Diastereomers chemical shift

In determination of the absolute configuration of a-chiral primary amines, BINOL derivatives were used as chiral derivatizing agent.10 In this procedure, the chiral substrate was derivatized with R and S enantiomers of the 2,-methoxy-l,l -binaphthalene-8-carbaldehyde and the XH spectra of both diastereomers were compared. Comparison of the chemical shift differences of the diastereomers has allowed determination of the absolute configuration of the chiral substrate [5]. 

Treatment of all four diasteromers of 72 with sodium or potassium bases yielded stilbenes with high stereoselectivity (Scheme 15) <2003T255>. Two of the diastereomers gave rise to some retro-aldol products. In two cases, hexacoordinate species were identified by the upheld 31P chemical shift (5—112 ppm). It was noted that phosphoranes that ring-closed to form hexacoordinate tricyclic species were the ones that did not undergo the retro-aldol reaction to produce aldehydes. The ring closure was disfavored for intermediates with steric repulsion between trifluoromethyl and phenyl groups. 

Additionally, information about the absolute configuration of the analyte can be obtained from the sign of the chemical shift differences of the diastereomers. This will be discussed here. Use of other substituted phenylacetic acids fulfilling a similar purpose to Mosher s reagent will be described partly in this section and in the following one. 

The optical purity of MTPA is checked conveniently using both readily available optically pure forms of phenylethylamine as a CDA15c. The H-NMR signal of the methyl group of the amine part is shifted for the (R,R) or (5,5)-diastereomer by 5 — 0.06 to higher field than for the (R.S) or (S, R) form (<5 = 1.46). A larger chemical shift difference of 5 = 0.15 can be observed in the l9F-NMR spectrum the trifluoromethyl group of the (R,R)- or (S.S)-diastereomer appears at S = 7.55 versus 7.30 for the (R,S) or (S,R) form, both dowmfield from trifluoroacctic acid. 

Racemic phenylmethanethiol reacts with endo-MBF-OH to give a-anomeric thioacetals as a mixture of diastereomers. The differences between the chemical shift of two pairs of singlets are used to measure the diastereomeric ratio (2-H AS = 0.51 7a-H AS = 0.12). 

Table 14 presents the chemical shifts of l9F-signal pairs of selected diastereomeric imidazolidines. The advantages of the above method can be summarized as follows high chemose-lectivity-ketones do not react C2 symmetry means a diastereomeric control excess reagent can be easily removed generally, an excellent separation of H-, l3C- and, 5F-NMR signals of the diastereomers is observed. 

Another way of evaluating 13C chemical shifts in oximes, especially if only one diastereomer is available, is a comparison with the corresponding ketone. For example, a comparison of 2-butanone and the Z- and E-oximes (the <5 values in parentheses are differences relative to the ketone) shows that the s>n-oriented a-carbon is affected much more strongly than the other. 

The first series involves cc-methyl-/Miydroxycarbonyl compounds where C-3 (carbinol) and the methyl carbon arc always shielded in the syn- compared to the aw/i-diastcrcomcr the chemical shift difference A d being 1.1 to 5 ppm. The results are less clearcut for C-2 (methine). The results can be explained after an inspection of the conformations for the diastereomers. In the syw-form the a-methyl group is partly in the axial position leading to a smaller chemical shift due to a higher number of gauche interactions. This is not the case for the w//-form because anti-1 (p 330) is the dominating conformation. 

These y-effect values are taken as perturbation increments and added to the parent compound of 7-9 (which is methylbutane C-l to C-4 and C-8), considering that the increments have to be averaged for the two conformations a and b in each case. The resulting calculated chemical shifts can be compared with the experimental values (Table 5)380. There is reasonable agreement indicating that the 13C chemical shift differences in these three diastereomers can be reproduced rather well by this procedure. 

A series of sesquiterpenoid cyclohexenones (1-abolones) has been studied395, and it was found empirically that in the naturally occurring diastereomer 14 the C-7 methyl protons resonate at <5 = 0.80, whereas in the synthetic diastereomer this value is 3 = 0.93. The same sequence and chemical shift difference was found for another eleven pairs of diastereomers with the same skeleton. 

When all substituents A and C are alkyl groups, there is no difference in the 1H chemical shifts for the two diastereomers. 

In addition to H and 13C chemical shifts, vicinal 1H,1H coupling constants provide another tool for the stereochemical assignment of disubstituted cyclopropanes, as the cis or trans relationship of the substituents can be determined from the absolute values, often without the need to compare diastereomers. The relationship is ... 

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