Calixarene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared with the behavior of a monofunctional model compound, the catalytic superiority of the calixarene derivatives points to a high level of cooperation between catalytic groups. Combination of acidity measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator. The additional guanidinium (guanidine) group in the diprotonated (monoprotonated) trifunctional calixarene acts as a more or less innocent spectator. This is not the case with the tetrasubstituted calixarene, whose mono-, di-, and triprotonated forms are slightly less effective than the corresponding di- and triguanidinocalixarene derivatives, most likely on account of a steric interference with HPNP caused by overcrowding.
Upper Rim Guanidinocalixarenes as Artificial Phosphodiesterases / Laura Baldini;Roberta Cacciapaglia;Alessandro Casnati;Luigi Mandolini;Riccardo Salvio;Francesco Sansone;Rocco Ungaro. - In: JOURNAL OF ORGANIC CHEMISTRY. - ISSN 0022-3263. - 77(2012), pp. 3381-3389. [10.1021/jo300193y]
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