Interference between peri-substituents at positions 3 and 9 in purines and positions 1 and 8 in pteridines, shown by nuclear magnetic resonance spectroscopy. Proposal of a steric model

In purines, including uric acids, steric interference between 3- and 9-methyl substituents leads to a marked downfield shift of their n.m.r. signals, of 0.20-0.40 p.p.m. In 1,8-dimethylpteridine-2,4,7-triones, the displacement of the methyl bands is less marked (0.12-0.18 p.p.m.). Two models are discussed to explain these downfield shifts: either moving the methyl groups out-of-plane, or spreading them apart within the plane of the heterocyclic structure. Calculations show that only the latter model is in accordance with the measured values. The smaller change of the δ-values in pteridine-2,4,7-triones can be explained by the observation that in 1- or 8-monomethyl derivatives the 7, 8- or 1,2-lactam group, respectively, is partially lactimised.