Abstract: The reactions of 6-benzylthiopyrimidine-4,5-diamines and carboxylic acids in presence of polyphosphoric acid were investigated. Three products, 6-benzylthiopurines 2, 6-mercaptopurines 3 and 7-aminothiazolo[5,4-d]pyrimidines 4 might be obtained selectively depending on the reaction conditions. When an aliphatic acid was employed, 6-benzylthiopurine 2 could be obtained by using toluene as the solvent at 60—80 ℃. An aromatic acid might yield the debenzylated product 6-mercaptopurine 3 and thiazolo[5,4-d]pyrimidine 4 at an elevated temperature(refluxing toluene or xylene). Increasing in the molar ratio of polyphosphoric acid to starting 6-benzylthiopyrimidine-4,5-diamine 1 could promote the formation of 6-mercaptopurine 3. Thiazolo[5,4-d]pyrimidine 4 could be prepared as the product of choice if both hydrogen atoms of the 4-amino group in compound 1 were blocked by an aryl or alkyl. These results could be rationalized by a mechanism involving 5-acylatedaminopyrimidine 7 as the key intermediate. Compound 7 may undergo either an intramolecular reaction followed by loss of water to yield 6-benzylthiopurine 2 which may in turn be debenzylated to
6-mercaptopurine 3, or loss of benzyl to 6-mercapto intermediate 8 leading to 7-aminothiazolo[5,4-d]pyrimidine 4 due to the superior nucleophilicity of
6-sulfur group to 4-amino one.

Key words: 6-Benzylthiopyrimidine, Polyphosphoric acid, Debenzylation, Mercaptopurine, Thiazolo[5,4-d]pyrimidine