Laurine A. LaPlanche

1H-, 13C- and 31P-NMR studies of dioctanoylphosphatidylcholine and dioctanoylthiophosphatidylcholine

Coupling constants and chemical shifts were measured for dioctanoylphosphatidylcholine and its thio analogue in a CDCl3/CD3OD solvent mixture. Replacing the bridging oxygen atom of the CH-CH2-O-P portion of the phosphatidylcholine molecule with a sulfur atom affects chemical shifts and coupling constants in the glycerol backbone portion of the molecule as well as in the choline head group region. Preferred conformations about selected bonds in the phospholipids were determined from the vicinal 1H-1H, 31P-1H and 31P-13C coupling constants. A reduction of the 31P T2* (effective spin-spin relaxation time) for the thio analogue, as well as changes in the relative chemical shifts of 13C nuclei in the acyl chains, suggest a somewhat greater degree of aggregation for the thio analogue. The quadrupolar coupling constant 1J(14N-13C) for the choline methyls of either analogue, however, indicates that aggregation of these phospholipids in the CDCl3/CD3OD solvent mixture is not significant. Differences in conformation between dioctanoylphosphatidylcholine and its thio analogue may be responsible for their differences in chemical and physical properties.

NMR Studies of Hindered Rotation of Unsubstituted Bridgehead Phenyl Rings in the Phencyclone - Norbornadiene Adduct

Abstract NMR studies of the Diels-Alder adduct of phencyclone with norbornadiene have provided evidence for hindered rotation of the two unsubstituted bridgehead phenyl groups at ambient temperatures. In d6-DMSO solution, all 14 of the unique proton signals of this molecule are resolved at 200.1 MHz. 1H chemical shift assignments have been made using the COSY two-dimensional homonuclear shift correlation experiment.

Assignment of 13C Resonances in the Phencyclone-Norbornadiene Adduct Via 2D NMR. 13C Evidence for Hindered Rotation of Unsubstituted Bridgehead Phenyl Rings

Abstract 13C NMR chemical shift assignments were obtained for the Diels-Alder adduct of phencyclone with norbornadiene in CD2Cl2 and in CDCl3 solution. The 13C spectrum at 50.3 MHz, as well as the 1H spectrum at 200.1 MHz, show evidence for hindered rotation of the two unsubstituted bridgehead phenyl rings of the adduct at ambient temperatures. In CD2Cl2 solution, all 19 of the unique 13C nuclei of this molecule give rise to individual 13C resonances. The 1H assignments which were made earlier, together with one-bond and long-range 2D heteronuclear correlation experiments, allowed the assignment of all 13C chemical shifts in the molecule.

Unusual Hindered Rotation of an Unsubstituted Phenyl Group. Variable Temperature 1H NMR Studies and Preliminary 13C Assignments in Ketazolam

Abstract An unusual hindered rotation of an unsubstituted phenyl group in the drug, ketazolam, has been found at ambient temperature. Variable temperature lH NMR has been employed to examine the two- fold rotation about the C(sp2)-C(sp3) bond. Chemical shift as- signments of the IH NMR spectra and partial assignments of the 13C spectra are supported by off-resonance decoupling, gated decou- pling, and low temperature 2D l3C-1H heteronuclear chemical shift correlation experiments at 200.1 MHz for IH and 50.3 MHz for 13C.

LOW-TEMPERATURE TWO-DIMENSIONAL HETERONUCLEAR SHIFT CORRELATION SPECTROSCOPY OF A 1,4-BENZODIAZEPINE

Abstract The unsubstituted phenyl ring in ketazolam, a 1,4-benzodiazepine derivative, has a two-fold rotational barrier about the aryl to tert-alkyl bond connecting this phenyl group to the rest of the molecule. At 50.3 MHz, all twenty of the 13C resonances of ketazolam are resolved below -40°C, including two distinct resonances for the ortho carbons and two for the meta carbons. Twelve of the twenty 13C nuclei resonate between 138 and 125 ppm, necessitating assignment via a combination of one-bond and long-range heteronuclear shift correlation experiments.

Improved Method for Optical Purity Determination of Ketazolam with Chiral Shift Reagent

Abstract An improved method for direct determination of optical purity of the novel benzodiazepine analog, ketazolam, 1, is reported. Adding the chiral lanthanide NMR shift reagent, tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato] ytterbium (III), Yb(HFC)3, to racemic ketazolam in CDCl3 solution allows observation of enantiomeric shift differences for six of the ketazolam proton resonances. The C(2) methyl 1H signal is especially appropriate for the determination of optical purity since the C(2) methyl resonance of each enantiomer are clearly resolved (for accurate peak intensity determination) at 200 MHz. For racemic 1, using a molarratio of Yb(HFC)3:1 of 0.20, the valley height between the peaks of the two enantiomers′ CCH3 signals was only 4.8% of the average peak heights. Detection of as little as 2–3% of a minor enantiomer should be possible. Some aspects of the hindered phenyl rotation in 1 and the LSR-induced broadening of certain NMR signals are also discussed.

NMR Studies of Drugs. 1H and 13C NMR Chemical Shift Assignments in Etidocaine and Etidocaine Hydrochloride Determined by Two-Dimensional NMR Spectroscopy

Abstract 1H and 13C NMR chemical shift assignments were obtained for the local anesthetics etidocaine (1) and etidocaine hydrochloride (2) in CDCl3 solution, as well as for 2 in D2O solution. The COSY experiment was employed for proton-proton correlation, while onebond and long-range 2D heteronuclear techniques allowed the assignments of all 13C chemical shifts in each molecule. Etidocaine has a chiral carbon; etidocaine hydrochloride has, in addition to the natural chiral center, an acid-induced chirality at the protonated amine nitrogen, resulting in solvent-dependent diastereomers. Ten of the fourteen magnetically nonequivalent 13C nuclei of 2 exhibit doubled 13C resonance peaks (50.3 MHz, 20°C, CDCl3 solution) due to the presence of the two diastereomers.

13C NMR Chemical Shift Assignments in Dibucaine and Dibucaine Hydrochloride Determined by Two-Dimensional NMR Spectroscopy

Abstract 13C NMR chemical shift assignments were obtained for dibucaine in CDCl3 and dibucaine hydrochloride in D2O solution using one-bond and long-range two-dimensional heteronuclear correlation techniques. The 13C chemical shift assignments, together with the fully coupled 13C spectrum, allowed the measurement of one-bond, and long-range 13C1H coupling constants.

Calculation of steric interactions between a lanthanide shift reagent and substituted pyridines

Steric interactions between a lanthanide-shift reagent (LSR) and ligands have been examined in detail by calculating the intramolecular non-bonded energy of interaction in Eu(dpm)3py2, (where py = pyridine or one of its methyl-substituted derivatives) as py is translated and/or rotated with respect to the remainder of the Eu(dpm)3py complex. The various calculations performed indicate that pyridine and 3,5-dimethylpyridine bind well to the LSR with a minimum amount of steric interference in the crystal conformation; however, for 2-methylpyridine and especially for 2,6-dimethylpyridine, steric interference of the methyl groups with the LSR are significant. Furthermore, the non-bonded energy of interaction becomes very high when the unsubstituted pyridine ring is moved away from its crystal conformation. This shows that only a small pocket is available for the pyridine molecule or its derivatives in the octahedral LSR complex. The lanthanide-shift reagent studied here is widely used in conformational analysis by n.m.r.; our results show that steric interactions between this LSR and substrate molecules can be severe and hence may be expected to cause conformational alterations of some types of flexible ligands.

1H and 13C NMR Studies of Etidocaine Free Base and Hydrochloride Salt. Solvent and Temperature Dependence

Abstract The 1H and 13C NMR spectra of etidocaine free base, 1, and its hydrochloride salt, 2, have been examined at 200 MHz in a variety of solvents. Variable temperature 1H NMR spectra of 2 were measured between 0° and 90°C at 500 MHz. Both 1H and 13C spectra of the salt were strongly dependent on temperature, solvent and spectrometer frequency. Results are interpreted primarily as a consequence of acid-induced chirality at the amine nitrogen, resulting in solvent-dependent diastereoisomerism. Spectral complexity is explained as the result not only of diastereotopic nuclei due to the fixed “natural” chiral center at carbon, but also, in 2, to the presence of diastereomeric racemates resulting from the second chiral center (at nitrogen).