Diastereotopic atoms

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

Fig. 7. Bridges containing diastereotopic atoms in Me2CH or PhCH2 groups in compounds of the type Os3H(/j-X)(CO)10.

Bonded to a group that withdraws part of the electron density from around the nucleus. The absorptions of deshielded nuclei are moved downfield, resulting in larger chemical shifts, (p. 568) Nuclei that occupy diastereomeric positions. The replacement test for diastereotopic atoms gives diastereomers. Diastereotopic nuclei can be distinguished by NMR, and they can split each other unless they are accidentally equivalent, (p. 592)... 

Stereoheterotopic A term that includes both enantiotopic and diastereotopic atoms, groups and faces. Equivalent atoms, groups and faces would be homotopic. 

Some of the compounds shown below contain enantiotopic or diastereotopic atoms or groups. Which possess this characteristic For those that do, indicate the atoms or groups that are diastereotopic and assign the groups as pro-R and pro-S. 

Two atoms or groups are said to the diastereotopic if the two atoms or groups appear to be similar but are not actually related by a symmetry operation. Selectively labeling one diastereotopic atom or group wUl not produce the mirror image of the molecule in which the other atom or group is labeled. For example, the methylene protons in phenylalanine are diastereotopic. The resonances from diastereotopic atoms and groups will normally be anisochronous, unless they happen to fortuitously have the same chemical shift. 

Diastereotopic atoms are nonequivalent in all environments, so they have different chemical shifts. These differences can lead to complex splitting of the signals of diastereotopic H atoms, especially those adjacent to a chiral center. 

Draw a three-dimensional representation of 4-methylheptane. Identify a pair of enantiotopic atoms. Identify a pair of diastereotopic atoms. 

Diastereotopic are those heterotopic atoms or groups that during substitution result in a pair of diastereomers. When substimtion of two H-atoms with deuterium or another atom/ group (i.e., desymmetrization) forms diastereomers, those two H-atoms are not equivalent, and give two signals in the NMR spectmm. Several examples of organic compounds with diastereotopic atoms are provided in Figure 1.33. 

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