Purine studies. Part XVI. Occurrence of syn- and anti-isomers in 4-amino-5-formamidopyrimidine derivatives

Abstract

The multiple nature of the resonances in the ¹H n.m.r. spectra of a series of derivatives of 4-amino-5-formamido-6-methyl-2-methylthiopyrimidines, with special reference to those due to the formyl and C-methyl groups, is attributed to the presence of syn- and anti-isomers. The isomeric ratio, which varies with the solvent employed, almost always has the syn-conformer as the equal or predominant species. Following ethylation the resulting 5(N-ethylformamido)pyrimidines also show two isomeric modifications but in these derivatives the anti-form is the major one present. While some analogies can be drawn between the spectra of the above derivatives and those of formanilide analogues, significant differences are found also. Thus, the reversal of the positions of the syn- and anti-formyl resonances is taken to reflect the essentially planar conformation of the amide group with the ring plane in the case of the simple formanilides compared with the orthogonal position assumed by this group in the formamidopyrimidines.