purine bases

https://doi.org/10.1351/goldbook.P04953
Purine and its substitution derivatives, especially naturally occurring examples. The customary numbering shown below is not systematic.
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purine
Source:
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1361 [Terms] [Paper]