Synthesis and Properties of Cholesteryl Esters Bearing 32- and 16-Membered Crown Ethers

Abstract

The cholesteryl esters (3b and 3a) of 5-carboxy-1,3-phenylene-16-crown-5 and 5-carboxy-4,6-dichloro-1,3-phenylene-16-crown-5 show cholesteric liquid crystalline behavior but only upon heating samples that were rapidly cooled from the isotropic melt in the case of 3a and a monotropic phase for 3b. The dichloro compound 3a was formed by treatment of the corresponding acid 2 with SOCl2 first and then cholesterol; it is believed that the SOCl2 was contaminated with SO2Cl2, leading to the chlorination of the aromatic ring. The dichloro compound 3a was structurally characterized using single crystal X-ray diffraction. 3a crystallizes in the orthorhombic space group P212121 with unit cell parameters of a = 12.76(4) Å, b = 17.511(5) Å, and c = 18.213 Å. Use of freshly opened SOCl2 produced 3b. The reaction of the acid 2 and cholesterol in the presence of dicyclohexylcarbodiimide yielded the acylisourea 4 as the major product (64%) along with 3b (36%) upon treatment with cholesterol. The dicholesteryl ester 9 of bis(5-carboxy-1,3-phenylene)-32-crown-10 (8a) was also synthesized, and by differential scanning calorimetry (DSC) and polarized optical microscopy no liquid crystalline behavior was observed with this system. Apparently the presence of the semirigid crown in the molecule prevents the two cholesteryl moieties from organizing in independent helices. The complexation ability of this cholesteryl crown (9) with methyl viologen bis(hexafluorophosphate) ([paraquat]·2[PF6]) (12) in acetone has been examined by 1H NMR spectroscopy; it showed weaker binding than its simple dimethyl ester analog 8b.