Synthetic Studies of Spiroschincarins: Construction of 1-Oxaspiro[6.6]tridecanone Skeleton with All-<i>cis</i> Cyclopropane Moiety

Spiroschincarins are triterpenoids characterized by a hexacyclic structure with a unique 1-oxaspiro[6.6]tridecanone skeleton and an all-<i>cis</i> substituted cyclopropane moiety. In the present study, this core skeleton was convergently synthesized by coupling all-<i>cis</i>-substituted cyclopropane and cycloheptanone fragments. The keys for its construction were the use of a conformationally limited substrate for lactonization and control of chelation for reducing the double bond on the spiro-lactone ring.