Isomerization through cleavage and recombination of imidazolide linkage in the condensed tricyclic system related to hypermodified bases of phenylalanine transfer ribonucleic acids

Article abstract of DOI:10.1248/cpb.47.1464

1-Benzyl-4,6-dimethyl-4,9-dihydro-1H-imidazo[1,2-a]purin-9-one bearing an alkyl, a 1-alkenyl, a hydroxymethyl, a methoxymethyl, or a formyl group at the 7-position (3a - e) underwent rearrangement through fission and recyclization of the pyrimidine ring, attaining equilibrium with the corresponding positional isomer 4a - e in MeONa-MeOH at 25 °C, whereas 7- methoxycarbonyl and 7-halogeno compounds 3f - i were irreversibly converted into the rearranged products 4f - i under identical conditions. The position of equilibrium appears to be affected by the electronic factor of the substituent rather than the steric one. The pseudo-first-order rate constants measured for the reactions of 3a,b,d,f - i suggest that the reaction is accelerated by the electron-withdrawing substituent. However, the reactions of this series of compounds do not always obey the Hammett equation. On the other hand, a linear free energy relationship (p = +3.2) was observed for the rates of rearrangement of a 6-demethyl series of compounds 9a,b,d,f,g, when σ(p/⁰) values were employed. The deviations from this relationship for the reactions with the 7-hydroxymethyl compound 9c and the 7-carbamoyl compound 9e are explicable in terms of the accelerative effect through intramolecular hydrogen bonding with the carbonyl oxygen at the 9-position.

Full text of DOI:10.1248/cpb.47.1464

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