Rearrangements of tricyclic ketones: [3.3.3]propellane formation via a γ-enolate revealed in an approach to the pentalenene skeleton through β-enolization

Abstract

The known isomerization of the tricyclo[5.3.1.01,5]- to the [6.3.0.01,5]undecane skeleton by β-homoenolization led to examination of 2,9,9-trimethyl-6-methoxymethyltricyclo[5.3.1.01,5]undecanone (6) as a potential precursor for the synthesis of pentalenene, a naturally occurring sesquiterpenoid, utilizing a β-enolate rearrangement as a key step. While the anticipated rearrangement of 6 occurs, it is a minor process. The major rearrangement product arises by generation of a γ-enolate intermediate from a tricyclic enone formed by loss of methanol from 6. This constitutes a new route to [3.3.3]propellanes and provides a third example of γ-enolate rearrangement. Structures of the products were established primarily from their 1H and 13Cmr spectra.