SYNTHESES OF ALL POSSIBLE CONFORMATIONAL ISOMERS OF O-ALKYL-PARA-TERT-BUTYLCALIX[4]ARENES

In p-t-butylcalix[4]arene (1H4) and its mono-, di-, tri-, and tetra-O-alkyl derivatives (1HR3, 1H2R2, 1HR3, and 1R4 respectively), 23 different homologs can exist (including 1H4). We found that the OH group in the unmodified phenol unit is permeable through the calix[4]arene ring. Thus, several conformational isomers become equivalent after the oxygen-through-the-annulus rotation of the OH group and the number of the possible homologs is reduced to 13 (including 1H4). We here report the syntheses of all of these possible conformational isomers by using a protection-deprotection method with a benzyl group and metal template effects. The results are very useful for understanding the conformer distribution in 1H2R2, 1HR3, and 1R4, which has confused calixarene chemists for a long time and is still a matter of discussion.