The 1H NMR Method for the Determination of the Absolute Configuration of 1,2,3-prim,sec,sec-Triols

An ingenious method for the determination of the relative and absolute configurations of compounds containing aryl-glycerol fragments by 1H NMR spectroscopy

A concise method was established to determine the relative and absolute configurations of aryl-glycerols that depend on the chemical shift differences (Δδ) of the diastereotopic methylene protons (H-3) by ¹H NMR spectroscopy. When using DMSO-d₆ as the preferred solvent, the threo configuration corresponded to a larger Δδ_H3a–H3b value (>0.15 ppm), whereas the erythro configuration (<0.07 ppm) corresponded to a smaller value. Furthermore, the absolute configurations were determined with the aid of a simple acylation reaction through camphanoyl chloride. In the threo enantiomers, the Δδ value of the 1R,2R configuration was <0.15 ppm, and that of the 1S,2S configuration was >0.20 ppm. In the erythro enantiomers, the Δδ value of 1R,2S was >0.09 ppm, and that of 1S,2R was <0.05 ppm. Remarkably, this empirical rule is invalid in CDCl₃. In addition, this method was also verified by a quantum ¹H NMR calculation.

A concise method was established to determine the relative and absolute configurations of aryl-glycerols that depend on the chemical shift differences (Δδ) of the diastereotopic methylene protons (H-3) by ¹H NMR spectroscopy.

A chiral sensor for recognition of varied amines based on 19F NMR signals of newly designed rhodium complexes

A novel chiral octahedral rhodium complex containing fluorine has been developed to be an excellent chiral sensor for a variety of amines including diamines, monoamines, amino alcohols and amino acids, showing well distinguishable 19F NMR signals and an accurate measurement of enantiomeric determination.

Determination of the absolute stereostructure of a cyclic azobenzene from the crystal structure of the precursor containing a heavy element

Single crystal X-ray diffraction has been used as one of the common methods for the unambiguous determination of the absolute stereostructure of chiral molecules. However, this method is limited to molecules containing heavy atoms or to molecules with the possibility of functionalization with heavy elements or chiral internal references. Herein, we report the determination of the absolute stereostructure of the enantiomers of molecule (E)-2, which lacks the possibility of functionalization, using a reverse method, i.e., defunctionalization of its precursor of known stereostructure with bromine substitution (S-(-)-(E)-1). A reductive debromination of S-(-)-(E)-1 results in formation of one of the enantiomers of (E)-2. Using a combination of HPLC and CD spectroscopy we could safely assign the stereostructure of one of the enantiomers of (E)-2, the reduced product R-(-)-(E)-1.

Enantio- and diastereocontrolled conversion of chiral epoxides to trans-cyclopropane carboxylates: application to the synthesis of cascarillic acid, grenadamide and L-(-)-CCG-II

An efficient high yielding improved method for the enantio- and diastereoselective cyclopropanation of chiral epoxides using triethylphosphonoacetate and base (Wadsworth-Emmons cyclopropanation) is reported. The utility of this protocol is illustrated by concise and practical synthesis of cascarillic acid, grenadamide and L-(-)-CCG-II, a cyclopropane containing natural products.

13C NMR as a general tool for the assignment of absolute configuration

(13)C NMR, alone or in combination with (1)H NMR, allows the assignment of the absolute configuration of chiral alcohols, amines, carboxylic acids, thiols, cyanohydrins, sec,sec-diols and sec,sec-aminoalcohols, derivatized with appropriate chiral auxiliaries. This extends the assignment possibilities of NMR to fully deuterated and to nonproton containing compounds.

Relative and Absolute Stereochemistry of Secondary/Secondary Diols: Low-Temperature 1H NMR of Their bis-MPA Esters

Comparison of the room- and low-temperature 1H NMR spectra of the bis-(R)- or bis-(S)-MPA ester derivative of an open chain sec,sec-1,2-diol allows the easy determination of its relative stereochemistry and in some cases absolute configuration. If the diol is anti, its absolute configuration can be directly deduced from the signs of DeltadeltaT1T2 for substituents R1/R2, but if the relative stereochemistry of the diol is syn, the assignment of its absolute configuration requires the preparation of two derivatives (both the bis-(R)- and bis-(S)-MPA esters), comparison of their room-temperature 1H NMR spectra, and calculation of the DeltadeltaRS signs for the methines Halpha(R1) and Halpha(R2) and R1/R2 protons. The reliability of these correlations is validated with 17 diols of known absolute configuration used as model compounds.

The Assignment of the Absolute Configuration of Diethyl Hydroxy- and Aminophosphonates by 1H and 31P NMR Using Naproxen as a Reliable Chiral Derivatizing Agent

The assignment of the absolute configuration of hydroxy- and aminophosphonates by their double derivatization with commercially available naproxen is presented. The correlation between the spatial arrangement around the stereogenic carbon center and the signs of the DeltadeltaRS allows determination of the absolute configuration of hydroxy- and aminophosphonates by simple comparison of the 1H and 31P NMR spectra of the (R)- and (S)-naproxen ester or amide derivatives. Extensive conformational analysis (theoretical calculations, low-temperature experiments) supported by the NMR studies of structurally diverse naproxen esters and amides of hydroxy- and aminophosphonates proved that a simplified model can be successfully used.