The synthesis of 5-[(2, 4-dioxo-6-methyl-5, 6, 7, 8-tetrahydropyrido [3, 2-d] pyrimidin-5-yl) methy1]-2r-deoxyuridine and a number of related compounds as models of potent inhibitors of thymidylate synthetase has been accomplished. The 'H and 13C NMR spectra of certain of these molecules show unusual and remarkably stable conformational effects. A partial conformational analysis was undertaken in order to determine the origins of these effects. ...