STRUCTURAL STUDIES OF SOME 5-(PERFLUOROALKENYL) URACILS

Structures of the most stable conformers of derivatives of 5-vinylurac il and fluorinated analogues have been determined using MM and semiemp irical MNDO calculations. It has been found that the most stable confo rmers of 5-trifluorovinyl and 5-E/Z-pentafluoropropenyl groups are alm ost perpendicular to the plane of the uracil ring. These findings are in good agreement with X-ray data obtained for 1,3,6-trimethyl-5-trifl uorovinyluracil. The calculated energy barrier for rotation of exocycl ic 5-substituents depends on the structure of the fluorinated fragment s as well as on the nature of the substituent at the 6-position of ura cil. A methyl group at the 6-position causes steric hindrance and sign ificantly disturbs the rotation of fluorinated groups.