Homoenolization and Rearrangement of Bicyclic Monoketones and Diketones

Abstract

<p>The H-D exchange and rearrangement of 3,3,6,6-tetramethyl[2.2.1]-heptane-2,5-dione (63a), 3,3,6,6-tetramethylbicyclo[2.2.1]heptan-2-one (63d) and 1,3,3-trimethylbicyclo[2:2.2]octane-2,5-dione (64) in 0.7 -1.3 M K⁺ṯ-BuŌ/ṯ-BuOD(H) at 175°C were examined. Preliminary work on the dithione 63b and monothiomonoketone 63c has also been carried out.</p> <p>The substrates 3,3,6,6-tetramethylbicyclo[2.2.1]heptan-2-one (63d), 3,3,6,6-tetramethylbicyclo[2.2.1]heptane-2,5-dithione (63c), and 3,3,6,6- tetramethylbicyclo[2.2.1]heptane-2,5-dithione(63b), which have not been reported in the literature previously, were synthesized.</p> <p>Approximate pseudo-first-order rate constants for the exchange at different sites were calculated from the H-D exchange data. The results suggested that the effect of the second carbonyl group in the exchange of the diones under homoenolization conditions, appeared to be due largely to an inductive and field effect, with small homoconjugative participation.</p> <p>In addition, homoenolization of 3,3,6,6-tetramethylbicyclo[2.2.1]-heptane-2,5-dione (63a) resulted in rearrangement to 1,3,3-trimethylbicyclo-[2.2.2]octane-2,5-dione (64), via a novel β-homoenolate switch. This is the first report of such a rearrangement when the homoenolate is generated by deprotonation of a ketone. When dione 64 was treated under homoenolization conditions, it underwent unusually rapid H-D exchange of the hydrogens of the bridgehead methyl group. The facile exchange can be explained if a carbon analogue of the acyloin rearrangement is invoked, the first example of such a rearrangement in a non-aromatic system. Monoketone 63d underwent H-D exchange at the bridgehead and the syn-H-7 position as well as at the hydrogens of the C-3 methyl group and the hydrogens of the endo-11-methyl group. It was also found that 1,3,3-trimethylbicyclo[2.2.1)heptan-2-one, (fenchone), (29), previously studied by others, also underwent exchange at the syn-7-position.</p>