Studies on Terpenoids and Related Alicyclic Compounds. III. Bromination-Dehydrobromination of 2-Oxo-5α-santanolide

Bromination of 2-oxo-5α-santanolide (I) gave a bromide (II) together with III as a minor bromide. This equatorial bromide (III) epimerized into the axial bromide (II). Dibromination of I gave dibromoketones (IV and V). Configuration and conformation of the bromoketones (II-V) were assigned from their infrared, ultraviolet proton magnetic resonance, and circular dichroism spectra. Dehydrobromination of II gave an enone (VI), whose treatment with ethanolic hydrochloric acid afforded an equilibrium mixture of VI and VII. Hydrogenation of VI gave a C-4 axial epimer of I. Dehydrobromination of dibromoketones (IV and V) under several conditions gave a mixture of IX-XII, and XIV-XV, and their structures were confirmed from their spectroscopic data. A mechanism of the bromination of I, and dehydrobromination of IV and V are discussed.