Polycyclic Cyclopropanes from Reactions of Alkene-Containing Fischer Carbene Complexes and Alkynes: A Formal Synthesis of (±)-Carabrone

The polycyclic cyclopropanation reaction of several alkene-containing Fischer carbene complexes with various alkyne partners to provide bicyclo[4.1.0] enol ethers and ketones was examined. A number of features were probed, including the role of carbene donor substituent (amino vs alkoxy), the presence of one vs two pendant alkenes in the carbene complex, the level and sense of diastereoselectivity, and the impact of the substituents present on the alkyne partner. Relative configurations in cyclization products were established by ¹H NMR and difference NOE experiments. Using the above investigations as a guide, the tricyclic sesquiterpene lactone carabrone (5) was formally synthesized. The key cyclization of a symmetrical tetraene-containing carbene complex (8) with propyne provided ketone 42 in good yield as a single diastereomer. Six additional steps gave the tricyclic lactone 6, thus constituting a formal total synthesis of carabrone (5). Notable features of the synthesis are that (i) the desymmetrizing cyclopropanation reaction between propyne and the symmetrical, tetraene-containing carbene complex is both efficient and highly diastereoselective, (ii) the alkenes remaining after the desymmetrizing cyclopropanation serve very effectively as protected carbonyl groups, and (iii) this work constitutes the first use of a Fischer carbene polycyclic cyclopropanation reaction in natural product synthesis.