Concise asymmetric synthesis of (−)-bilobalide

Abstract

The Ginkgo biloba metabolite bilobalide is widely ingested by humans but its effect on the mammalian central nervous system is not fully understood1–4. Antagonism of gamma-aminobutyric acid A receptors (GABAARs) by bilobalide has been tied to rescue of cognitive deficits in mouse models of Down syndrome⁵. A lack of convulsant activity coupled with these neuroprotective effects have led some to postulate an alternative, unidentified target.⁴ However, steric congestion and the instability of bilobalide1,2,6 have prevented pull-down of biological targets other than the GABAΑRs. A concise and flexible synthesis of bilobalide would provide a platform to generate probes for identification of potential new targets; analogs with differential selectivity between insect and human GABAΑRs; and stabilized analogs with enhanced serum half-life⁷. Here we exploit the unusual reactivity of bilobalide to affect a late-stage oxidation that symmetrizes the molecular core and allows oxidation states to be embedded in the starting material. The same overall strategy disclosed here may be applicable to G. biloba congeners including the ginkgolides, some of which are glycine receptor (GlyR)-selective antagonists⁸. The therapeutic potential of bilobalide and its incompletely understood effects can now be interrogated through chemical synthesis.

Full text

Nature | Vol 575 | 28 November 2019 | 643
Article
Concise asymmetric synthesis of
(−)-bilobalide
Meghan A. Baker1,3, Robert M. Demoret1,3, Masaki Ohtawa1,2* & Ryan A. Shenvi1*
The Ginkgo biloba metabolite bilobalide is widely ingested by humans but its eect on
the mammalian central nervous system is not fully understood1–4. Antagonism of
γ-aminobutyric acid A receptors (GABAARs) by bilobalide has been linked to the
rescue of cognitive decits in mouse models of Down syndrome5. A lack of convulsant
activity coupled with neuroprotective eects have led some to postulate an
alternative, unidentied target4; however, steric congestion and the instability of
bilobalide1,2,6 have prevented pull-down of biological targets other than the GABAΑRs.
A concise and exible synthesis of bilobalide would facilitate the development of
probes for the identication of potential new targets, analogues with dierential
selectivity between insect and human GABAΑRs, and stabilized analogues with an
enhanced serum half-life7. 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 antagonists8. A chemical synthesis of bilobalide should
facilitate the investigation of its biological eects and its therapeutic potential.
The leaves of Ginkgo biloba have been used historically as insecticides
and helminthicides
9,10
, and this activity has been attributed to their con-
stituent terpene trilactones, including bilobalide11,12 (1, Fig.1). Ginkgo
extracts have also been used in traditional Chinese medicine to treat
senility, a practice that has penetrated the Western world—although not
without controversy, owing to opposing claims of efficacy
1
and serious
adverse effects associated with Ginkgo toxin (4-O-methylpyridoxine)
2
or the inhibition of platelet aggregating factor
13
by ginkgolides. Animal
models demonstrate some credible effects on impaired cognition: in a
mouse model of Down syndrome (Ts65DN), in which mice show deficits
in declarative learning and memory, normalized novel object recog-
nition was exhibited after treatment with pure bilobalide
5
. Rescue of
learning and memory is proposed to arise through neuronal excitation
by antagonism of GABA
A
Rs. Unlike the plant metabolite picrotoxinin
(2, Fig.1), bilobalide is not acutely toxic, and unlike the gingkolides,
bilobalide does not affect platelet aggregating factor. Despite their
disparate toxicity, bilobalide and picrotoxinin exhibit similar inhibitory
potencies at recombinant GABA
Α
Rs, with half-maximum inhibitory
concentrations (IC
50
) of4.6 µM and 2.0 µM, respectively (α
1
β
2
γ
2L
recep-
tor, Xenopus laevis oocytes), yet the two compounds show differential
inhibition of GABAΑR-positive modulators14.
In addition to incomplete approaches
15–17
, two previous syntheses
of bilobalide have been completed (24 steps enantioselective
18,19
; 17
steps racemic20), both of which established the cyclopentane core
with efficiency but required 8–11 subsequent redox steps to reach
the target compound. Guided by these challenges, we realized that
a single oxidation transform might reduce synthetic complexity by
unmasking a pseudosymmetric fused dilactone, ultimately leading
to a symmetric starting material (Fig.1). However, late-stage installa-
tion of the deep C10 hydroxyl (Fig.1, highlighted in red) presented a
problem. Neither hydrogen of its precursor inner lactone (shown in
green) seemed accessible, whereas a hydrogen of the outer lactone
(shown in blue) resided at the surface of the bowl-like scaffold. Here
we utilize unexpected properties of the bilobalide architecture in a
concise synthesis of (−)-des-hydroxybilobalide (>99% enantiomeric
excess (e.e.), 5, Fig.1), which relies on stereocontrol transmitted from
an unusual oxetane acetal. A late-stage oxidation is rendered regiose-
lective using skeletal rearrangement and acidification, completing the
synthesis of (−)-bilobalide in a single additional step.
The synthesis commenced with a methodological challenge: an asym-
metric Reformatsky reaction between 6a and 6b (which are produced
in two steps and one step respectively, seeSupplementary Informa-
tion section 3). Reformatsky conditions proved necessary owing to
the tendency of 7 to undergo retro-aldol cleavage under basic condi-
tions, whereas zinc, chromium and samarium alkoxides were stable
at −78°C. To our knowledge, there have been no examples of catalytic
enantio- and diastereoselective zinc Reformatsky reactions21, nor the
use of simple, chiral -type bisoxazoline (BOX) ligands. A previous
study demonstrated the use of related, electron-rich hemiaminals
for the control of single stereocentres
22
, and the simplicity of these
conditions provided a foundation to explore. After a ligand screen, we
found that a combination of diethylzinc and indabox (10 mol% A) pro-
vided secondary alcohol 7 in 97:3 enantiomeric ratio (e.r.) in favour of
syn-diastereomer 7 (2.3:1). The combined yield of both diastereomers
was determined to be 64% by NMR, and the crude reaction mixture
could be carried forward efficiently (purification by chromatography
https://doi.org/10.1038/s41586-019-1690-5
Received: 10 June 2019
Accepted: 8 August 2019
Published online: 16 October 2019
1Department of Chemistry, The Scripps Research Institute, La Jolla, CA, USA. 2Present address: Graduate School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan. 3These authors
contributed equally: Meghan A. Baker, Robert M. Demoret. *e-mail: ohtawam@pharm.kitasato-u.ac.jp; rshenvi@scripps.edu
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