Stereoselective construction of cis-transoid-cis-tricyclo[7.3.0.0]dodecanes by an intramolecular Diels–Alder reaction: a formal total synthesis of (±)-Δ9(12)-capnellene

Abstract

(1R*,2S*)-2-(2-Oxobut-3-enyl)-1-(1-oxoprop-2-enyl)-1,3,3-trimethylcyclopentane 5 was prepared stereoselectively from 4,4-dimethylcyclopent-2-en-1-one 6 and then converted into the conjugated silyl enol ether 17. Intramolecular cycloaddition of 17, followed by base-catalysed equilibration, provided (1S*,2R*,7R*,9R*)-3-tert-butyldimethylsiloxy-9,12,12-trimethyltricyclo[7.3.0.0]dodec-3-en-8-one 18a, which was transformed, after contraction of the cyclohexene ring, into the synthetic intermediate 32 for (±)-Δ⁹⁽¹²⁾-capnellene 1.