A lactone-fused cyclohexadiene, which can be readily prepared by ruthenium(III)-catalyzed [2 + 2 + 2] cyclization of an enediyne, functioned as a versatile platform for the stereoselective synthesis of differently functionalized 5,6,5-tricyclic scaffolds.
We
established a method for installing a methyl group at the β-position
of a butenolide ring. The methylated position is located at the congested
ring juncture of a 5,6,5-tricyclic lactone, which is common to neurotrophic seco-prezizaane-type sesquiterpenes. The samarium(II)-mediated
conjugate addition of the halomethylsilyl ethers tethered to the proximal
hydroxy groups efficiently formed the desired C–C bond. Subsequent
fluoride-free Tamao oxidation and Barton–McCombie deoxygenation
converted the resultant cyclic silyl ether into the corresponding
methyl group.
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