Elimination and hydrolysis reactions of 4-alkoxyimino-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H)-ones and 4-alkoxyimino-5-fluoro-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H)-ones in strong acid media

Abstract

The rate of elimination of hydroxylamines from 4-alkoxyimino-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H)-ones (Ia) in a number of acids is shown to be maximal at a H₀ value of –1.8, with the exception of trifluoroacetic acid for which a linear increase in rate constant is observed with increasing acid strength. The results are explained by a mechanism involving the doubly protonated form of the substrate. With 4-alkoxyimino-5-fluoro-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H)-ones (Ib), acid-catalysed hydrolysis to form 5-fluoro-5,6-dihydro-6-alkoxyaminopyrimidine-2,4(1H,3H)-diones (V) occurs in preference to the elimination reaction which in this case is ca. 750 times slower than with (Ia). The rate differences for elimination are explained by differences in conformation between the two substrates (Ia and b).