Nuclear magnetic resonance spectra of xanthines and thioxanthines

Abstract

The n.m.r. signal of the 8-proton in xanthines is shifted downfield more strongly by introduction of a 2- than of a 6-thioxo-group. The signals of N-methyl groups are also shifted to lower field, but the effect depends strictly on the distance between the methyl and the thioxo-group. Thus in 2-thioxanthines, the displacement decreases in the following order: 1-methyl = 3-methyl > 7-methyl, and in 6-thioxanthines the sequence is 1-methyl > 7-methyl > 3-methyl > 9-methyl.