Synthesis of (±)-kuwanon V and (±)-dorsterone methyl ethers via Diels–Alder reaction

The mulberry Diels–Alder adducts, the pentamethyl ethers of kuwanon V 1a and dorsterone 2a were synthesised via a biomimetic intermolecular [4+2] cycloaddition reaction between a highly electron-rich dienophile and a Lewis acid sensitive diene derived from chalcone. Cycloaddition under thermal conditions afforded 1a and 2a (in 3:2 ratio). Cycloaddition catalysed by AgOTf/Bu4NBH4 gave higher yields of the adducts.