A Series of Novel Acyclic Nucleosides. IV. Synthesis of N1-Sulfur Analogues of Acyclovir, Directed toward Improved Antivial Activities

Novel imidazothiazine acyclic nucleoside analogues (9a-d, 12a-d and 3c, d) in which N1 of the purine base is replaced by a sulfur atom were synthesized. 5-Substituted imidazo[4, 5-d][1, 3]thiazine-7(3H)-thiones (7a-d) were prepared from 5(4)-substituted amino-4(5)-ethoxycarbonyl-1(3H)-imidazoles with Lawesson reagent and then 7a-d were alkylated with 2-oxa-1, 4-butanediol diacetate or with 2-acetoxyethoxymethyl halide to give 9a-d and 10a, d in moderate yields. Compounds 9a-d were led to the corresponding 7-one derivatives (12a-d) by KMnO4 oxidation. Deprotection of the acetyl group in 9a-d and 12c, d was achieved by means of the Zemplen procedure.