CHIRAL CALIX[4]ARENE

A cone-shaped, asymmetrically substituted p-tert-butylcalix[4]arene has been synthesized and optically resolved. The oxygen-through-the-annulus rotation which causes racemization of these ring-originating optical isomers is suppressed by four O-propyl substituents. In the synthesis the use of Ba(OH)2 as the base for the O-propylation is a key point: it affords only a cone-shaped, conformationally immobile tri-O-propylated p-tert-butylcalix[4]arene. After O-substitution with the fourth propyl group, the racemic product can be optically resolved by HPLC. This is the first example of a successful optical resolution of an asymmetrically substituted calix[4]arene.