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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: 26 February 2021

Practical Aspects of the Preparation of the Derivatives

Practical Aspects of the Preparation of the Derivatives

Chapter:
2 Practical Aspects of the Preparation of the Derivatives
Source:
The Assignment of the Absolute Configuration by NMR using Chiral Derivatizing Agents
Author(s):

Josi M. Seco

Emilio Quiqoa

Ricardo Riguera

Publisher:
Oxford University Press
DOI:10.1093/oso/9780199996803.003.0005

Most of the NMR spectra shown in this book and in the literature have been recorded at 250 or 300 MHz, with a few being obtained at 500 MHz for 1H NMR (the equivalent for 13C NMR). No special pulse sequences are necessary, just standard one-dimensional (1D) spectra although two-dimensional (2D) experiments (e.g., correlation spectroscopy; COSY) may be necessary in some cases in order to get an unambiguous identification of the signals relevant for the assignment. In general, 5–10 mg or less of CDA derivative dissolved in 0.5 mL of deuterated solvent are sufficient to obtain a good NMR spectrum. Temperature, solvent, and concentration used in the NMR experiments should be adequate for each CDA-substrate pair and methodology, because the method is based on the conformational composition of the AMAA derivatives in precise conditions. With the exception of the low-temperature procedure (single derivatization), a NMR probe temperature around 300 K has always been used. In general, the spectra for double-derivatization assignments should be taken in deuterated chloroform. Different NMR solvents are required only in two of the single-derivatization methods. In the assignment by low-temperature NMR, the most convenient solvent is a CS2/CD2Cl2 (4:1) mixture, which allows the use of temperatures low enough (i.e., 213 K) to obtain relevant shifts. In the procedure based on the complexation with Ba2+, the NMR solvent should be deuterated acetonitrile. The barium salt is anhydrous Ba(ClO4)2, which can be added directly to the tube by using a spatula. No weighing is necessary after shaking, as the excess salt will remain at the bottom of the NMR tube and will not disturb the experiment. (R)- and (S)-MPA, MTPA, and Boc-phenylglycine (BPG) are commercially available and can be used without further purification. The first two (MPA and MTPA) can also be purchased as acid chlorides. When using MTPA or the corresponding acid chloride [85] for the derivatization of an alcohol or amine, it should be noted that the Cahn-Ingold-Prelog priority rules assign different R/S descriptors to the acid and to the corresponding chloride; this is due to the different priority order generated by the substituents [i.e., (R)-MTPA generates the (S)-acid chloride and vice versa].

Keywords:   acid chlorides, carboxylic acids, diols, mix-and-shake, preparation, resin loading, secondary alcohols, triols

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