Stereoselective synthesis of spirocyclohexadiene-pyrazolones via organic base and/or hydrogen bonding assisted [3 + 3] annulation reactions

We have reported the 1,4-diazabicyclo[2.2.2]octane (DABCO)-catalyzed [3 + 3] cycloaddition reaction of α-arylidene pyrazolinones and 2-benzylidenemalononitriles under mild reaction conditions, which afforded the spirocyclohexadiene-pyrazolones in good yields with moderate to good diastereoselectivities. By the use of a cinchona alkaloid derived bifunctional squaramide-tertiary amine catalyst, an asymmetric variant of this [3 + 3] cycloaddition reaction has also been developed to provide optically active spirocyclohexadiene-pyrazolones bearing an all-carbon quaternary chiral center in good yields with excellent enantioselectivities and moderate diastereoselectivities.