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The Assignment of the Absolute Configuration by NMR using Chiral Derivatizing AgentsA Practical Guide$
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Josi M. Seco, Emilio Quiqoa, and Ricardo Riguera

Print publication date: 2015

Print ISBN-13: 9780199996803

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199996803.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 27 October 2021

Assignment of the Absolute Configuration of Polyfunctional Compounds

Assignment of the Absolute Configuration of Polyfunctional Compounds

(p.111) 5 Assignment of the Absolute Configuration of Polyfunctional Compounds
The Assignment of the Absolute Configuration by NMR using Chiral Derivatizing Agents

Josi M. Seco

Emilio Quiqoa

Ricardo Riguera

Oxford University Press

From a practical point of view, the assignment of the absolute configuration of sec/sec 1,2- and 1,n-diols does not require the separate derivatization (two different steps with the CDA of choice) of each one of the two hydroxyl groups present in the substrate; on the contrary, it can be carried out by simultaneous derivatization of the two hydroxyls (a single step), leading to the corresponding bis-(R)- and bis-(S)-CDA esters [13, 59–61]. The most used CDAs are 9-AMA and MPA [59, 60], although 1-NMA, 2-NMA, and MTPA are also appropriate [59, 60]. This assignment has an important difference compared to that of monofunctionalized compounds [15]; this is due to the presence in the bis-(R)- and bis-(S)-derivatives of two CDA units that produce distributions of ΔδRS and ΔδSR signs that do not follow the trends found in monoderivatized compounds [13, 15, 82]. This means that the NMR spectra of the bis-CDA derivatives cannot be interpreted as if they had originated from two isolated mono-CDA derivatives [82]. Thus, the correlations described for secondary alcohols [35–39] cannot be applied to diols [59–61] because the chemical shifts and ΔδRS values result from the combination of the anisotropic effects—usually shielding—from the two CDA units and not from a single unit, as happens with monoalcohols. A result of the combination of aromatic shielding effects [59, 60] in diols is that the diagnostic protons/signals for assignment are not always the same as in isolated monoalcohols (i.e., L1/L2). For instance, in acyclic syn-1,2-diols, the diagnostic signals [59, 60] are those corresponding to the protons at the alpha positions of the OH groups (i.e., the hydrogens linked directly to the asymmetric carbons) Hα(R1) and Hα(R2) exclusively. On the other hand, in acyclic anti-1,2-diols, the diagnostic signals are from Hα(R1)/Hα(R2) together with those from R1 and R2. As in the case of monofunctional compounds, the assignment consists [13, 59, 60] in the preparation of two bis-CDA derivatives from the two enantiomers of the chosen CDA, followed by comparison of the corresponding NMR spectra and calculation of the ΔδRS (or ΔδSR in the case of MTPA) signs for Hα(R1), R1, Hα(R2), and R2.

Keywords:   correlation model, diagnostic signals, polyfunctional compounds, shielding effects, structural types, validation structures

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