Synthesis of 1-aryl-1,4,5,6-tetrahydropyrimidines and 1-aryl-3-substituted1,4,5,6-tetrahydropyrimidinium salts

A synthetic approach to 1-aryl-1,4,5,6-tetrahydropyrimidines (1) is describ ed, by ring closure of N-aryl-N'-formyl-1,3-propanediamines (5) with trimet hylsilyl polyphosphate (PPSE). Quaternization of compounds (1) with methyl (or ethyl) iodide led to the corresponding cyclic amidinium salts (2), whil e treatment of compound (la) with 2,4-dinitrochlorobenzene yielded an open chain product resulting from hydrolysis of the salt. An alternative method was employed for the synthesis of 1-aryl-1,4,5,6-tetrahydropyrimidinium sal ts bearing a branched alkyl or an aryl substituent on N3, not accessible th rough alkylation. Such compounds were obtained in high yields by dehydrogen ation of the corresponding hexahydropyrimidines (3). The scope and limitati ons of both procedures are discussed.