A Tautomeric Pair of 2,2-Dimethyl-6-carbamoyl-9-phenyldihydropurines

Two tautomeric dihydropurine derivatives, C 14 H 15 N 5 O, namely 2,2-dimethyl-9-phenyl-1,2-dihydropurine-6-carboxamide, (A) (coloured orange-red), and 2,2-dimethyl-9-phenyl-2,3-dihydropurine-6-carboxamide, (B) (yellow), are confirmed to have hydrogen substituents at N1 and N3, respectively. A different pattern of observed bond lenghts in the purine rings of (A) and (B) illustrates differences in conjugation in accordance with the different bond alternation. The tautomerism also gives rise to differences in the hydrogen bonding, although both tautomers have an internal hydrogen bond from an amide N-H to an N atom of the five-membered ring, with N...N distances 2.84 (2) A in (A) and 2.836(6)A in (B).