Studies on the mass spectra of 6‐methylthiopurine and its C‐and N‐methyl derivatives

6‐Methylthiopurines which bear a 9‐NH‐ or a 9‐NCH₃‐group (class A) form an [M—1]‐ion with much higher abundance than do the 1‐, 3‐, or 7‐methyl derivatives (class B). The higher stability of the [M—1]‐ion in class A may be explained by ring closure to N‐7. Methyl radicals are cleaved from N‐, but not from S‐ or C‐methyl groups, with the exception of the 7‐methyl derivative, in which the S‐CH₃‐group can also split off a methyl radical. The methylthio group may lose all of the following fragments: S, SH, SCH, SCH₂ and SCH₃. In the remaining purine skeleton, in general first the pyrimidine and subsequently the imidazole ring breaks down with elimination of HCN.