Cyclic Urea-Based Heterocalixarenes

The urea unit in the cyclic urea component of these heterocalixarenes is capped with -(CH2)2-, -(CH2)3- and -CH2-N(Bu )-CH2- moieties. In a straightforward synthetic approach, the bisanions of cyclic ureas, tetrahydropyrimidin-2-one (88JCS(P1)13), 5-t-butyltetrahydro-l,3,5-triazin-2-one (92TL1021, 95JOC6946) and imidazolidin-2-one (92TL1021) on reaction with 1,3-Z)A(bromomethyl)benzene 

Uracil, benzimidazol-2-one, and quinazolin-2,4-dione units constitute sub-cycles of these heterocalixarenes where, unlike the flrst category, the urea moiety is extensively conjugated. This category represents both odd- and even-membered heterocalixarenes having varied combinations of cyclic ureas and arenes and also of methylene and carbonyl bridges. 

Trimeric heterocalixarenes 170, 171 and 172 characterized by one cyclic urea, two arenes and a carbonyl bridge have been obtained by TEA HSO4 catalysed condensations of uracils, benzimidazolin-2-ones and quinazolin-2,4-dione with the dibromide 169 formed by A -bromosuceinamide bromination of benzophenone derivative 168, in turn obtained by Friedel-Crafts aroylation of 2-methylanisole with 

3-methylbenzoyl chloride (2000JCS(P1)2295). In case of the uracil-based systems, of the two possibilities 170 and 173, structures 170 have been confirmed by NOE (nuclear overhauser effect) studies and single crystal X-ray analysis. However, structure 172 in the case of the quinazolin-2,4-dione based system, has been assigned by analogy only. 

In the H-NMR spectrum of 17, a bridged CH2 shows two AB quartets and one OAc appears upheld. Similar prohles of H-NMR spectra reveal inward flattened partial cone conformations in solution for all 176 having OAc or OMe at C-2 of phenylene attached at N-l, iV-1 of uracil. But 176a, 176c and 176j which have H or OH at this site and show broad signals for -CH2, have flexible structures in solution. However, their variable temperature H-NMR studies show the existence of two or more conformers which equilibrate at room temperature. 

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Based on the rational of binding modes of 18-crown-6, heterocalixarenes 185-188 were designed for RNHs" /K binding selectivity. Their synthesis involved a straightforward condensation of respective trinuclear systems 175 and 177 (R, R = H) with the dibromides 183 and 184, formed from the corresponding cyclic urea and l,3- )w(bromomethyl)benzene (2000MI2). 

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