The Dimroth rearrangement. Part IX. The formation and isomerisations of propynyl (and related)-iminopyrimidines

Abstract

The insertion of a 5-allyl, a prop-2-ynyl, or a prop-1-ynyl substituent into 1,2-dihydro-2-imino-1,4,6-trimethyl-pyrimidine progressively decreases the basic strength and increases the rate of Dimroth rearrangement into the corresponding 4,6-dimethyl-2-methylaminopyrimidine; similar effects follow replacement of the 1-methyl group in the same imine by such substituents. Condensation of 3-(prop-2-ynyl)acetylacetone with guanidine gives the expected 2-amino-4,6-dimethyl-5-(prop-2-ynyl)pyrimidine, but also the isomeric 5-(prop-1-ynyl)- and 5-allenyl-pyrimidines; other prop-2-ynylpyrimidines also suffer such prototropic changes which are unprecedented in the series. An alkaline solution of 1,2-dihydro-2-imino-4,6-dimethyl 1-(prop-2-ynyl)pyrimidine undergoes two parallel isomerisations at comparable rates; one is a normal Dimroth rearrangement, and the other a cyclisation to 2,4,6-trimethyl-1,3a,7-triazaindene. Evidence for some of the structures was obtained from ¹H n.m.r. and ultraviolet spectra, which were also used to measure rates of rearrangement.