Enantiomeric Recognition of d- and l-Amino Acid Methyl Ester Hydrochlorides by New Chiral Bis-pyridino-18-crown-6 Substituted with Urea, and Diphenyl Groups

The article reports the synthesis and chiral recognition properties of a new chiral bis-pyridino-18-crown-6 (7), having urea, diphenyl, and allyloxy groups. The chiral bis-pyridino-18-crown-6 was prepared by a thirteen-steps procedure from the commercially available (S)-(+)-mandelic acid and chelidamic acid. The association constants (K _a) (1.33 × 10³–3.20 × 10³) for enantiomeric recognition of d- and l-amino acid methyl ester hydrochlorides using the chiral bis-pyridino-18-crown-6 have been examined by ¹H-NMR titration method in CDCl₃ at 25 °C. The chiral bis-pyridino-18-crown-6 showed higher association constants for the d-series amino acid methyl ester (d-AlaOMe, d-LeuOMe, d-MetOMe) hydrochlorides as compared to the corresponding l-series (l-AlaOMe, l-LeuOMe, l-MetOMe) hydrochlorides.