Stereochemical studies. Part 103. Saturated heterocycles part 107. Preparation of 3-mono- and 2,3-di-substituted pyrimidin-4(3H)-ones in retro- Diels–Alder reactions. The correct 1,2-disubstituted structure of the compounds previously described as 2,3-disubstituted derivatives

Abstract

The orthoformate cyclization of carboxamides (4) obtained from 3-exo-aminobicyclo[2.2.1]hept-5-ene-2-exo-carboxylic acid (1) yielded intermediate 8,9,10-trinorbornene-fused pyrimidinones (5) and hence 3-substituted pyrimidin-4(3 H)-ones (6) through the splitting off of cyclopentadiene. Analogously, the reaction of carboxamides (4) with orthoacetate or orthopropionate led to the formation of 2,3-disubstituted pyrimidin-4(3H)-ones (8). A comparative i.r. and n.m.r. study of compounds (8) and of the previously described derivatives (9) gave unambiguous evidence for the structure of the new compounds. It was also established unequivocally that the products obtained from propiolate with amidines are not the previously assumed 2,3-disubstituted 4(3H)-ones (8), but are in fact the 1,2-disubstituted pyrimidin-4(1H)-ones (9).