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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.