An 15n-nmr study of isomeric n 1 and n 3 substituted 7-methyl-10-oxo-9,10-dihydro pyrimido[l,2-a]purines and 9-oxo-8,9-dihydro-5-alkyl-imidazo[1,2-a] purines in neutral and acidic medium.

Abstract

An 15N-NMR study in neutral and acidic solutions of isoaeric N 1 and N 3 substituted 7-methy1-10-oxo-9,10-dihydro-pyrimido[l,2-a]purines, 4 and 5 , and 9-oxo-8,9-dihydro-5-alky'l-imidazo[l,2-a'] purines, 6 and 7 respectively, have shown the electronic implications of building an additional six-membered ring with two double bonds, as in 4 and 5 , and a five-membered ring with one double bond, as in 6 and 7 , involving 1-NH and exocyclic 2-NH 2 substituent of the guanine moiety 1a . The ease of formation of N 1 or N 3 protonated species and the magnitude of their 15N chemical shifts in compounds 4 to 7 have established that the π-electron rich imidazole system is more deactivated 1n pyrimido[l,2-a]purine derivatives, 4 and 5 , than in the imidazo[l,2-a]purines 6 and 7 . It has also emerged that the N 3 of N 1 isomers, 5 and 7 , are more strongly protonated than the N 1 of N 3 isomers 4 and 6 . A consideration of 2 J N 8(N 9)H 7(H 8) and the resonances of N 9(N 8) and N 5 in compounds 4 to 1 has shown that the N 5,N 9-fused six-membered ring of the pyrimido[1,2-a ] purines is π-electron deficient and is not coplanar with the rest of the molecule while the geometry of the N 5,N 8-fused five-membered ring of the imidazo[l,2-a] purines allows the participation of the N 5 ione pair to activate the imidazole system as the exocyclic 2-NH 2 or 2-NHCOR groups of N 9-substituted guanine moiety.