Using NMR spectroscopic methods to determine enantiomeric purity and assign absolute stereochemistry

Discrimination of α-amino acids using green tea flavonoid (-)-epigallocatechin gallate as a chiral solvating agent

We report a special, hitherto-unexplored property of (-)-epigallocatechin gallate (EGCG) as a chiral solvating agent for enantiodiscrimination of α-amino acids in the polar solvent DMSO. This phenomenon has been investigated by (1)H NMR spectroscopy. The mechanism of the interaction property of EGCG with α-amino acids has been understood as arising out of hydrogen-bonded noncovalent interactions, where the -OH groups of two phenyl rings of EGCG play dominant roles. The conversion of the enantiomeric mixture into diastereomers yielded well-resolved peaks for D and L amino acids permitting the precise measurement of enantiomeric composition. Often one encounters complex situations when the spectra are severely overlapped or partially resolved hampering the testing of enantiopurity and the precise measurement of enantiomeric excess (ee). Though higher concentration of EGCG yielded better discrimination, the use of lower concentration being economical, we have exploited an appropriate 2D NMR experiment in overcoming such problems. Thus, in the present study we have successfully demonstrated the utility of the bioflavonoid (-)-EGCG, a natural product as a chiral solvating agent for the discrimination of large number of α-amino acids in a polar solvent DMSO. Another significant advantage of this new chiral sensing agent is that it is a natural product and does not require tedious multistep synthesis unlike many other chiral auxiliaries.

Determination of absolute configuration and conformation of a cyclic dipeptide by NMR and chiral spectroscopic methods

Increasing precision of contemporary computational methods makes spectroscopies such as vibrational (VCD) and electronic (ECD) circular dichroism attractive for determination of absolute configurations (AC) of organic compounds. This is, however, difficult for polar, flexible molecules with multiple chiral centers. Typically, a combination of several methods provides the best picture of molecular behavior. As a test case, all possible stereoisomers with known AC (RS, SR, SS, and RR) of the cyclic dipeptide cyclo(Arg-Trp) (CAT) were synthesized, and the performances of the ECD, infrared (IR), VCD, Raman, Raman optical activity (ROA), and nuclear magnetic resonance (NMR) techniques for AC determination were investigated. The spectra were interpreted with the aid of density functional theory (DFT) calculations. Folded geometries stabilized by van der Waals and electrostatic interactions between the diketopiperazine (DKP) ring and the indole group are predicted to be preferred for CAT, with more pronounced folding due to Arg-Trp stacking in the case of SS/RR-CAT. The RS/SR isomers prefer a twist-boat puckering of the DKP ring, which is relatively independent of the orientation of the side chains. Calculated conformer-averaged VCD and ECD spectra explain most of the experimentally observed bands and allow for AC determination of the tryptophan side-chain, whereas the stereochemical configuration of the arginine side-chain is visible only in VCD. NMR studies provide characteristic long-range (2)J(C,H) and (3)J(C,H) coupling constants, and nuclear Overhauser effect (NOE) correlations, which in combination with either ECD or VCD also allow for complete AC determination of CAT.

Analytical contribution of NAD 2D-NMR spectroscopy in polypeptide mesophases to the investigation of triglycerides

In this work, we report and discuss on the use and limitations of the natural abundance deuterium two-dimensional NMR spectroscopy in polypeptide chiral and achiral aligning media in the studies of homogenous triglycerides at 14.1 T. As illustrative examples, two triglycerides with short and long alkyl chains were investigated: the 1,3-di(butanoyloxy)propan-2-yl butanoate or tributyrin (TB) and the 1,3-di(tetradecanoyloxy)propan-2-yl tetradecanoate or trimyristin (TM). If both flexible compounds are theoretically of C(s) symmetry on average, according to the Altmann's definition (Proc. Roy. Soc., 1967, A298, 184.), the analysis of spectral data in terms of enantiotopic and diastereotopic discriminations shows noticeable differences related to their orientational ordering behavior inside the mesophases. Although from NMR analysis viewpoint, TB behaves as a C(s) symmetry molecule as expected, the NMR results obtained for TM suggest a behavior that could be formally predicted for a C(3v) symmetry molecule on average. This conclusion was nicely supported by the comparison with the tri-n-propylorthoformate, a real C(3v) symmetry solute on average on the NMR timescale. This difference of effective orientational behavior could originate from the difference of size and shape between lateral and central alkyl chains of the solute molecule.

The use of exchangeable nuclei to observe enantiomers through deuterium NMR in chiral liquid crystalline solvents

It is demonstrated that (2)H NMR in chiral liquid crystalline solvents can be used to measure enantiomeric excesses using exchangeable deuterons in alcohols. This is performed in a trivial way at low temperature (260-270 K) where a slow exchange regime was reached. Among the various alcohols used to explore the possibilities of this technique, an unusually large isotopic effect on molecular orientation between two isotopomers has been observed.

Phenylalanine-based polyarylacetylenes as enantiomer-differentiating alignment media

Lyotropic liquid crystalline phases of a phenylalanine-based polyacetylene are introduced as new enantiodifferentiating alignment media. Based on the unusual temperature dependence of the quadrupolar splitting of the (2)H-signal of the solvent (CDCl(3)), three distinct states of the phase with different orientational properties can be identified. This offers the opportunity to measure multiple alignment data sets without changing the sample. Unexpectedly, the largest difference in the orientation of the enantiomers of isopinocampheol was found in the high temperature domain of the phase. This is even more astonishing because the helical structure of the polymer backbone seems to break down at temperatures above approximately 10 °C, leaving the stereogenic centers of the amino acid moieties in the repeating units as the only reason for the enantiodifferentiation.

The structure of MLn(hfbc)4 and a key to high circularly polarized luminescence

The heterobimetallic complex CsEu[(+)-hfbc](4) (hfbc = 3-heptafluorobutyryl camphorate), prepared by Kaizaki and co-workers, displays the highest ratio of polarization versus total luminescence (measured by the g(lum) factor), i.e., ~85% of the emitted photons at 595 nm are left-circularly polarized. We present a detailed structural analysis in solution, based on paramagnetic nuclear magnetic resonance (NMR), and discuss the possible dynamic processes, where its analogues are involved. We demonstrate that the first coordination sphere is very close to the achiral regular square antiprism (SAPR) with D(4d) symmetry, which rules out the intrinsic dissymmetry of the Eu environment for rationalizing the g(lum). In contrast, the dynamic coupling between the f-f transitions, responsible for the emission, to the ligand-centered π-π* transition at 310 nm displays almost ideal geometry to justify g(lum) and discloses a key to high circularly polarized emission.

Two new nonacosanetriols from Ginkgo biloba sarcotesta

Two new fatty alcohols named as (7S,8R,11S)-nonacosanetriol (1) and (10R,12R,15S)-nonacosanetriol (2), along with eight known compounds including ginkgolic acid (3), hydroginkgolic acid (4), sciadopitysin (5), ginkgetin (6), isoginkgetin (7), ginkgolide A (8), ginkgolide B (9) and ginkgolide C (10) have been isolated from the petroleum ether extract of Ginkgo biloba sarcotesta. Their structures were elucidated by means of chemical and extensive spectroscopic analysis. The absolute stereochemistry of compounds 1 and 2 was elucidated on the spectroscopic analysis of the R- and S-MTPA esters. Compounds 1 and 2 exhibited slight activity of antithrombin and moderate activity of antiplatelet aggregation in vitro. This was the first report regarding the anticoagulative activities of biflavonoids in G. biloba, and isoginkgetin (7) showed significant antithrombin and antiplatelet aggregation activity.

Synthesis and NMR characterization of (Z,Z,Z,Z,E,E,ω)-heptaprenol

We describe a practical, multigram synthesis of (2Z,6Z,10Z,14Z,18E,22E)-3,7,11,15,19,23,27-heptamethyl-2,6,10,14,18,22,26-octacosaheptaen-1-ol [(Z(4),E(2),ω)-heptaprenol, 4] using the nerol-derived sulfone 8 as the key intermediate. Sulfone 8 is prepared by the literature route and is converted in five additional steps (18% yield from 8) to (Z(4),E(2),ω)-heptaprenol 4. The use of Eu(hfc)(3) as an NMR shift reagent not only enabled confirmation of the structure and stereochemistry of 4, but further enabled the structural assignment to a major side product from a failed synthetic connection. The availability by this synthesis of (Z(4),E(2),ω)-heptaprenol 4 in gram quantities will enable preparative access to key reagents for the study of the biosynthesis of the bacterial cell envelope.

Simple and efficient methods for discrimination of chiral diacids and chiral alpha-methyl amines

The three-component chiral derivatization protocols have been developed for (1)H, (13)C and (19)F NMR spectroscopic discrimination of chiral diacids by their coordination and self-assembly with optically active (R)-α-methylbenzylamine and 2-formylphenylboronic acid or 3-fluoro-2-formylmethylboronic acid. These protocols yield a mixture of diastereomeric imino-boronate esters which are identified by the well-resolved diastereotopic peaks with significant chemical shift differences ranging up to 0.6 and 2.1 ppm in their corresponding (1)H and (19)F NMR spectra, without any racemization or kinetic resolution, thereby enabling the determination of enantiopurity. A protocol has also been developed for discrimination of chiral alpha-methyl amines, using optically pure trans-1,2-cyclohexanedicarboxylic acid in combination with 2-formylphenylboronic acid or 3-fluoro-2-fluoromethylboronic acid. The proposed strategies have been demonstrated on large number of chiral diacids and chiral alpha-methyl amines.

Chiral discrimination of aliphatic amines and amino alcohols using NMR spectroscopy

Two methods are compared for analyzing the enantiomeric purity of aliphatic amines and amino alcohols using NMR spectroscopy. The first employs (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral NMR solvating agent in methanol-d(4) . The second involves a derivatization scheme in which the amine is reacted with naphtho[2,3-c]furan-1,3-dione to form the corresponding amide. The naphthyl amide is then mixed with a chiral calix[4]resorcinarene in deuterium oxide. The crown ether only produces sufficient enantiomeric discrimination to determine enantiomeric purity for three of the nine substrates studied. The system with the naphthyl amide and a calix[4]resorcinarene produces enantiomeric discrimination of sufficient magnitude to determine enantiomeric purity for all nine substrates. The H1 and H4 resonances of the naphthyl ring are especially suitable to monitor for enantiomeric discrimination. The order of the (R)- and (S)-enantiomers of the H1 and H4 resonances exhibit specific trends for aliphatic amines and amino alcohols that correlate with the absolute configuration.