Biomimetic Studies Towards the Cardinalins: Synthesis of (+)-ventiloquinone L and an Unusual Dimerisation

Studies towards the biomimetic synthesis of cardinalin 3 are described. Despite the successful enantioselective synthesis of the monomeric pyranonaphthoquinone ventiloquinone L, it subsequently failed to undergo a proposed biomimetic homodimerisation to cardinalin 3 using a range of oxidants. However, treatment of a related naphthopyran with cerium ammonium nitrate (CAN) facilitated a tandem biaryl bond formation-oxidative demethylation sequence furnishing a dimeric pyranonaphthoquinone that had exclusively dimerised at C6. The nature of this unusual sequence is discussed and the product subsequently converted to the C6 regioisomer of cardinalin 3.