Enantioselective Approach
to Polycyclic Polyprenylated Acylphloroglucinols via Catalytic Asymmetric
Intramolecular Cyclopropanation
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Abstract
The formal enantioselective
total synthesis of nemorosone, garsubellin A, clusianone, and hyperforin
is described. The catalytic asymmetric intramolecular cyclopropanation
(CAIMCP) of an α-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 α-diazo β-keto sulfone afforded the
desired product in 79% yield with 84% ee. Investigation of the CAIMCP
of the α-diazo β-keto sulfone demonstrated the formation
of a rearrangement product in the presence of molecular sieves 4 Å,
whereas, in the presence of H<sub>2</sub>O, 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
