Evaluation of the use of mandelate derivatives to determine the enantiomeric purity and the absolute configuration of secondary cyclohexenols

Enantiomerically pure chiral auxiliary agents are often used in NMR spectroscopy to facilitate the differentiation of enantiomers. Chiral derivatizing agents are covalently bound to the substrate and differences in chemical shifts of the resulting diastereomeric complexes are used in the analysis. Macrocycles such as cyclodextrins, crown ethers, and calix[4]resorcinarenes are chiral solvating agents that associate with the substrate through non-covalent interactions. Enantiomeric differentiation occurs in the NMR spectrum because of the diastereomeric nature of the associated complexes and/or because of the differences in association constants between the two enantiomers and the chiral reagent. Metal complexes are Lewis acids that bind to suitable Lewis base donor compounds. Exchange of substrate can be slow or fast depending on the particular metal ion, mimicking the behavior of a chiral derivatizing or solvating agent, respectively. Chiral liquid crystals undergo a partial alignment in an applied magnetic field and enantiomers dissolved in the liquid crystal undergo a partial alignment as well. If the alignment of the two enantiomers is different, enantiomeric differentiation can potentially be observed by differences in chemical shifts, differences in dipolar coupling constants, and different magnitudes of splitting of quadrupolar nuclei such as deuterium. The chiral reagents described herein can be used to determine enantiomeric composition and sometimes to assign absolute configuration. Significant discoveries as well as recent findings with each of these types of systems are described.

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Alkylhalovinylboranes: a new class of Diels–Alder dienophiles

Pisano

Pellegrinet

2018

RSC Adv.

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The Diels-Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally. Alkylhalovinylboranes presented higher reactivity than the corresponding dialkylvinylboranes. Although endo/exo selectivities were high for the reactions with cyclopentadiene, facial selectivities for the chiral analogues were low. Our results demonstrate that the replacement of an alkyl group on the boron atom by a halogen increases the dienophilicity considerably.Scheme 1 Synthesis and Diels-Alder reaction of chiral dialkylvinylborane 1. † Electronic supplementary information (ESI) available: Experimental section, NMR spectra, computational methods, FMOs and global and local reactivity indices of reactants, optimized geometries of the transition structures (TSs) not included in the paper and charge transfer (GEDT) of TSs, cartesian coordinates, absolute energies including zero-point energy corrections, free energies and number of imaginary frequencies of all the stationary points reported in the paper and values of imaginary frequencies of all transition structures. See

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Evaluation of the use of mandelate derivatives to determine the enantiomeric purity and the absolute configuration of secondary cyclohexenols

Cited by 3 publications

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Chiral Derivatizing Agents, Macrocycles, Metal Complexes, and Liquid Crystals for Enantiomer Differentiation in NMR Spectroscopy

Alkylhalovinylboranes: a new class of Diels–Alder dienophiles

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