Synthesis and rearrangement of cycloalkyl[1,2-e]oxazolo[3,2-a]pyrimidin-8/9-ones: an access to cycloalkyl[1,2-d]oxazolo[3,2-a]pyrimidin-5-ones

Abstract 2-Substituted-4a-hydroxy-9 H -cycloalkyl[1,2- e ]oxazolo[3,2- a ]pyrimidin-9-ones 2a – c were synthesized by an one-step cyclocondensation from the 5-substituted-2-amino-2-oxazolines 1a – c with ethyl 2-oxocyclohexanecarboxylate in ethanol at room temperature, and easily dehydrated to provide 2-substituted-9 H -cycloalkyl[1,2- e ]oxazolo[3,2- a ]pyrimidin-9-ones 3 . In refluxing xylene, the reaction conducted with various ethyl 2-oxocycloalkanecarboxylates led to the two isomeric 2-substituted-8/9 H -cycloalkyl[1,2- e ]oxazolo[3,2- a ]pyrimidin-8/9-ones 3 and 2-substituted-5 H -cycloalkyl[1,2- d ]oxazolo[3,2- a ]pyrimidin-5-ones 4 . The structure of some compounds was unambiguously established using X-ray crystallography. According to results from the DSC analysis of compound 2a , formation of the thermodynamically stable pyrimidinones 4 could be related to an intramolecular rearrangement of kinetically controlled pyrimidinones 3 .