An easy route to 2-substituted-2,3-dihydro-5(7)H-oxazolo[3,2-a]pyrimidin-5-ones and 7-ones starting from the corresponding 2-amino-2-oxazolines

The isomeric 2-substituted-7 (5)-methyl-2,3-dihydro-5(7)H-oxazolo [3,2-a] p yrimidin-5-ones 3a-b and 7-ones 2a-b,7a were synthesized by cyclocondensati on from the 5-substituted-2-amino-2-oxazolines 1a-b with biselectrophiles. In boiling ethanol, the reaction of 1a-b with acetylenic esters led to a mi xture of 2a-b,7a with a small amount of (E)-2-N-(2-ethoxycarbonylethylene)- 5-substituted-2-iminooxazolines 5a-b. The ring annulation between 1a-b and diketene gave the 2-substituted-7-hydroxy-7-methyl-2,3,6,7-tetrahydro -5H-o xazolo [3,2-a] pyrimidin-5-ones 4a-b which can be easily dehydrated to prov ide the 2-substituted-7-methyl-2,3-dihydro-5H-oxazolo[3,2-a]pyrimidin-5-one s 3a-b.