Asymmetric syntheses of (+)- and (–)-collybolide enable reevaluation of kappa-opioid receptor agonism

The fungal metabolite collybolide attracted attention as a non-nitrogenous, potent and biased agonist of the kappa-opioid receptor (KOR). Here we report a 10-step asymmetric synthesis of this complex sesquiterpene that enables facile access to either enantiomer. The synthesis relies on a diastereoselective α-benzoyloxylation to install the buried C6 benzoate and avoid irreversible translactonization of the congested, functionally dense core. Neither enantiomer, however, exhibited KOR agonism, raising the specter of a yet-unidentified contaminant responsible for the reported activity.