Rearrangement Approaches to Cyclic Skeletons. XII. Acid-Catalyzed Isomerization of Bicyclo[3.2.1]oct-6-en-2-ones into Bicyclo[3.2.1]oct-3-en-2-ones without Racemization
The treatment of a bicyclo[3.2.1]oct-6-en-2-one (3) with Amberlyst® (H-15) in boiling benzene gave a mixture of bicyclo[3.2.1]oct-3-en-2-one (5) and bicyclo[2.2.2]oct-5-en-2-one (6). The ratio of 5 to 6 was influenced by the substituent at the C-1 position of the ketone 3. Chiral ketones 3 were transformed into the chiral α,β-unsaturated ketones 5 without racemization.
Both enantiomers of (P)-(+)-2- and (M)-(−)-2-acetoxy-11,14-dimethyl[5]helicenes 8 were synthesized by asymmetric aromatic oxy-Cope rearrangement of the corresponding chiral bridged bicyclic compounds, which were obtained by enzymatic resolution. The absolute configurations of 8 were assigned by their circular dichroism spectra.