Diastereotopic pair

As an analytical method for determining the diastereoisomeric purity, thin-layer chromatography is occasionally used, but most frequently H-NMR spectroscopy is employed. Enantiomers give identical spectra. Thus, the H-NMR spectrum of a mixture of diastereoisomeric pairs of enantiomers contains only two sets of signals, one set for each pair of enantiomers this is analogous to the situation with a pair of optically active diastereoisomers described in Section VI. Hence, by H-NMR spectroscopy, the separation of diastereotopic pairs of enantiomers can be monitored, and the diastereoisomeric purity can be controlled (143-MS, 151, 171-179). 

Compound B has no symmetry element in the planar conformation. C-5 is a chiral center, and the protons of each CH2 group are diastereotopic pairs. Each proton of the pair has its own chemical shift. The H-4 proton adjacent to the chiral center is distinctly separated, but the H-3 protons are not, at 300 MHz. Each proton of a diastereotopic pair couples geminally with the other and independently (different coupling constants) with the vicinal protons to give complex multiplets. 

We have recently discussed the situation where one of the members of a diastereotopic pair is exchanged with a stereochemical label, e.g. a generic Z -label in place of C(l) or C(6).1 In this case, four isomers (two diastereomeric D,L-pairs 3,3-bar... 

In ipsenol, there are four methylene groups, all of which possess diastereotopic pairs of protons. Resonance for two of these methylene groups occurs in the carbon spectrum at 41 and 47 ppm. Note with which protons these carbon atoms are correlated and compare these results with what we have found with COSY. As we expect, the results here confirm our assignments from COSY and help build an ever-strengthening basis for our assignments. The other two methylene carbon... 

Pro-R. An atom or functional group that is part of an enantiotopic or diastereotopic pair that, upon (isotopic) labeling such that it acguires a higher precedence, generates an R chiral center. 

Formation of chiral supramolecular complexes results in the loss of an effective symmetry plane changing pairs of enantiotopic protons into diastereotopic pairs, which is clearly seen in the NMR spectra. 

When the aromatic group of the sulfoxide is replaced by a heteroaromatic group (e.g., N-methylimidazole), the internal coordination between Li—N to form a five-membered metallocycle apparently predominates over Li—O coordination to form a four-membered metallocycle . Reaction of imidazole (S)-sulfoxide 16 with benzaldehyde produces aldol 17 as the major product in which the a-H and the sulfoxide lone pair are syn (equation 14) imidazole (R)-sulfoxide 18 reacts similarly (equation 15). The stereochemical outcome of these reactions is rationalized in terms of a-lithiosulfoxides in which the reactive diastereomer (i.e., 20 and 21) is that having one diastereotopic face of the five-membered Li—N metallocycle carrying both H and sulfoxide lone pair. 

Diastereotopic proton/group A proton (or group) which if replaced by another hypothetical group (not already found in the molecule), would yield a pair of diastereoisomers. 

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