Ring transformation reactions of C ‐nucleosides: Facile synthesis of pyrazolo[1,5‐ a ]pyrimidine and pyrazolo[1,5‐ a ]triazine C ‐nucleosides

1,3‐Dimethyluracil ( 1 ), a versatile synthon for the synthesis of various heterocycles, reacted readily with 3‐aminopyrazoles 2 in sodium ethoxide to give pyrazolo[1,5‐ a ]pyrimidines 3 . Under similar conditions, 3‐aminopyrazole C ‐nucleosides 4 and the synthon 1 gave a mixture of pyrazolo[1,5‐ a ]pyrimidine C ‐nucleosides, which was separated on a silica gel column. Attempts to remove the protecting groups yielded pyranose derivative 10 . Another synthon 1,3‐dimethyl‐5‐azauracil and 3‐aminopyrazoles 12 gave pyrazolo[1,5‐ a ]triazines 13 . In a similar reaction with 3‐aminopyrazole C ‐nucleosides 4 gave the corresponding pyrazolo[1,5‐ a ]‐triazine C ‐nucleosides 14 and 15 .