Total synthesis of (+)-α-onocerin in four steps via four-component coupling and tetracyclization steps

A remarkably short (four steps, 31% overall yield) enantioselective synthesis of the structurally unique C2-symmetric tetracyclic triterpene (+)-α-onocerin (1) has been developed. The brevity of this mechanism depends on the assembly of four fragments (two molecules of a chiral epoxy ketone and two molecules of vinyllithium) to generate the chiral bis-epoxide 4 in one step, and on the efficient formation of all four carbocyclic rings in one step by cation-olefin tetracyclization. New and general methodology for the conversion of vinyl tert-butyldimethylsilyl ethers to vinyl triflates with retention of E or Z olefinic geometry also was utilized in the synthesis of 1. A short enantioselective synthesis of a non-C2-symmetric diastereomer of 1 is also described which uses the new methodology. Copyright © 2002 American Chemical Society.