Details, please?
In the meantime, I will venture to blow some smoke. :-)
Is your statement necessarily true...? That is, let us agree that an environment conducive to making purine and pyrimidines is incompatible with synthesis of ribose.
Could the environment *first* have been suitable for purine and pyrimidine, cranked out a bunch of them, and then changed in such a way that ribose got made?
Two sub-questions.
1) How stable are *existing* purine and pyrimidine in a ribose-making environment? (and vice versa)?
2) How hard is it to change the environment between favoring making purine/pyrimidine vs. ribose?
Cheers!
Another possibility--different compounds were made in different areas, run-off carried some away and mixed them together.