for his valuable help in obtaining the 500-MHz NMR and high-resolution mass spectra, molecular mechanics calculations, and elemental analysis determinations. We also thank CONICET (Consejo Nacional

Dynamic 'H NMR measurements of the tetramethyl ether of p-tert-butylcalix[4]arene (2) show for the first time that all four possible conformations of one particular calix[4]arene are present, including the 1 ,balternate conformation. The thermodynamically most stable partial cone conformation readily interconverts to a cone or to a 1,3-alternate conformation; the interconversion to a 1,2-alternate conformation is much slower. The 1,2-alternate conformation of 2 is the kinetically stable conformation at the I H NMR time scale. The 1,a-alternate conformation was confirmed by comparison of its 'H NMR spectrum with that of the newly synthesized tetraethyl ether of p-tert-butylcalix[4]arene in a fixed 1,2-alternate conformation (6), of which the X-ray structure was determined. Partial rigidification of the calix[4]arene moiety in four different ways was achieved by replacing two of the methoxy groups of the tetramethyl ether 2 by ethoxy groups. The relative thermodynamic stabilities of the conformations of the calix[4]arene are influenced strongly by this relatively small change; in particular the 1,2-alternate conformation becomes much more stable. For the anti-1,3-diethyl-2,4-dimethyl ether 7b the 1,2-alternate is even the thermodynamically most stable conformation. Molecular mechanics calculations indicate that this is caused by the combined favorable effects on the electrostatic energy of the inside orientation of the methoxy groups and the relative large distance between the two ethoxy groups. The tetraethyl ether of p-tert-butylcalix[4]arene is not flexible at room temperature, but it equilibrates in solution at temperatures above 100 "C to a mixture of also all the four possible conformations.

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Methods are described for the Selective diametrical functionaCization of calix[4]arenes at the uppeT@ trarufer of functionality and selective substitutions at the para positions of the phenol rings. The crystal structure of 26,28-dimethoxy-11,23-dinitrocalix[4]arene is described. Calix[4]arenes1 are able to form host-guest complexes in the solid state2 as well as in solution3. Most work has been directed to the complexation of cations in the 'polyoxygen' cavity (lower rim)4. Recently, we have used 26,28-dimethoxy-l1,23-bis[(dimethyla~o)methyl]calix[4]a 12 could be isolated in 91% yield (mp >330

Abstract. The development of a facile, single-step method for the synthesis of 1,4-disubstituted piperazine-2,6-diones (4) in excellent yields from the corresponding aliphatic, aromatic and heteroaromatic primary amines and iminodiacetic acids (1) is described. A possible mechanism is discussed and the presence of intermediate 6a is confirmed by high-resolution proton NMR spectroscopy. The subsequent two-step conversion of dione 4a into the deoxygenated 4-unsubstituted piperazine 10 in high yield provides an attractive alternative to the currently available (heter0)arylpiperazine synthetic methods.

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The calixarene (I) is selectively substituted at the two facing OH groups, producing the bis(phenol ethers) (III).