Restricted rotation in 6-methylaminopurine and analogues. Intramolecular hydrogen bonding

Abstract

The preferred conformations of 9-alkyl-6-methylaminopurines (m⁶A), the corresponding 1-oxides, and 1-alkyl-4-methylaminopyrazolo[3,4-d]pyrimidines (4-MeAPP) and the corresponding 5-oxides in chloroform arise from intramolecular hydrogen bonding. m⁶A has the syn-conformation; m⁶A 1-oxide exists mainly as the anti-rotamer; 4-MeAPP has similar proportions of syn- and anti-rotamers, and 4-MeAPP 5-oxide has just the anti-conformation. The possible consequences for hydrogen bonding at the nucleic acid level are discussed.