SYNTHESIS, COMPLEXING PROPERTIES AND MOLECULAR MODELING OF OPEN-CHAINRECEPTORS OF BARBITURATES DERIVED FROM 2,6-DIAMINO PYRIDINE

Three new derivatives of 2,6-diacyldiaminopyridine are reported. NMR s hift titrations were performed in CDCl3 with barbiturates. The diamide 1 affords a greater complexation energy (-13.00 kJ mol(-1)) with beme gride than the dithioamide 2 (-9.15 kJ mol(-1)). This result, unexpect ed on the basis of the proton acidities, is explained by the great tor sion energy induced in 2 by the bulky sulfur atom. Compounds 3 and 4 p resent unusual four and five H-bond features with barbital and relativ ely weak complexation energies (-9.53 and -16.34 kJ mol(-1), respectiv ely). Molecular mechanics indicates that ligand 4 displays a helical s econdary structure which is disrupted by complexation. Calculations of the H-bond energies (Delta E(calc.)) of the intermolecular assemblies with barbital or phenobarbital and other host-guest complexes given i n the literature give a good correlation (r = 0.98) with experimental values: Delta E(calc.) = 1.07 Delta G(a)-42.0. Limitations of this rel ation are discussed.