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Oxazolo[5,4- d ]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- d ]pyrimidine-7(6 H )-ones (9-oxa-guanines) have been shown to inhibit ricin. The preparation of a series of 2-substituted 5-aminooxazolo[5,4- d ]pyrimidin-7(6 H )-ones and related 5-thio-oxazolo[5,4- d ]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 50   ca.  1–3 mM.

beilstein journal of organic chemistry

Synthesis of 2-substituted 9-oxa-guanines {5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones} and 9-oxa-2-thio-xanthines {5-mercaptooxazolo[5,4-d]pyrimidin-7(6H)-ones}