Synthesis of DEFG ring system of cneorins

The cneorins have been isolated from the xerophytic shrub Cneorum pulverulentum, which is native to the Canary Islands. They are natural products containing a [4.3.1]propellane ring system (DEFG rings) as the northern part of the molecule and a 5,5-spiroketal unit and a butenolide moiety (A ring) as the southern part. The synthesis of the DEFG ring system of the cneorins is described. The key steps include: intramolecular cyclopropanation of a diazomalonate providing the EFG ring fragment and an anionic cyclization of a sulfone yielding the [4.3.1]propellane ring system.