Pyrimidine reactions. XVI. Thermal rearrangement of substituted 2- and 4-alkoxypyrimidines

Thermal rearrangement of methyl, bromo, and nitro derivatives of 2- and 4-alkoxy-pyrimidines produced their N-alkyl isomers. These were identified by p.m.r. spectral comparison with synthetic specimens of unambiguous or proven structure. The rates for such isomerizations were measured conveniently by changes in ultraviolet spectra. The C-methyl derivatives rearranged more slowly than the parent alkoxypyrimidines, but the bromo, and especially the nitro, derivatives did so much more quickly. Among the derivatives of each methoxy-pyrimidine, the rate of rearrangement increased as the basic pKa value fell and as the methoxyl protons moved downfield, thus affording a qualitative correlation with properties reflecting the electronic effect of each substituent.