Enantioselective Approach to Polycyclic Polyprenylated Acylphloroglucinols via Catalytic Asymmetric Intramolecular Cyclopropanation

The formal enantioselective total synthesis of nemorosone, garsubellin A, clusianone, and hyperforin is described. The catalytic asymmetric intramolecular cyclopropanation (CAIMCP) of an a-diazo ketone, a common synthetic intermediate for the above four polycyclic polyprenylated acylphloroglucinols previously reported by us, exhibited low enantioselectivity. However, CAIMCP of the corresponding alpha-diazo beta-keto sulfone afforded the desired product in 79% yield with 84% ee. Investigation of the CAIMCP of the alpha-diazo beta-keto sulfone demonstrated the formation of a rearrangement product in the presence of molecular sieves 4 angstrom, whereas, in the presence of H2O, the byproduct derived from ring-opening of the desired cyclopropane was observed. X-ray crystallographic analysis suggested that the above two products are derived from the same chiral intermediate. The product derived from ring-opening of the cyclopropane was successfully transformed to the respective synthetic intermediates for the total syntheses of nemorosone, garsubellin A, clusianone, and hyperforin, which had previously been reported by us.