Inverted carbon atoms

Interchanging any two substituents on an asymmetric carbon atom inverts its (ft) or (S) configuration. If there is only one chirality center in a molecule, inverting its configuration gives the enantiomer. 

A phase change takes place when one enantiomer is converted to its optical isomer. As illustrated in Figure 9, when the chiral center is a tetra-substituted carbon atom, the conversion of one enantiomer to the other is equivalent to the exchange of two electron pairs. This transformation is therefore phase inverting. 

Figure 9. The phase-inverting transformation of chiral system with a tetra-substituted carbon atom.

See maltose structure (p. 618) invert OH and H on carbon atom at lower right... 

It is particularly appealing that the configuration at the a-carbon atom in the w-diastereo-mers m-18 can be inverted by trifluoroacetylation and migration of the 2,2-dimethylpropanoyl group24. Furthermore, A-deprotection is possible through simple operations24. 

In certain small-ring systems, including small propellanes, the geometry of one or more carbon atoms is so constrained that all four of their valences are directed to the same side of a plane (inverted tetrahedron), as in 98. An example is 1,3-... 

This shows that compounds (76) is optically stable at tin, i.e. that tin does not invert even after long periods. However, pyridine causes a slow epimerization. This was an example of the case where the asymmetric carbon atom was not resolved and where two diastereomeric racemic mixtures could be separated. 

The spray paint can was inverted and a small amount of product was dispensed into a 20 mL glass headspace vial. The vial was immediately sealed and was incubated at 80°C for approximately 30 min. After this isothermal hold, a 0.5-mL portion of the headspace was injected into the GC/MS system. The GC-MS total ion chromatogram of the paint solvent mixture headspace is shown in Figure 15. Numerous solvent peaks were detected and identified via mass spectral library searching. The retention times, approximate percentages, and tentative identifications are shown in Table 8 for the solvent peaks. These peak identifications are considered tentative, as they are based solely on the library search. The mass spectral library search is often unable to differentiate with a high degree of confidence between positional isomers of branched aliphatic hydrocarbons or cycloaliphatic hydrocarbons. Therefore, the peak identifications in Table 8 may not be correct in all cases as to the exact isomer present (e.g., 1,2,3-cyclohexane versus 1,2,4-cyclohexane). However, the class of compound (cyclic versus branched versus linear aliphatic) and the total number of carbon atoms in the molecule should be correct for the majority of peaks. 

The configuration of the carbon atom becomes inverted during Sn2 reaction. 

But when a molecule contains two or more asymmetric carbon atoms and the configuration of only one is inverted, the process is called epimerisation. 

The electrochemical behaviour of PBN +-cyanide ion is identical to that found in the two cases of inverted spin trapping described above, namely that attack at PBN + occurs via the softer carbon atom of CN". This contrasts with an observation of the cyano adduct to PBN formed by irradiation of Mo(CN) -... 

A classical application of cyclopropane rings in "reactivity inversion", in which a previously negatively polarised y-carbon atom (see 29) to a carbonyl group is inverted by formation of a cyclopropane ring (30) and is then intramolecularly attacked by a nucleophilic aromatic ring, is found in the synthesis of hydrophenanthrene system 11 (Scheme 5.19) developed by Stork in 1969 [21]. 

From the results described above it was clear that our synthetic plan had to be modified significantly if the synthesis of kelsoene was to be successful. We were willing to take on the challenge and started to think about a modification. Key to our plan was the transformation rac-59 rac-6l in the model studies. The trans-anti-cis configuration of the product would be acceptable if we succeeded in inverting the stereogenic center at carbon atom C-6... 

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