Concise asymmetric synthesis of (-)-bilobalide

The Ginkgo biloba metabolite bilobalide is widely ingested by humans but its effect on the mammalian central nervous system is not fully understood(1-4). Antagonism of gamma-aminobutyric acid A receptors (GABA(A)Rs) by bilobalide has been linked to the rescue of cognitive deficits in mouse models of Down syndrome(5). A lack of convulsant activity coupled with neuroprotective effects have led some to postulate an alternative, unidentified target(4); however, steric congestion and the instability of bilobalide(1,2,6) have prevented pull-down of biological targets other than the GABA(A)Rs. A concise and flexible synthesis of bilobalide would facilitate the development of probes for the identification of potential new targets, analogues with differential selectivity between insect and human GABA(A)Rs, and stabilized analogues with an enhanced serum half-life(7). Here we exploit the unusual reactivity of bilobalide to enable a late-stage deep oxidation that symmetrizes the molecular core and enables oxidation states to be embedded in the starting materials. The same overall strategy may be applicable to G. biloba congeners, including the ginkgolides-some of which are glycine-receptor-selective antagonists(8). A chemical synthesis of bilobalide should facilitate the investigation of its biological effects and its therapeutic potential.