Rearrangements of some polycyclic hydroxy ketones in strong protic acids

Abstract

The polycyclic hydroxy ketones (1)–(3) are unaffected by treatment with 1 mol dm^–3 aqueous hydrochloric acid. Hydroxy ketone (5) is also stable to fluorosulphonic acid–antimony pentafluoride in liquid sulphur dioxide at 0 °C. ¹H and ¹³C NMR spectroscopy shows that (5) is protonated on both alcohol and carbonyl oxygens in this medium, and an activation energy, ΔG^‡, of 14.7 ± 0.8 kcal mol^–1, for geometric isomerism of the protonated carbonyl has been determined by dynamic NMR methods. The rate, k= 1.17 s^–1(23.3 °C) of degenerate rearrangement of (5) by 1,4-hydride shift from alcohol methine to carbonyl carbon in trifluoromethanesulphonic acid in liquid sulphur dioxide solution was determined by spin-saturation transfer experiments. The primary kinetic isotope effect was 2.30 ± 0.35. In trifluoromethanesulphonic acid, (5) also rearranges more slowly to (7) or (8). In strongly acidic media, hydroxy ketone (6) rearranges rapidly to a mixture of (9) and (10). The X-ray crystal structure of (10) has been determined.