Mass‐spectrometric studies on alkylated 6‐thiopurines

6‐Thiopurine and its N‐ or C‐alkyl derivatives all form an [M – 1]‐ion upon fragmentation. In the 7‐alkyl derivatives, this ion represents the major component of the spectrum. This is ascribed to formation of a five‐membered thiazoline‐like ring. Similar ring formation stabilises the [M – 1]‐ion in the 7‐methyl derivatives of hypoxanthine, adenine and 6‐selenopurine.