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Guanine 11 was converted, in a one pot reaction, into 2-amino-6-[(4-chlorophenyl)sulfanyl]purine 9a in 88% isolated yield. 4-Acetoxy-3-(acetoxymethyl)butanol 23 was prepared from 2-chloroethanol in five steps and in 46% overall yield. The mesylate ester of compound 23 reacted with 9a in the presence of potassium carbonate with a high degree of regioselectivity (89%) to give the N−9 alkylated product 26 which was isolated in 80% yield. Acidic hydrolysis of the latter compound 26 gave penciclovir 4 in virtually quantitative yield. Penciclovir 4 and famciclovir 5 were prepared from 2-amino-6-[(4-chlorophenyl)sulfanyl]purine 9a in four and five steps, respectively, by procedures involving initial alkylation with 1,2-dibromoethane. The overall yields obtained were 65 and ca. 60%, respectively. |
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