R)-l-bromo-2-butanol is reduced to 2-butanol without disturbing the chiral center, the product is the (S) isomer, even though the configuration is unchanged, because CH3CH2 ranks lower than BrCH2 but higher than CH3.
2. Conversion at the chiral center if the mechanism is known. Thus, the Sn2 mechanism proceeds with inversion of configuration at an asymmetric carbon. It was by a series of such transformations that lactic acid was related to alanine:
3. Biochemical methods. In a series of similar compounds, such as amino acids or certain types of steroids, a given enzyme will usually attack only molecules with one kind of configuration. If the enzyme attacks only the l form of eight amino acids, say, then attack on the unknown ninth amino acid will also be on the l form.
4. Optical comparison. It is sometimes possible to use the sign and extent of rotation to determine which isomer has which configuration. In a homologous series, the rotation usually changes gradually and in one direction. If the configurations of enough members of the series are known, the configurations of the missing ones can be determined by extrapolation. Also, certain groups contribute more or less fixed amounts to the rotation of the parent molecule, especially when the parent is a rigid system such as a steroid.
5. The special X-ray method of Bijvoet gives direct answers and has been used in a number of cases.
6. One of the most useful methods for determining enantiomeric composition is to derivatize the alcohol with a chiral nonracemic reagent and examine the ratio of resulting diastereomers by gas chromatography. There are many derivatizing agents available, but the most widely used are derivatives of ɑ-methoxy-ɑ -trifluoromethylphenyl acetic acid (MTPA, or Mosher's acid, 38).Reaction with a chiral nonracemic alcohol (R*OH) generates a Mosher's ester (39) that can be analyzed for diastereomeric composition by 1H or 19F NMR as well as by chromatographic techniques.
Alternatively, complexation with lanthanide shift reagents allow the signals of the MTPA ester to be resolved and used to determine enantiomeric composition. The effectiveness of this method, and other related methods, have been evaluated and found to be good for determining the absolute configuration of the alcohol of interest (R*OH). Two, of many other reagents that have been developed to allow the enantiopurity of alcohols and amines to be determined include 40 and 41. Chloromethyl lactam (40) reacts with R*OH or R*NHR (R*NH2), forming derivatives that allow analysis by 1H NMR and 41 reacts with alkoxides (R*O-)100 to form a derivative that can be analyzed by 31P NMR.
Other methods have also been used, including optical rotatory dispersion, circular dichroism (CD), and asymmetric synthesis.