Asymmetric Synthesis of Tetrahydrofurans through Palladium(0)-Catalyzed [3 + 2]-Cycloaddition of Vinylcyclopropanes with Ketenes

In
this article we describe a new asymmetric synthesis of highly
substituted tetrahydrofurans through a Pd­(PPh3)4-catalyzed formal [3 + 2]-cycloaddition of donor–acceptor
cyclopropanes and ketenes. The desired structural motif was formed
in moderate to excellent yields (42–84% for 16 examples), with
excellent Z:E isomer diastereoselectivity
(≥19:1 for 16 examples), and with good to excellent enantioselectivity
in all cases examined (83–97% ee for 6 examples). The synthetic
utility of the products was illustrated by a number of diastereoselective
transformations into reduced tetrahydrofurans and spirocyclic tetrahydrofuran-barbiturate
derivatives.