A Concise, Efficient and Scalable Total Synthesis of Thapsigargin and Nortrilobolide from (R)-(?)-Carvone

Article abstract of DOI:10.1021/jacs.7b01734

A concise, efficient and scalable synthesis of thapsigargin and nortrilobolide from commercially available (R)-(?)-carvone was developed. Our synthetic strategy is inspired by nature’s carbon-carbon bond formation sequence, which facilitates the construction of a highly functionalized sesquiterpene lactone skeleton in five steps via an enantioselective ketone alkylation and a diastereoselective pinacol cyclization. We envision that this strategy will permit the construction of other members of the family, structural analogs and provide a practical synthetic route to these important bioactive agents. In addition, we anticipate that the prodrug Mipsagargin, which is currently in late-stage clinical trials for the treatment of cancer, will also be accessible via this strategy. Hence, the limited availability from natural sources, coupled with an estimated demand of one metric ton per annum for the prodrug, provides a compelling mandate to develop practical total syntheses of these agents.

Full text of DOI:10.1021/jacs.7b01734

Communication
pubs.acs.org/JACS
A Concise, Efficient and Scalable Total Synthesis of Thapsigargin and
Nortrilobolide from (R)‑(−)-Carvone
Dezhi Chen and P. Andrew Evans*
Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada
*
S Supporting Information
intriguing structural complexity and biological importance of
ABSTRACT: A concise, efficient and scalable synthesis of
thapsigargin and nortrilobolide from commercially avail-
able (R)-(−)-carvone was developed. Our synthetic
strategy is inspired by nature’s carbon−carbon bond
formation sequence, which facilitates the construction of
a highly functionalized sesquiterpene lactone skeleton in
five steps via an enantioselective ketone alkylation and a
diastereoselective pinacol cyclization. We envision that this
strategy will permit the construction of other members of
the family, structural analogs and provide a practical
synthetic route to these important bioactive agents. In
addition, we anticipate that the prodrug Mipsagargin,
which is currently in late-stage clinical trials for the
treatment of cancer, will also be accessible via this strategy.
Hence, the limited availability from natural sources,
coupled with an estimated demand of one metric ton
per annum for the prodrug, provides a compelling
mandate to develop practical total syntheses of these
agents.
thapsigargin (1) have attracted degradation and derivatization
1
9−11
studies, in addition to total synthesis efforts.
Nevertheless,
the development of a commercially feasible synthetic route to this
complex chemical structure still represents a significant and
ongoingchallenge.Herein,wenowdiscloseaconcise,efficientand
scalablestrategytoaccessthehighlyoxidizedguaianolideskeleton,
which enables the total synthesis of thapsigargin (1) and
nortrilobolide (3) from commercially available (R)-(−)-carvone
(
10).
Thetotalsynthesisofthapsigargin(1)hasanumberofinherent
challenges, namely, the stereoselective construction of a hexaoxy-
genated 5−7−5 tricyclic guaianolide skeleton functionalized with
eight stereogenic centers, the installation of four different ester
groups, a trans-vicinal tertiary diol and an internal tetrasubstituted
olefin.Anefficientsynthesisof1requiresthestrategicintroduction
of these oxygen substituents to minimize redox chemistry,
protecting group and functional group manipulations. To this
end, we devised a divergent strategy, wherein the common
synthetic intermediate 6 would access both thapsigargin (1) and
the other members of the thapsigargin family, for example,
nortrilobolide (3) (Scheme 1A). Inspired by the proposed
12
hapsigargin (1) was isolated from the Mediterranean plant
ThapsiagarganicaL.byChristensenandco-workersin1978,
biosynthetic pathway for thapsigargin (1), we envisioned that
the disconnection of the carbon−carbon bond at C-6/C-7 would
provide a novel approach and minimize functional group
transformations (Scheme 1B). The assembly from (R)-
(−)-carvone (10) and the methylerythritol derivative 9 (see
Supporting Information) combines a ten- and a five-carbon
T
which along with a number of structurally related guaianolides are
1
,2
collectively called thapsigargins (Figure 1A). The structure of 1
was elucidated through extensive spectroscopic studies and X-ray
crystallographic analysis of various derivatives, which revealed a
hexaoxygenated guaianolide core functionalized with acetyl,
butyryl, angelyl and octanyl esters in addition to two free tertiary
13
fragment, which is consistent with the logic of nature’s strategic
combinationofbuildingblocksinterpenebiosynthesis.Hence,the
proposed abiotic approach would utilize a transition-metal-
catalyzed enantioselective alkylation reaction to forge the 15-
carbon framework 8 necessary for a metal-mediated aldehyde/
ketone pinacol coupling reaction to construct the guaianolide
skeleton 7 en route to the common synthetic intermediate 6
(Scheme 1C). This biosynthetically inspired process could
incorporate the necessary stereochemistry and functionalities for
the synthesis of thapsigargin (1) in a concise manner. Our
hypothesis would provide rapid and scalable access to 6, which
serves as a key intermediate for the preparation of 1 and related
members of the family that should permit detailed structure−
activity relationship studies on these important agents.
3
hydroxyl groups. The significance of thapsigargin (1) in
molecular biology and oncology is attested to by over 17 000
4
publications over the past 40 years. For instance, 1 is categorized
as a highly selective subnanomolar inhibitor of intracellular
calcium ion transport enzymes, which are termed sarco/
2
+
5
endoplasmic reticulum Ca -ATPases (SERCAs). The remark-
ablyhighlevelofselectivitytowardSERCAsmakes1aparticularly
2
+
useful tool to investigate a variety of Ca -dependent cellular
6
processes. More significantly, the induction of cell apoptosis is
2
+
also dependent on Ca signals, in which the strategic application
of thapsigargin (1) to promote the induction of programmed
cancer cell death in a proliferation independent manner has led to
7
The synthesis commences with the regioselective allylic
the development of novel cancer therapeutics. For example, the
14
chlorination of the commercially available monoterpene, (R)-
−)-carvone (10), followed by a one-pot reduction and in situ
prodrug,Mipsagargin(5),whichhasapolypeptidechainattheC-8
positionof1,iscurrentlyinlatestageclinicaltrialsforthetreatmen8t
of liver, brain, prostate and kidney cancer (Figure 1B).
Furthermore, it is anticipated that the clinical demand for this
(
Received: February 18, 2017
1
,9
agent is likely to exceed one metric ton per annum. The
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b01734
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Journal of the American Chemical Society
Communication
Figure 1. Structurally and biologically interesting thapsigargin derivatives.
Scheme 1. Biosynthetic Inspiration, Strategic and Retrosynthetic Analysis
protection of the secondary alcohol to provide the tert-
accomplished via selective ozonolysis of the more electron-rich
olefin followed by an in situ intramolecular aldol condensation
15
butyldimethylsilyl ether 11 in excellent overall yield (Scheme
). The asymmetric alkylation of 11 with the lithium enolate of
13b,17
2
catalyzedbypiperidiniumacetate,
toaffordthecyclopentene
ketone 9, which was generated with lithium hexamethyldisilazide,
was achieved in the presence of lithium chloride and the chiral
catalystderivedfromPd (dba) ·CHCl and(S)-BINAPtofurnish
derivative 8 in moderate yield. This one-pot transformation
permits the installation of the internal tetrasubstituted double
bond present in the natural product and simultaneously provides
the intermediate required for the sequential pinacol coupling/
lactonization cascade reaction. To this end, a range of reaction
2
3
3
the coupled product 12 in 93% yield and with 8:1 diastereose-
1
6
lectivity. The ring contraction of the cyclohexene in 12 was then
B
DOI: 10.1021/jacs.7b01734
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Journal of the American Chemical Society
Communication
Scheme 2. 12-Step Total Synthesis of Thapsigargin (1) from (R)-(−)-Carvone
conditions were investigated to implement the proposed
sequence, in which the well-established samarium(II) iodide
mediatedpinacolcouplingwitharangeofadditivesprovidednone
oxidation of the resulting secondary hydroxyl group with 2-
iodoxybenzoic acid (IBX) followed by sodium borohydride
reduction, afforded 6 in 94% overall yield and with ≥19:1
diastereoselectivity on gram scale. In addition, these conditions
conveniently cleave the trimethylsilyl protecting group. The
selectiveacylationof6proceededcleanly,whichpermitstheinsitu
Jones oxidation of the tert-butyldimethylsilyl ether to furnish the
keyα,β-unsaturated cyclopentenone15in87%yieldandthussets
18
of the desired adduct. Alternatively, treatment of 8 with low
19
valent titanium reagents delivered reduced side products.
Gratifyingly, more encouraging results were obtained when 8
was slowly added to a solution of the dimeric vanadium complex,
[
V Cl (THF) ] [Zn Cl ], which was prepared in situ in the
2 3 6 2 2 6
22
presence of hexamethylphosphoramide (HMPA), to afford the
sesquiterpene lactone 7 in 60% yield and with ≥19:1
the stage for the completion of the synthesis.
The advancement of 15 to thapsigargin (1) was achieved via a
2
0
9
diastereoselectivity. Hence, this five-step sequence provides a
scalable and stereochemically versatile approach to the highly
oxidizedguaianolideskeleton7,whichwillenabletheconstruction
of a wide array of structural analogs.
modification of Christensen’s three-step protocol. The octanoy-
loxy side chain at the C-2 position in 15 was introduced
stereoselectively using manganese(III) acetate [Mn(OAc) ] as
3
theoxidantinbenzeneandoctanoicacidasamixedsolventsystem
to afford 16 in modest yield. Diastereoselective reduction of the
α,β-unsaturated ketone in 16 with zinc borohydride at −20 °C,
followedbytheangeloylationofthestericallyhinderedC-3alcohol
usinganhydride17inthepresenceofsodiumbicarbonateafforded
The installation of the tertiary acetate at the C-10 position was
then accomplished by a selective cobalt-catalyzed Mukaiyama
hydration of the less sterically hindered olefin and acetylation of
21
the resulting tertiary alcohol. Treatment of 7 with cobalt(II)
10b
acetylacetonate [Co(acac) ] catalyst and excess phenylsilane
the natural product, thapsigargin (1), in 64% yield.
2
(
PhSiH ) under an oxygen atmosphere provided 13 in 79% yield
Nortrilobolide(3),whichdiffersfromthapsigargin(1)attheC-
3
and with ≥19:1 diastereoselectivity. Single-crystal X-ray diffrac-
tion analysis of 13 confirmed both the relative configuration and
structural assignment of this key tricyclic intermediate. The
selective acetylation of the C-10 tertiary alcohol in 13 at elevated
temperature then afforded the desired tertiary acetate 14 in
excellent yield. The stereochemistry at the C-8 position was then
adjustedandtherequisiteesterwasintroducedusingthefollowing
sequence. Selective hydrogenation of the benzyl ether in 14 with
palladium hydroxide under a hydrogen atmosphere and the
2position,wasisolatedfromthesameplantThapsiagarganicaL.by
SmittandChristensenin1991. Althoughthereisnooxygenated
2b
substituent at the C-2position, ithas beenreported that 3 exhibits
23
equipotent inhibition of SERCAs to that of thapsigargin (1).
Hence, nortrilobolide (3) represents an important lead for the
development of new anticancer agents, which can be prepared
using the rapid and scalable synthesis of the common synthetic
intermediate 6. The selective acylation of the C-8 secondary
alcohol in 6 followed by the in situ deprotection of the tert-
C
DOI: 10.1021/jacs.7b01734
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Journal of the American Chemical Society
Communication
(
2) For the isolation of the thapsigargins, see: (a) Rasmussen, U.;
Scheme 3. 10-Step Total Synthesis of Nortrilobolide (3) from
Christensen,S.B.;Sandberg,F.ActaPharm.Suec.1978,15,133.(b)Smitt,
U. W.;Christensen, S.B.PlantaMed. 1991,57,196.(c)Liu,H.;Jensen,K.
G.;Tran,L.M.;Chen,M.;Zhai,L.;Olsen,C.E.;Søhoel,H.;Denmeade,S.
R.; Isaacs, J. T.; Christensen, S. B. Phytochemistry 2006, 67, 2651.
(R)-(−)-Carvone
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C. J. Org. Chem. 1982, 47, 649. (b) Christensen, S. B.; Norup, E.
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(
4) SCIFINDER, American Chemical Society (Searched by research
topic ‘thapsigargin’ on February 18, 2017).
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J.; Fu, X.; Kunnen, K. Methods of Making Cancer Compositions. WO
(
(
1
(
butyldimethylsilyl protecting group under mild acidic conditions
furnished an allylic alcohol intermediate, which was subjected to
the same Yamaguchi acylation described above to provide
9
(
10a
nortrilobolide (3) in 78% yield over two steps (Scheme 3).
Overall, the total syntheses of thapsigargin (1) and
nortrilobolide (3) were accomplished in 12 and 10 steps,
respectively (longest linear sequence: 5.8% and 13.3% overall
yield) from commercially available (R)-(−)-carvone (10).
Notably, the total syntheses were accomplished in less than one-
third of the number of steps required by Ley and co-workers (42
and36steps, respectively)andsignificantlymoreefficient(40and
2
014145035A1, September 18, 2014.
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(
̈
a) Oliver, S. F.; Hogenauer, K.; Simic, O.; Antonello, A.; Smith, M. D.;
Ley, S. V. Angew. Chem., Int. Ed. 2003, 42, 5996. (b) Ball, M.; Andrews, S.
P.; Wierschem, F.; Cleator, E.; Smith, M. D.; Ley, S. V. Org. Lett. 2007, 9,
3
0 times, respectively) than the syntheses recently reported by
6
63. (c) Andrews, S. P.; Ball, M.; Wierschem, F.; Cleator, E.; Oliver, S.;
Baran and co-workers. Furthermore, the five-step synthesis of 7
Hogenauer,K.;Simic,O.;Antonello,A.;Hunger,U.;Smith,M.D.;Ley,S.
̈
̈
represents one of the shortest approaches to the guaianolide
V. Chem. - Eur. J. 2007, 13, 5688. (d) Chu, H.; Smith, J. M.; Felding, J.;
Baran, P. S. ACS Cent. Sci. 2017, 3, 47.
24
skeleton developed to date, which we anticipate will allow the
rapid preparation of alibrary of simplifiedthapsigargin analogs for
detailed structure−activity relationship studies. In addition, it is
envisioned that this approach could provide the basis for a
manufacturingroutetothisimportantagent,particularlygiventhe
brevity and scalability of the synthesis. Finally, we believe that this
strategy will provide a useful guide for the construction of related
polyoxygenated terpenes.
(
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Antonello, A.; Balskus, E. P.; Booth, D. T.; Christensen, S. B.; Cleator, E.;
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ASSOCIATED CONTENT
Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/jacs.7b01734.
■
*
S
Experimental procedures, spectral data and copies of
spectra for all new compounds (PDF)
Crystallographic data (CIF)
(
13) (a) Yang, H.; Gao, Y.; Qiao, X.; Xie, L.; Xu, X. Org. Lett. 2011, 13,
670. (b) Hu, X.; Xu, S.; Maimone, T. J. Angew. Chem. Int. Ed. 2017, 56,
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3
AUTHOR INFORMATION
Corresponding Author
Andrew.Evans@chem.queensu.ca
ORCID
P. Andrew Evans: 0000-0001-6609-5282
Notes
The authors declare no competing financial interest.
■
*
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ACKNOWLEDGMENTS
■
Dedicated to Professor Stephen F. Martin for his outstanding
contributionstonaturalproductchemistry.Wesincerelythankthe
National Sciences and Engineering Research Council (NSERC)
foraDiscoveryGrantandQueen’sUniversityforgenerousfinancial
support. NSERC is also thanked for supporting a Tier 1 Canada
Research Chair (P.A.E.).
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DOI: 10.1021/jacs.7b01734
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX