Perchloric acid induced epimerisation of the thevinones: an improved synthesis of 7β-dihydrothevinones

The region above and away from C7 in the orvinols is known to be of particular importance in determining the μ-opioid receptor profile of this important class of opioids. However it has been difficult to explore this site due to the relative inaccessibility of 7β-substituted compounds. Here we report that perchloric acid induced epimerisation of the 7α-ketones (dihydrothevinones) allows considerably improved access to a series of β-ketones (β-dihydrothevinones). The extent of epimerisation of the 7α-ketone is determined by the degree of steric bulk in both the 6,14-bridge and in the ketone side chain.