Purine studies. Part XI. Condensation of tetraethoxymethane and similar orthocarbonates with ortho-diamines to give 8-ethoxypurines and related fused imidazoles

Abstract

4,5-Diaminopyrimidines are converted by boiling tetraethoxy-, tetrapropoxy-, and tetrakismethylthio-methane into the corresponding 8-alkoxy- or 8-methylthio-purines. By conducting the first stage of the reaction at 25° in the presence of acetic acid, the intermediate diethoxymethyleneaminopyrimidines (3) may be isolated prior to thermal cyclization. 5,6-Diaminopyrimidine-2(and 4)-thiones normally undergo S-alkylation by the orthocarbonate as well as cyclization but, under acidic conditions, the 4-thiones yield 2-alkoxy-7-aminothiazolo[5,4-d]pyrimidines (4) instead of purines. Treatment of o-phenylenediamine, 2,3-diaminonaphthalene, and 2,3-diaminopyridine with a tetra-alkoxymethane gives the appropriate 2-alkoxylated benzimidazole (5; X = CH), naphth[2,3-d]imidazole (6), or imidazo[4,5-b]pyridine (5; X = N). Ionization constants, u.v. absorption, and ¹H n.m.r. data are recorded and discussed.