title: Use of alpha-secondary isotope effects in nucleophile-promoted reactions of pyrimidine derivatives; evidence for transient 5,6-dihydropyrimidine intermediates.
abstract: alpha-Secondary isotope effect provides a useful tool for detection of transient 5,6-dihydropyrimidine intermediates which have been indirectly implicated in a number of chemical and enzymic conversions of pyrimidine heterocycles. The method involves the use of kinetic secondary alpha-hydrogen isotope effects which are expected to accompany sp2 to sp3 rehybridization of C-6 of the pyrimidine ring if they occur prior to or at the rate determining step. Thus, using 6-tritiated pyrimidines, and measurement of the isotopic ratio of reactant and products, kT/kH values of 1.15 or greater would be indicative of rehybridization. Using this method, we have studied the inhibition of thymidylate synthetase by 5-fluorodeoxyuridylate and 5-nitro-deoxyuridylate as well as the chemical and enzymic dehalogenation of 5-bromodeoxyuridine and 5-bromodeoxyuridylate. Large secondary isotope effects observed in all of these reactions provided strong evidence for the formation of 5,6-dihydropyrimidine intermediates.
mesh_terms: D001973:Bromodeoxyuridine; D055598:Chemical Phenomena; D002621:Chemistry; D003856:Deoxyuracil Nucleotides / Q000494:pharmacology; D005468:Fluorodeoxyuridylate / Q000031:analogs & derivatives / Q000494:pharmacology; D007553:Isotope Labeling / Q000379:methods; D011742:Pyrimidine Nucleotides / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors; D014316:Tritium
chemical_list: D003856:Deoxyuracil Nucleotides; D011742:Pyrimidine Nucleotides; D014316:Tritium; D005468:Fluorodeoxyuridylate; C017563:5-nitro-2'-deoxyuridine 5'-monophosphate; C017089:bromodeoxyuridylate; D013940:Thymidylate Synthase; D001973:Bromodeoxyuridine