Biomimetic reactions in organic synthesis: Semi-pinacol rearrangements of some spirocyclic epoxyalcohols derived from Juliá-Colonna asymmetric epoxidations

Epoxy tert-alcohols have been prepared from (E)-enones in a two-step approach consisting of Juliá–Colonna asymmetric epoxidation followed by Grignard alkylation of the epoxyketone. On treatment with sub-stoichiometric amounts of Yb(OTf) 3 these trans-epoxyalcohols underwent efficient stereoselective semi-pinacol rearrangement to afford anti-α-phenyl-β-hydroxy-ketones (aldols). Under the same conditions, spirocyclic epoxyalcohols derived from 1-tetralone and 1-benzosuberone undergo either ring contraction (via semi-pinacol rearrangement) or fragmentation. A mechanistic rationale is presented to explain the formation of the various products.