The problem goes deeper than that. If the prebiotic chemistry was conducive to the synthesis of purines and pyrimidines then it is incompatible with the synthesis of ribose. Tough nut to crack there even with a nutcracker.
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!