Artificial genetic systems: exploiting the "aromaticity" formalism to improve the tautomeric ratio for isoguanosine derivatives

The tautomerism of 2'-deoxy-7-deaza-isoguanosine (2) was studied and compared to that of 2'-deoxyisoguanosine (1). The fixed (1)N-methyl (8) and O-methyl (4) derivatives were synthesized to represent the pure extremes of each tautomer. The replacement of the imidazole ring in 1 with a pyrrole ring in 2 makes the keto form in the latter more favored by 2 orders of magnitude (K(TAUT) for 2 approximately 10(3), as opposed to K(TAUT) for 1 approximately 10).