A Biomimetic Strategy to Access the Silybins: Total Synthesis of (-)-Isosilybin A

Article abstract of DOI:10.1021/ol503303w

(Figure Presented). We report the first asymmetric, total synthesis of (-)-isosilybin A. A late-stage catalytic biomimetic cyclization of a highly functionalized chalcone is employed to form the characteristic benzopyranone ring. A robust and flexible app

Full text of DOI:10.1021/ol503303w

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Letter
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A Biomimetic Strategy to Access the Silybins: Total Synthesis
of (−)-Isosilybin A
Benjamin R. McDonald, Antoinette E. Nibbs, and Karl A. Scheidt*
Dept. of Chemistry, Dept. of Pharmacology, Center for Molecular Innovation and Drug Discovery, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: We report the first asymmetric, total synthesis of (−)-isosilybin A. A late-stage catalytic biomimetic cyclization of
a highly functionalized chalcone is employed to form the characteristic benzopyranone ring. A robust and flexible approach to
this chalcone provides an entry to the preparation of the entire isomeric family of silybin natural products.
ilk thistle (Silybum marianum) has been used for centuries
Mfor the treatment of liver disorders and as a hepato-
protectant.1 The extracts of the fruit (achenes) are known as
silymarin and consist of a complex mixture of flavonolignans and
flavonoids including silybin A, silybin B, isosilybin A, isosilybin B,
silychristin, isosilychristin, silydianin, and dihydroquercetin, also
known as taxifolin.2 Our interest in these compounds is driven by
the limited efficient asymmetric approaches for their con-
struction,3 their promising activity against prostate cancer,4 and
our ongoing translational interest in this area.5 The members of
the silybin family share a functionalized benzopyranone core, but
differ in a benzodioxane core that is differentially substituted
to produce regioisomers with complementary diastereomers
(Figure 1).6 While gram quantities of each isomer can be isolated
via arduous processes,7 no general and flexible stereoselective
routes to these molecules exist, which prohibits a detailed,
molecularly driven understanding of their promising pharmacol-
ogy.8 Flavanolignans such as the silybin isomers originate from
the cyclization of a chalcone to a stereodefined flavanone by
chalcone isomerase (CHI) in an estimated S/R ratio of
100 000:1 (ee = 99.998%).9 In nature, these simple flavanones
are then further processed and functionalized to produce a vast
array of flavonoids.10 Given that the diversity-generating bio-
synthetic pathway occurs after CHI cyclization, our initial
interest was to explore a general synthesis strategy that could
regio- and stereoselectively produce each silybin member. In
contrast to a general route following an early cyclization to the
flavanone core followed by functionalization (biosynthesis), we
considered that a late-stage, biomimetic cyclization could enable
the stereoselective synthesis of the silybin isomers (chemical
synthesis).11 A major challenge to our approach would be the
limited stereoselective methods to access complex flavanones
from 2′-hydroxy chalcones in a manner similar to CHI.3,12
Our unified strategy to construct each member of the silybin
family follows a biomimetic approach inspired by CHI, specifically
the cyclization of an appropriate 2′-hydroxy chalcone substrate to
forge the characteristic benzopyranone.13
Of the two different regioisomeric pairs (silybins A, B and
isosilybins A, B), we selected the isosilybins as initial targets to
pursue due to their superior activity against prostate cancer. The
chalcone precursor requires (1) an aldol condensation between a
sterically congested 2′,4′,6′-trisubstituted acetophenone and a
1,4-benzodioxane aldehyde and (2) careful orchestration of the
alcohol/ether stereocenters in the aldehyde fragment 3 (Figure 1).
Subsequent chalcone cyclization studies by Hintermann14
toward flavanones and additional investigations by our
group15 have also provided key information about the subtle
yet crucial aspects of conformational and electronic factors of
the 2′,4′,6′-trisubstituted aryl ring vs typical 2′-monosubsti-
tuted substrates.3 Reported herein is the first asymmetric, total
synthesis of (−)-isosilybin A.
The synthesis of the requisite benzodioxane fragment began
from commercially available vanillin (4), which was elaborated to
cinnamate 5 via Boc-protection and the Horner−Wadsworth−
Emmons olefination in 86% yield over two steps (Scheme 1). A
reduction with DIBAL-H and subsequent dihydroxylation with
AD-mix β yielded enantioenriched triol 6 in 69% yield.16
A
regioselective mesylation followed by treatment with a mild base
afforded multigram quantities of enantioenriched epoxide 7 in
58% yield and 99:1 er over two steps.17 Aldehyde 8 was coupled
with the epoxy alcohol via a robust Mitsunobu reaction in 91%
yield (>20:1 dr) on gram scale.18 We found that the protection of
the phenol of 4 with an electron-withdrawing group (i.e., Boc)
was essential for stereochemical transfer. The use of a benzyl
protective group led to incomplete conversion and low
diastereoselectivity (2.5:1), presumably via partial or complete
Received: November 13, 2014
© XXXX American Chemical Society
A
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With epoxy aldehyde 9 in hand, the removal of the allyl pro-
tective group under Pd(0) conditions occurred (55%, unoptimized)
to provide a phenol that, upon subsequent treatment with
potassium carbonate in MeOH, would yield the desired aldehyde
12 (Scheme 1). Unexpectedly, epoxide 10 underwent cyclization
to provide preferentially the seven-membered benzodioxipine
(11) vs the desired six-membered benzodioxane (12) under all
conditions examined.19 This challenging ring closure step was
obviated by a Co(III)-catalyzed hydrolytic epoxide opening20 of
9 and subsequent regioselective TIPS protection to provide
secondary alcohol 13 on gram scale and in excellent yield
(84% over two steps) (Scheme 2). A palladium(0)-catalyzed
Scheme 2. Generation of Chalcone 15
deallylation followed by a Mitsunobu ring closure provided the
completed aldehyde fragment 3 in 66% yield over two steps. The
subsequent aldol condensation with acetophenone 14 yielded
the desired chalcone 15.
With a robust route to the key chalcone substrate in place,
attention was turned to the development of a catalytic system to
promote formation of the benzopyranone ring with control at
the newly established C-2 stereocenter. Based on previous
reports14,21 and our own knowledge with related cyclizations,15
we envisioned bifunctional (thio)urea cinchona alkaloid catalysts
would activate the carbonyl of the chalcone through hydrogen
bonding, deprotonate the phenol with the quinuclidine nitrogen,
and organize the chalcone complex, thereby allowing for
asymmetric intramolecular conjugate addition of the phenol.
Preliminary studies were carried out on the cyclization of
naringenin dimethyl ether chalcone 16 (Table 1). Notably, the
2′- and 6′-phenolic oxygens must remain unprotected for
successful cyclizations under these conditions.22 Quinine and
quinidine stereodivergently produced the desired flavanone with
good stereoselectivity, but the overall process was extremely slow
(Table 1, entries 1−2). The addition of Schreiner’s thiourea23
(C) accelerated reaction times, but resulted in lower stereo-
selectivity (Table 1, entries 3−4). Merging thiourea functionality
and cinchona alkaloid scaffolds into a single compound based on
Hiemstra’s reports24 (catalysts D and E) improved reaction
times (<6 h). These pseudoenantiomeric catalysts25 led to good
diastereoselectivity (85:15) for the desired (R)-flavanolignan
(with F) yet an almost racemic product (56:44) with D (Table 1,
entries 5−6). Further studies to understand the stereochemical
complexities of this cyclization (i.e., matched vs mismatched)26
and the investigation of new catalysts to favor the (S) antipode in
a manner that is complementary to selectivities observed with
catalyst F are ongoing.27
Figure 1. Synthetic strategy for the total synthesis of 1.
Scheme 1. Preparation of Epoxy Ether 10
Chalcone 17, possessing an unsubstituted benzodioxane ring
and unprotected 4′-phenol, was then explored as a simplified
analog. Switching the solvent to MeCN allowed for a lower
catalyst loading (15 mol %) without significant loss of
formation of a para-quinone methide intermediate and
subsequent SN1-type addition of 8.
B
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Table 1. Optimization of Biomimetic Chalcone Cyclization
Scheme 3. Completion of the Total Synthesis of
(−)-Isosilybin A
directly with DMDO to smoothly furnish a mixture of α-hydroxyl
20 and α-siloxy 21 in a combined 73% yield (5:1 ratio of 20:21).
The α-siloxy flavanone (21) was cleanly converted to
α-hydroxy flavanone 20 with treatment of dilute aqueous HCl.
Unfortunately, minor erosion of the stereochemistry at the
C2-position of 20 was observed, with enantiopure MOM-
flavanone 19 yielding a 9:1 mixture of trans diastereomers. We
attributed this minimal epimerization to ring opening during the
enolization/silylation protocol. Unfortunately, exploration of
soft enolization conditions resulted in complex mixtures of
chalcone, flavanones, and silyl enol ether. Global MOM-
deprotection was carried out using pTsOH·H2O conditions to
obtain isosilybin A (1).
Gratifyingly, our synthetic material matched data reported for
the natural material by high-field NMR spectroscopy (1H and
13C).31 Electronic circular dichroism (ECD) has been used
extensively in the assignment of the absolute stereochemistry of
flavanones32 as well as the silybins.2 Oberlies and co-workers
have further validated the use of ECD to assign the stereo-
chemistry of silybins via X-ray crystallography of a heavy atom
analog of natural (+)-isosilybin A.33 Through ECD and optical
rotation measurements, synthetic 1 was determined to be the
unnatural (−) antipode (see Supporting Information for this
analysis). The application of this approach to other silybins is
ongoing and ultimately should provide a general and unique
method to fashion new silylbin analogs for structure−biological
activity relationships.
In summary, the first synthesis of isosilybin A, a member of the
silybin flavanolignans, was achieved in 16 steps (longest linear).
The strategy employs a late-stage biomimetic cyclization of a
chalcone to install the benzopyranone ring, which was inspired
by the biosynthesis route employing chalcone isomerase (CHI).
The applications of an asymmetric Sharpless dihydroxylation and
Mitsunobu inversion transformations further provide a robust and
flexible approach to selectively synthesize the benzodioxane ring
systems of the silybin isomers. These stereoselective operations
combined with our biomimetic cyclization to install the flavanone
core ring system provide a general platform to access all silybins.
This pursuit and the use of these molecules in biological investiga-
tions are ongoing and will be reported in due course.
a
Time required for complete consumption of starting material; yield
b
not determined. 1:1 ratio of catalysts (50 mol % each).
stereoselectivity. The subjection of the fully elaborated substrate
1828 to the optimized conditions using catalyst E necessitated
increasing the catalyst loading to 30 mol %, but afforded the
product with 83:17 dr. Notably, the use of achiral thiourea C with
chalcone 18 led to a 50:50 mixture of diastereomers, indicat-
ing the existing stereocenters in the benzodioxane portion do not
exert stereocontrol over ring closure (Table 1, entry 10).
Ultimately, urea analog F was determined to be the optimal
catalyst with reduced reaction times (36 h vs 48 h) and a small
improvement in stereoselectivity (entry 13).
Following our carefully optimized biomimetic cyclization,
the free phenols and alcohols of the flavanone were globally
protected for the Rubottom oxidation sequence.29 While global
Boc-protection was high-yielding, the resulting tert-butyl
carbonates unfortunately impeded the oxidation of the resulting
silyl enol ether. Presumably, these groups significantly decreased
the nucleophilicity of the silyl enol ether and rendered it un-
reactive toward a wide variety of electrophilic oxidizing reagents
(e.g., DMDO, MoOPh, mCPBA, and Davis oxaziridine). Thus,
conditions were developed for an unusual global MOM-
protection, which was challenging due to the stereoelectronic
differences of the four free hydroxyl groups. Based on reports
by Mander,30 DMAP was a key additive to enable efficient
global MOM-protection (72% yield, Scheme 3). At this point,
flavonolignan 19 and its diastereomer were separated by
semipreparative HPLC. The careful treatment of enantiopure
MOM-protected 19 with TMSCl/Et3N, followed by LiHMDS
at −78 °C, provided the trimethylsilyl enol ether, which was treated
C
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Letter
(10) Ngaki, M. N.; Louie, G. V.; Philippe, R. N.; Manning, G.; Pojer, F.;
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ASSOCIATED CONTENT
■
S
* Supporting Information
(11) For recent syntheses of polyaromatic phenol-containing natural
products, see: Zhu, Z.−Q.; Beaudry, C. M. J. Org. Chem. 2013, 78, 3336.
Jepsen, T. H.; Thomas, S. B.; Lin, Y. Q.; Stathakis, C. I.; de Miguel, I.;
Snyder, S. A. Angew. Chem., Int. Ed. 2014, 53, 6747.
Experimental procedures, characterization data, and spectra,
1
including H and 13C NMR, ORD, and CD. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) Almasi, D.; Alonso, D. A.; Najera, C. Tetrahedron: Asymmetry
2007, 18, 299.
AUTHOR INFORMATION
■
(13) The feasibility of this approach was demonstrated by the
pioneering work of Ishikawa: Tanaka, T.; Kumamoto, T.; Ishikawa, T.
Tetrahedron Lett. 2000, 41, 10229. Sekino, E.; Kumamoto, T.; Tanaka,
T.; Ikeda, T.; Ishikawa, T. J. Org. Chem. 2004, 69, 2760.
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Corresponding Author
*E-mail: scheidt@northwestern.edu.
Notes
The authors declare no competing financial interest.
(15) Due to this increased substitution, our efforts to access the
appropriate alkylidene β-ketoesters were unsuccessful and warranted the
development of a protocol for the cyclization of chalcones to flavanones.
Biddle, M. M.; Lin, M.; Scheidt, K. A. J. Am. Chem. Soc. 2007, 129, 3830.
(16) At this stage, the use of AD-mix α could lead to the corresponding
enantiomer which maps directly onto the absolute configuration
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X. F.; Chan, A. S. C.; Yang, T. K. Tetrahedron Lett. 2000, 41, 6079.
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(19) Seven member ring confirmed by single crystal X-ray analysis of a
downstream intermediate (see Supporting Information). For similar
cyclizations, see: Ganesh, T.; Krupadanam, G. L. D. Synth. Commun.
1998, 28, 3121. Reference 17. Banwell, M. G.; Chand, S.; Savage, G. P.
Tetrahedron: Asymmetry 2005, 16, 1645.
ACKNOWLEDGMENTS
■
Financial support has been provided by the American Chemical
Society (Research Scholar Award 09-016-01 CDD to K.A.S.).
A.E.N. acknowledges the NIH for a predoctoral fellowship
(F31CA132617) and the NU Malkin Scholars Program for
support. We thank Michael Wang (NU) for providing X-ray
crystallography support. Purification support was provided by
the Center for Molecular Innovation and Drug Discovery
ChemCore (NIH S10RR025690). We thank Regis Technologies
for chromatography assistance and Prof. Nicholas Oberlies
(University of North Carolina Greensboro) for providing
authentic samples of (+)-isosilybin A and (−)-isosilybin B. We
thank Tyler Graf (University of North Carolina Greensboro) for
helpful discussions regarding the characterization of silybins by
electronic circular dichroism.
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