Molecular design of a new class of spin-labeled ribonucleosides with N-tert-butylaminoxyl radicals

We designed a new type of spin-labeled nucleosides with an N-tert-butylaminoxyl radical which is introduced to the nucleobase directly. Purine and pyrimidine ribonucleosides containing the aminoxyl radical such as 1a-d, 2, 3, and 4 were synthesized to investigate the stability and behavior of the N-tert-butylaminoxyl radical on a nucleobase. Lithiation of tri-O-silylated 6-chloropurine ribonucleoside (5) followed by reaction with 2-methyl-2-nitrosopropane (MNP) gave the key compound 6a, which was further converted to 6b-d. Oxidation of the obtained 6a-d and their triols (7a-d) with Ag₂O led to formation of the corresponding stable spin-labeled nucleosides (8a-d and 1a-d), which were confirmed by EPR spectroscopy. Similarly, the precursors of spin-labeled pyrimidines (13, 20, and 23) were synthesized by site-selective lithiation of tri-O-protected pyrimidine derivatives (9, 18, and 21) followed by the reaction with MNP and deprotection. An EPR study showed that the aminoxyl radicals (2, 3, and 4) were stable and that their hyperfine structures were dependent on the position of the radical. Electron densities of pyrimidine also affected hyperfine structures.