The cation complexing abilities of a series of p-tert-butylcalix[4]arenes bearing ligating ester groups in the cone conformation have been assessed by stability constant measurements in methanol and extraction studies from water into dichloromethane. The cations studied were Na+ and K+ and variations in the ester function (C0,R) included R = methyl, ethyl, n-butyl, tert-butyl, benzyl, p henyl, p henacyl, met hoxyet hyl , trif luoroet hyl, met hylt h ioet hyl and prop-2ynyl. The effect of replacing one or two ester functions in the tetraethyl ester by methyl ester, carboxylic acid, ketone and amide functions was also studied. Selectivities for Na' relative to K+ in stability constants range from 2 to 2500, the phenacyl derivative having the highest selectivity. X-Ray diffraction analysis was used to probe the conformation of the trifluoroethyl ester 11. Crystals of I 1 are monoclinic, space group P2,/n, in a cell of dimensions a = 13.987( 2), b = 16.194(3), c = 27.630(5) A; p = 98.70(1)"; R = 0.077 for 3172 observed data. The compound possesses a distorted cone conformation.

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