Chiral Boron Complex-Promoted Asymmetric Diels–Alder Cycloaddition and Its Application in Natural Product Synthesis

An
efficient method for the asymmetric Diels–Alder cycloaddition
of 2′-hydroxychalcones with acyclic or cyclic dienes has been
successfully developed. The Diels–Alder cycloaddition is mediated
by a chiral boron complex with VANOL, affording the corresponding
products in high yields and with excellent diastereo- and enantioselectivities.
This reaction enabled the enantioselective construction of cyclohexene
skeletons crucial for the total synthesis of a number of Diels–Alder-type
natural products (−)-nicolaioidesin C, (−)-panduratine
A, (−)-kuwanon I, (+)-kuwanon J, and (−)-brosimones
A and B.