A Regioselective Biginelli‐like Reaction Controlled by the Size of Alicyclic Mono‐ketones

A regioselective Biginelli-like reaction of alicyclic mono-ketones, aromatic aldehydes, and urea in ionic liquid [BPY]BF4 has been investigated. The process is controlled by the size of alicyclic mono-ketones and the steric hindrance of aromatic aldehydes. The reaction of cyclopentanone with urea and aromatic aldehydes afforded 7-arylidene-3,4,6,7-tetrahydro-4-aryl-1H-cyclopenta[d]pyrimidin-2(5H)-ones (4). When cyclohexanone was used as the source of active methylene to react with urea and aldehydes with slight steric hindrance groups under the same condition, 8-arylidene-3,4,5,6,7, 8-hexahydro-4-arylquinazolin-2(1H)-ones (6), a homologue of 4, were yielded, whereas 4,8-bisaryloc-tahydro-1H-pyrimido[5,4-i]-quinazoline-2,10(3H,11H)-diones (7) were obtained via the simple one-pot reaction of cyclohexanone, urea, and aromatic aldehydes with high steric hindrance groups. The possible transitional states and mechanism of the regioselective process were discussed.