Synthetic studies of neoclerodane diterpenes from Salvia divinorum: Exploration of the 1-position

Modification of the C-1 ketone of salvinorin A ( 2a) produces analogues with opioid antagonist properties. Of particular significance is the finding that 1-deoxo-1,10-dehydrosalvinorin A ( 11a) is a moderately potent antagonist at all three opioid receptor subtypes, and that herkinorin ( 2b), a μ agonist, is converted to a weak antagonist by removal of the C-1 ketone ( 3b and 11b). These observations suggest that the ketone of 2b is a key structural feature responsible for μ agonist activity.