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From the NMR standpoint, the L1 substituent of an (S)-MTPA amide is shielded in conformer sp1 (see Figure 103a), while substituent L2 is virtually unaffected because it is deshielded in conformer ap1 and shielded in conformer ap3 (Figure 103a). The magnitude of the shielding/deshielding effects on substituent L2 depends on the difference in the populations of the two conformers. Similarly, in the (R)-MTPA amide, substituent L2 is shielded in conformer sp1 and substituent L1 is almost unaffected (deshielded in conformer ap1 and shielded in conformer ap3) (Figure 103b).
11 (Figure 87) were prepared by Claisen rearrangement and the relative stereochemistries (threo or erythro) were deduced by comparison of 1H and 13C NMR spectra (see Table 10) and application of the YasunaraYamaguchi method that established the configuration at C-25. Similar NMR studies of MTPA esters showed that there was a correlation between the shape of the signals due to the CH2OH (C-26) group in the (R)and (S)-MTPA esters and the absolute configuration at C-25. 65 It can be seen from Figure 88 that, in the spectra of the MTPA esters of a 25S isomer, the C-26 methylene protons resonate much closer to each other in the spectrum of the (R)-MTPA ester than in the (S)-MTPA derivative, whereas the reverse situation occurs for the MTPA esters of a 25R isomer.
Chemical-shift differences (13C NMR, ∆δC) in pyridine-d5 (∆δCp) and CDCl3 (∆δCc) of β-D- and β-Lfucofuranosides. CDCl3 data given in parentheses. β-position are considered; because the signs for the other C atoms are unreliable, the absolute values are very small and, furthermore, the degree of substitu- Assignment of Absolute Configuration by NMR Chemical Reviews, 2004, Vol. 104, No. 1 55 Table 8. The “b-pl” Rules for the Assignment of Configuration from Noe Lactols ∆δ ) δHAB - δHAA Figure 78.