Formation of Prenylated Pterocarpans from Synthetically Derived Demethylmedicarpin by Regioselective Gold(I)‐Catalyzed Intramolecular Hydroarylation and Related Reactions

Three prenylated pterocarpans including erybraedin A and erybraedin C have been synthesised from (±)‐demethylmedicarpin, itself prepared in four simple steps starting from resorcinol and 2,4‐dimethoxyphenylacetic acid. A gold(I)‐catalyzed and two‐fold intramolecular hydroarylation (IMHA) reaction of a readily‐derived bis‐propargyl ether of (±)‐demethylmedicarpin proceeds with remarkable regio‐selectivity to afford the racemate of a recently reported but yet‐to‐be named bis‐2H‐chromene‐annulated pterocarpan‐containing natural product. A possible explanation for this potentially very useful selectivity is advanced. In a related reaction sequence, a readily‐derived bis‐allyl ether of (±)‐demethylmedicarpin undergoes a thermally‐induced and two‐fold aromatic Claisen rearrangement (ACR) to give a 1:5 mixture of the racemic modifications of the isomeric and cytotoxic natural products erybraedin A and erybraedin C.