Configuration: absolute relative

Figure 9.5. Defining absolute, relative collection, and relative single configuration stereochemistry. The older convention depends on a "chiral" flag on the molecule to specify whether a given structure represents one or several stereoisomers. In the newer convention, collections of stereo centers can be defined, and they can be designated absolute, relative-part-of-a-mixture, or relative-single-configuration.

With a knowledge of the structures of the necines and necic acids and with the location of ester linkages ascertained, it is possible to write structural formulas to represent the pyrrolizidine alkaloids. Configurations (absolute and relative) at asymmetric carbons and double bonds are indicated when the author feels that these have been established with a reasonable degree of certainty. Structures have not been provided where the information is deemed insufficient, so that not all of the alkaloids listed in Table 1 will be given representations. The alkaloids are divided into three main categories monoesters, diesters (two different necic acids), and cyclic diesters. The amine iV -oxides are not given since their structures are obvious from the amines. 

Within the formulae (343)-(365) stereochemical detail relates to the relative configuration of centres in the same half of a molecule no absolute configurations or relative stereochemistries of centres in different halves of any molecule are implied. 

The structure of sertraline is characterized by two stereogenic centres with absolute (15,45) configuration and relative c -configuration. The compound can be obtained from an enantiomerically pure natural chiral compound, as presented in Sect. 7.7. However, a more effective approach was initiated from achiral materials by transforming them diastereoselectively, followed by chiral separations of the enantiomers [11]. 

Section 7 5 Relative configuration compares the arrangement of atoms m space to some reference The prefix as m as 4 methylcyclohexanol for example describes relative configuration by referencing the orientation of the CH3 group to the OH Absolute configuration is an exact description of the arrangement of atoms m space... 

Chemical conversion of compounds to intermediates of known absolute configuration is a method routinely used to determine absolute configuration (86). This is necessary because x-ray analysis is not always possible suitable crystals are required and deterrnination of the absolute configuration of many crystalline molecules caimot be done because of poor resolution. Such poor resolution is usually a function of either molecular instability or the complex nature of the molecule. For example, the relative configuration of the macroHde immunosuppressant FK-506 (105) (Fig. 8), which contains 14 stereocenters, was determined by x-ray crystallographic studies. However, the absolute configuration could only be elucidated by chemical degradation and isolation of L-pipecoUc acid (110) (80). 

Among the modem procedures utilized to estabUsh the chemical stmcture of a molecule, nuclear magnetic resonance (nmr) is the most widely used technique. Mass spectrometry is distinguished by its abiUty to determine molecular formulas on minute amounts, but provides no information on stereochemistry. The third most important technique is x-ray diffraction crystallography, used to estabUsh the relative and absolute configuration of any molecule that forms suitable crystals. Other physical techniques, although useful, provide less information on stmctural problems. 

Nitrogen chirality may also be produced by the action of an achiral peroxyacid on a Schiff base containing a chiral amine (75JOC3878). In this case the oxaziridine contains a configurationally known centre of chirality relative to this, absolute configurations of the centres of chirality at nitrogen and carbon, and thus the complete absolute configuration of the molecule, can be determined (see Section 5.08.2.2). 

Photoluminescence finds its greatest strengths as a qualitative and semiquantitative probe. Quantification based on absolute or relative intensities is difficult, although it is useful in applications where the sample and optical configurations may be carefully controlled. The necessary conditions are most easily met for analytical applica-... 

Although no absolute configuration was known for any substance until the midtwentieth century, organic chemists had experimentally determined the configurations of thousands of compounds relative to one another (their relative configurations) through chemical interconversion. To illustrate, consider (-l-)-3-buten-2-ol. Hydrogenation of this compound yields (i-)-2-butanol. 

An elaborate network connecting signs of rotation and relative configurations was developed that included the most important compounds of organic and biological chemistry. When, in 1951, the absolute configuration of a salt of (3-)-taitaiic acid was... 

Relative configuration (Section 7.5) Stereochemical conhgu-ration on a comparative, rather than an absolute, basis. Terms such as D, L, erythro, threo, a, and (3 describe relative conhguration. 

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. 

For atoms with more than two electrons, it is very difficult to obtain such a small absolute error in the energy as in the helium case, but, within an isoelectronic sequence, the relative error will, of course, go down rapidly with increasing atomic number Z. The method of superposition of configurations has been used successfully in a number of applications, particularly by Boys (1950-) and Jucys (1947-), and, for a more detailed survey of the work on atoms, we will refer to the special table on atomic calculations in the bibliography. This is a field of rapid development, where one can expect important new results within the next few years. 

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