Total Synthesis of Eucalyptusdimers A-C

The first total synthesis of eucalyptusdimers A-C, three dimeric phellandrene-derived meroterpenoids featuring an unprecedented 6-6-6-6-6-fused ring system, was accomplished starting from α-phellandrene and 4-bromo-2,6-dihydroxybenzaldehyde. A key feature is the exploitation of a cascade of double-hetero-Diels-Alder reactions to construct a pyrano[4,3,2-de]chromene framework with excellent regioselectivity and stereoselectivity, incorporating four chiral centers and one prochiral center in a single step. In addition, a Vilsmeier-Haack formylation followed by Friedel-Crafts alkylation was employed to complete the target molecules.