Synthesis of labelled [N-15(3)]-6-bromopurine, a useful precursor of N-15-labelled O-6-alkylguanines, to be used as internal standards for quantitative GC-MS analyses

[N-15(3)]-6-Bromopurine was synthesized using readily available labelled starting materials. The N-15-labelled precursor guanidine, obtained from (NH3)-N-15 and cyanogen bromide, was condensed with ethyl cyanoacetate to give 4-hydroxy[N-15(2)]diaminopyrimidine. An additional N-15 isotope was incorporated by nitrosation with N-15-labelled sodium nitrite. After reduction to the corresponding triaminopyrimidine and condensation with formamide, [N-15(3)]guanine was obtained in 96% yield and then converted to [N-15(3)]-6-thioguanine by reacting it with phosphorus pentasulfide. [N-15(3)]-6-Thioguanine was readily converted to the corresponding bromopurine using bromine in the presence of methanolic hydrobromic acid. The final product was characterized by mass spectrometry. [N-15(3)]-6-Bromopurine was the precursor for the subsequent synthesis of a series of O-6-alkylguanines which can be used in isotope dilution mass spectrometry.