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.