Crisamicin A

A strategy based on bidirectional Dötz benzannulation and the oxa-Pictet-Spengler reaction toward the synthesis of actinorhodin and γ-actinorhodin has been explored. This work has resulted in the synthesis of deoxyactinorhodin and deoxy-γ-actinorhodin. The latter is a regioisomer of crisamicin A (which has 10,10'-dihydroxy groups).

The first stereoselective total synthesis of the dimeric naphthoquinonopyrano-γ-lactone (-)-crisamicin A was realized (13 steps, 5% overall yield). 1,4,5-Trimethoxynaphthalene, reached in five known steps, was brominated at C-3 to install a but-3-enoic ester by an ensuing Heck coupling. An asymmetric Sharpless dihydroxylation followed and gave a β-hydroxy-γ-lactone with >99.9% ee. Its OH substituent and acetaldehyde established the dihydropyran ring in a completely diastereoselective oxa-Pictet-Spengler cyclization. The 2,3-fused anisole moiety allowed the C5-H bond under Hartwig's conditions to be borylated. This set the stage for engaging the resulting C5-B bond in an oxidative dimerization, which led to a binaphthohydroquinon-5-yl. The latter was advanced to synthetic crisamicin A by a double CAN oxidation (→ a binaphthoquinon-5-yl) and a double demethylation.

A full account of our efforts toward an asymmetric synthesis of crisamicin A are presented. The key steps include a Hauser-Kraus annulation of a cyanophthalide with a chiral enone-lactone, a stereoselective cyclization-reduction to install the pyran unit, and a Suzuki homocoupling to forge the key biaryl bond. This work has culminated in the asymmetric synthesis of a dimer bearing the complete carbon skeleton of the dimeric pyranonaphthoquinone natural product crisamicin A.