Preparation of pyridino-crown ether-based new chiral stationary phases and preliminary studies on their enantiomer separating ability for chiral protonated primary aralkylamines

Abstract

This paper reports the preparation and testing of three new pyridino-18-crown-6 ether-based chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20. Secondary amine (S,S)-7 was first transformed to triethoxysilyl derivative (S,S)-11, which contains a urea unit by treating the former with 3-(triethoxysilyl)propyl isocyanate. Next, (S,S)-11 was heated with spherical HPLC quality silica gel in toluene to obtain (S,S)-CSP-12. In order to acetylate the 3-aminopropylsilyl groups bonded to the silica gel during immobilization of the triethoxysilyl derivative (S,S)-11, we pumped acetic anhydride and triethylamine in DMF through the column to give the modified chiral stationary phase (S,S)-CSP-20.Triflate (S,S)-13 was first transformed to a pyridino-18-crown-6 ether derivative (S,S)-15, which contains a 4-(methoxycarbonyl)phenyl substituent at the 4-position of the pyridine ring by a Suzuki carbon–carbon coupling reaction. The hydrolysis of ester (S,S)-15 gave carboxylic acid (S,S)-21. Carboxylic acid (S,S)-21 was reacted with an excess of thionyl chloride to form the appropriate acyl chloride, which was treated with 3-(triethoxysilyl)propylamine in the presence of triethylamine in THF to furnish triethoxysilyl derivative (S,S)-16 containing an amide unit. Triethoxysilyl derivative (S,S)-16 was heated with spherical HPLC quality silica gel in toluene to give the chiral stationary phase (S,S)-CSP-17.The enantiomer separating ability of chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20 were tested by using mixtures of enantiomers of 1-(1-naphthyl)ethylamine hydrogen perchlorate (1-NEA), 1-(2-naphthyl)ethylamine (2-NEA), 1-(4-bromophenyl)ethylamine (Br-PEA) and 1-(4-nitrophenyl)ethylamine hydrogen chloride (NO2-PEA). Chiral stationary phase (S,S)-CSP-17 showed the best enantiomer separating ability for the mixtures of enantiomers of amine compounds amongst the pyridino-crown ether-based CSPs ever synthesized. The high enantioselectivity is probably due to the strong π–π interaction of the extended π system of the aryl–substituted pyridine unit.