Cystine-based symmetrical cyclic oligoureas, synthesized in a one-pot reaction from L-cystine dimethyl ester and triphosgene, are demonstrated to act as versatile neutral receptors for both inorganic and organic anions operating exclusively through hydrogen bonds; 1H NMR studies have shown that while the cyclic triurea prefers to complex with spherical (halides) and trigonal planar (nitrate) anions, the higher oligomer tetraurea can trap the tetragonal planar squarate dianion with modest efficiency.
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Ranganathan et al. (2001) studied this question.
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