Nucleosides 5(15): synthesis of novel 1,2,4-triazolo[3,4-c]-1,2,4-triazole nucleosides

Ribosylation of 3-methylthio-5-phenyl-1,2,4-triazole (1) with ribose derivative 2 gave the protected 1,2,4-triazole-nucleoside 3, which reacted with hydrazine hydrate to afford the 3-hydrazino-1,2,4-triazole derivative 5. Reaction of 5 with aromatic aldehydes yielded the corresponding hydrazones 6, which cyclized under bromination in acetic acid to give 8. Debenzoylation of 8 afforded novel 1,2,4-triazolo[3,4-c]-1,2,4-triazole nucleosides 9.

Synthesis of 3-aminoimidazo[4,5-c]pyrazole nucleoside via the N-N bond formation strategy as a [5:5] fused analog of adenosine

3-Amino-6-(beta-D-ribofuranosyl)imidazo[4,5-c]pyrazole (2) was synthesized via an N-N bond formation strategy by a mononuclear heterocyclic rearrangement (MHR). A series of 5-amino-1-(5-O-tert-butyldimethylsilysilyl-2,3-O-isopropylidene-beta-D-ribofuranosyl)-4-(1,2,4-oxadiazol-3-yl)imidazoles (6a-d), with different substituents at the 5-position of the 1,2,4-oxadiazole, were synthesized from 5-amino-1-(beta-D-ribofuranosyl)imidazole-4-carboxamide (AICA Ribose, 3). It was found that 5-amino-1-(5-O-tert-butyldimethylsilyl-2,3-O-isopropylidene-beta-D-ribofuranosyl)-4-(5-methyl-1,2,4-oxadiazol-3-yl)imidazole (6a) underwent the MHR with sodium hydride in DMF or DMSO to afford the corresponding 3-acetamidoimidazo[4,5-c]pyrazole nucleoside(s) (7b and/or 7a) in good yields. A direct removal of the acetyl group from 3-acetamidoimidazo[4,5-c]pyrazoles under numerous conditions was unsuccessful. Subsequent protecting group manipulations afforded the desired 3-amino-6-(beta-D-ribofuranosyl)imidazo[4,5-c]pyrazole (2) as a 5:5 fused analog of adenosine (1).