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Treatment of 5-amino-1-(9-methyl-5,6-dihydronaphtho[1',2':4,5]thieno[2,3-d]pyrimidin-11-yl)-1H-pyrazole-4-carbonitrile (1) with formic acid afforded pyrazolo[3,4-d]pyrimidin-4-one derivative 2. The sodium salt of the latter compound (generated in situ) was treated with some alkyl halides to afford the corresponding N-substituted compounds 3-7. The siloxy derivative 8 (generated also in situ from 2) was ribosylated and glycosylated to yield compounds 9 and 11, respectively. Deprotection of compounds 9 and 11 in methanolic ammonia produced the free nucleosides 10 and 12, respectively. Moreover, the prepared compounds were tested for antiviral activity against H5N1 virus [A/chicken/Egypt/1/2006] and some of them revealed moderate results compared with the other tested compounds.