Oxazolo[5,4-<i>d</i>]pyrimidines can be considered as 9-oxa-purine analogs of naturally occurring nucleic acid bases. Interest in this ring system has increased due to recent reports of biologically active derivatives. In particular, 5-aminooxazolo[5,4-<i>d</i>]pyrimidine-7(6<i>H</i>)-ones (9-oxa-guanines) have been shown to inhibit ricin. The preparation of a series of 2-substituted 5-aminooxazolo[5,4-<i>d</i>]pyrimidin-7(6<i>H</i>)-ones and related 5-thio-oxazolo[5,4-<i>d</i>]pyrimidines is described, including analogs suitable for further elaboration employing “click” chemistry utilizing copper-catalyzed Huisgen 1,3-dipolar cycloadditions. Two of the compounds prepared were found to inhibit ricin with IC<sub>50</sub> <i>ca.</i> 1–3 mM
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