Synthesis and Properties of Calixarene Analogs Modified in the Macrocyclic Ring
Kazuaki ITO, Yoshihiro OHBA and Tyo SONE*
Department of Materials Science and Engineering, Faculty of Engineering, Yamagata University;
Johnan, Yonezawa-shi 992-8510 Japan
In an attempt to introduce the site (s) available for modification into the macrocyclic ring of calixarenes, cyclic phenol-formaldehyde oligomers having such a group as carbonyl, sulfur, chiral amino acid residue (Val, Trp, Cys and Cys-S-S-Cys), or thiophene ring as bridge (s) were synthesized stepwise. The oxocalix[n]- (n=4--6) and thiacalix[4]arenes (Sn, n=1--4) form crystalline inclusion complexes with organic solvents. NMR studies (VT-NMR, 1H-NMR relaxation times (T1/s), etc.) indicated that the oxo- and thia-analogs preferentially existed in cone conformations in CDCl3, which are much more flexible than the parent calixarenes. The greater flexibility of the oxocalixarenes is ascribable to strong intramolecular hydrogen bonding between bridging CO and the neighboring phenolic OH groups; the flexibility is in the order, oxocalix[5]>[6]>[4]. Replacement of the methylene bridge by sulfur weakened the intramolecular hydrogen bonding characteristic for calixarenes. The mean T1 values for the aromatic protons in the molecules of the thiacalixarenes gave an excellent linear relationships with the strength of the hydrogen bonding (νOH, δOH) or with the energy barriers (ΔG†) of the conformational inversion of the diarylmethane units. The calixarene analogs incorporating the amino acid residue in the macrocyclic ring, as well as homoazacalixarenes with the chiral unit as side chain, were found to have chiral cavities. With the dihomoazacalixarenes constructed from the L-amino acid methyl esters and bischloromethylated p-cresole-formaldehyde tetramer, 1H-NMR chemical shift non-equivalence was observed for the methyl protons of racemic trimethyl (1-phenylethyl)ammonium iodide.
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