Synthesis of 1,5‐dihydro‐5‐(4‐methoxyphenyl)‐4 H ‐pyrazolo[3,4‐ d ]pyrimidin‐4‐ones and their ribosides and mannich bases

1,5‐Dihydro‐4 H ‐pyrazolo[3,4‐ d ]pyrimidin‐4‐ones 5 and 6 were prepared by reaction of the corresponding ethyl 5‐aminopyrazole‐4‐carboxylate with triethyl orthoformate and amines. Ethyl N ‐(4‐methoxyphenyl)formimidate and N,N ′‐bis(4‐methoxyphenyl)formamidine could also be used as the ring closing reagent. The Mannich bases 13 and 14 were produced in a reaction of 9 with formaldehyde and piperidine. The ribosides 15 were obtained by melting a mixture of 9 and 1,2,3,5‐tetra‐ O ‐acetyl‐β‐ D ‐ribofuranose in the presence of p ‐toluenesulphonic acid.