| Popis: |
The crystal and molecular structures of (Z)-4,4',6-triacetoxy aurone ( I ) and (Z)-3',5'-di-bromo-2',4,4',6-tetrahydroxyaurone monohydrate ( II ) have been studied by X-ray analysis and AM1 molecular orbital methods. Aurones are characterized by a methine bridge between the benzofuran and phenolic rings and thus have only one flexible bond, the torsion about C(2)-C(21)-C(1')-C(6'),θ'. In the crystal both aurones have a ( Z )-planar conformation defined by θ' values of 3.1(5)° and 0.7(5)°, for ( I ) and ( II ) respectively. Minimum energy conformations from AM1 molecular orbital calculations have θ' values of 12.5° for ( I ) and 3.3° and 31.3° for ( II ), depending on the orientation of the 2'-hydroxy hydrogen. The relative barrier to rotation about θ' for all AM1 conformations is less than 4 kcal mol −1 . When H2' 1 is in the plane of the phenolic ring cis to the methine bridge, it comes within 1.5 A of the methine hydrogen and causes the crystallographic conformation to be 2 kcal mol −1 higher than the minimum energy structure at 31°. When this hydrogen ia placed trans to the methine bridge, the minimum energy conformation is 3.3°, in agreement with the X-ray results. These observations suggest that caution should be used in the interpretation of AM1 results for such a sterically crowded system. Furthermore, these data indicate that, although a planar conformation is a minimum energy structure for aurones, less than 4 kcal mol −1 is required to adopt a nonplanar conformation suggested as the active form for their action as inhibitors of the enzyme iodothyronine deiodinase. These aurones also act as competitive inhibitors for the binding of thyroxine to transthyretin, where similar conformational requirements are postulated. |
