Synthesis of tetrahydropyrimidines with 2-oxoalkyl groups using pyrimidinium salts and enol silyl ethers: formation of diazabicyclo[3.3.1]nona-3,6-diene derivatives

Abstract

Reactions of N-silyl- or N-acyl-pyrimidinium salts with enol trimethylsilyl ethers were carried out to afford the adducts with a 2-oxoalkyl group, that is, 2-hydroxy-4-(2-oxoalkyl)-1,2,3,4-tetrahydropyrimidines (9) or the 3-acyl derivatives (8), respectively. Monoacylation of compounds (9) gave monoamides (8) in high yields. Exhaustive acylation of diamines (9) with a large excess of 2,2,2-trichloroethyl chloroformate in the presence of pyridine gave bicyclic compounds, 2-oxa-8,9-diazabicyclo[3.3.1]nona-3,6-dienes (14). By addition of trifluoroacetic acid, compounds (14) were reopened to afford 3,4-dihydropyrimidinium salts (11′), which were subjected to nucleophilic attack to give 1,2,3,4-tetrahydropyrimidines (16) in high yield.