Scalable Synthesis of (-)-Rasfonin Enabled by a Convergent Enantioselective α-Hydroxymethylation Strategy

Article abstract of DOI:10.1021/acs.orglett.8b02222

A scalable synthesis of the potent antitumor agent, (-)-rasfonin, has been achieved. The synthetic strategy features a highly convergent approach based on a single protocol construction of both major fragments via catalytic enantioselective α-hydroxymethylation of simple aliphatic aldehydes. The route described has been successful in the generation of gram quantities of the natural product and serves as the first synthetic strategy to provide sufficient material to continue studies related to its mechanism of action and potential as a cancer therapeutic.

Full text of DOI:10.1021/acs.orglett.8b02222

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Scalable Synthesis of (−)-Rasfonin Enabled by a Convergent
Enantioselective α‑Hydroxymethylation Strategy
Robert K. Boeckman, Jr.,* Justin M. Niziol, and Kyle F. Biegasiewicz
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: A scalable synthesis of the potent antitumor agent, (−)-rasfonin, has been achieved. The synthetic strategy
features a highly convergent approach based on a single protocol construction of both major fragments via catalytic
enantioselective α-hydroxymethylation of simple aliphatic aldehydes. The route described has been successful in the generation
of gram quantities of the natural product and serves as the first synthetic strategy to provide sufficient material to continue
studies related to its mechanism of action and potential as a cancer therapeutic.
he α-pyrone-containing natural product, (−)-rasfonin
(1), was isolated by Hayakawa and co-workers in 2000
truly scalable route that would provide enough material for
further biological testing has remained elusive. As a result of
our ongoing synthetic studies, in combination with our interest
in understanding the biological activity associated with 1, we
have now planned and executed a scalable route of 1. The
results of these studies are presented herein.
Retrosynthetically, as previously practiced, we envisioned
(−)-rasfonin (1) as the product of a Yamaguchi esterification
between alcohol 2 and acid 3. Alcohol 2 is envisioned to be the
product of a CBS-catalyzed vinylogous Mukaiyama addition of
2-(trimethylsiloxy)furan to aldehyde 4. A subsequent DBU-
catalyzed furanone-to-pyranone rearrangement would then
furnish 2.
As part of our new plan, we envisioned access to (E,E)-acid
3 through a Horner−Wadsworth−Emmons protocol starting
from aldehyde 5. We then anticipate rapid construction of both
key intermediate aldehydes 4 and 5 using the same synthetic
protocol involving a tandem catalytic α-hydroxymethylation/
Wittig olefination sequence from simple aliphatic aldehydes 6
and 7, respectively (Scheme 1).
We began our synthetic studies with an organocatalytic α-
hydroxymethylation sequence recently developed in our
laboratory.10 Propionaldehyde (6) was treated with formalin
(37% aq) in the presence of the (S)-Jørgensen−Hayashi
prolinol catalyst 8 (10 mol %) in buffered media (pH = 7),
f o l l o w e d b y t r e a t m e n t w i t h e t h y l 2 -
(triphenylphosphaneylidene)propanoate which provided un-
saturated ester 9 in high yield and enantioselectivity (81%,
94:6 er). The resulting unsaturated ester was subjected to
T
from the fermentation broth of Taleromyces sp. 3656-A1.1 The
same year, it was also isolated (and temporarily identified as
TT-1) by Ishibashi et al. from the fungi imperfecti Trichurus
terrophilus.2 The molecule is now most commonly referred to
as (−)-rasfonin, as it has gained a significant amount of
attention through its interactions with the G-protein, Ras. In
vivo studies show that 1 targets and downregulates Ras activity,
consequently causing a downregulation of phosphorylation
events in the c-Raf/MEK/ERK signaling pathway in proteins.
(−)-Rasfonin was also shown to reduce Son of sevenless
(Sos1) expression with no change in GEF or GAP activity. It
has also been demonstrated that 1 is a potent inhibitor of
pancreatic cancers with the K-ras mutation. It has also
successfully delayed the growth of xenograft tumors (30 mg/
kg) originating from Panc-1 cells, with an eventual reduction in
tumor mass after 20 days of treatment.3 (−)-Rasfonin has even
been shown to interact with the autophagic and necrotic
pathways of renal cancer cell lines, the former being of
extraordinarily high interest in the pursuit of cancer
therapeutics.4−6
Along with its significance from a biological activity
perspective, (−)-rasfonin (1) contains a series of synthetically
interesting moieties, exemplified by a pendant dihydrtoxylated
ester and a tetramethylated alkyl sidechain, both of which stem
from the chiral α,β-unsaturated pyranone core on carbon
atoms 4 and 5, respectively (Scheme 1).
Apart from our initial synthesis of (−)-rasfonin in 2006,
there have only been three other total syntheses reported to
date.1,7−9 While all of these synthetic endeavors have provided
uniquely interesting approaches to the construction of 1, a
Received: July 16, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.8b02222
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Retrosynthesis of (−)-Rasfonin (1)
Scheme 2. Synthesis of Aldehyde 4
As employed in our initial 2006 synthesis,7 completion of
the pyranone subunit of 1 from aldehyde 4 was carried out via
a CBS-catalyzed vinylogous Mukaiyama aldol reaction with
(furan-2-yloxy)trimethylsilane and the Corey oxazaborolidine
catalyst 13. The TMS-furan is easily generated from the Dakin
oxidation of furfural followed by soft enolization of the
resulting butenolide. Catalyst 13 is generated in situ from the
oxazaborolidine (obtained from the dehydration of diaryl
prolinol and o-tolylboroxine) and triflic acid at −78 °C.
Introduction of aldehyde 4 followed by (furan-2-yloxy)-
trimethylsilane cleanly provides nearly a single diastereomer of
aldol adduct 14 in good yield (74% yield, > 20:1 syn:anti, >
20:1 dr).7 Successive reduction of 14 with DIBAL-H, stirring
with DBU, and oxidation with MnO2 in a 3-step sequence
provide a mixture of the desired pyranone 15 (34% yield or
70% average yield over 3 steps at 45% conversion for a single
pass) along with starting material 14, as we have previously
described.7 The starting material 14 can be recycled, increasing
the overall conversion. Unsaturated pyranone 15 was then
treated with 1 M HCl in THF to provide the desired
unsaturated pyranone alcohol 2 in 95% yield (Scheme 3).
Our further optimized synthesis of the side chain carboxylic
acid 3 commenced with the mono-TBS protection of 1,4-
butanediol (16) followed by Swern oxidation, providing
aldehyde 7 in high yield (88% over two steps).15 An ensuing
α-hydroxymethylation of 7, followed by a one-pot successive
treatment with ethyl 2-(triphenylphosphanylidene)propanoate
and then TBS protection conditions, provided the expected
chiral ester 18 in high yield and enantioselectivity (94%, 93:7
er). Reduction with excess DIBAL-H followed by Parikh−
Doering oxidation16 furnished aldehyde 19 with the α-olefin
intact. Horner−Wadsworth−Emmons olefination17 gave meth-
yl ester 20 which was efficiently hydrolyzed with excess LiOH·
H2O in THF:CH3OH:H2O (2:2:1) to provide the acid chain 3
of (−)-rasfonin in excellent yield. Yamaguchi esterification18 of
asymmetric hydrogenation conditions developed by Evans
using [Rh(nbd)(+)BINAP]BF4 under 250 psi of H2 (nbd =
norbornadiene).11 It was found that the catalyst loading for
this reaction could be as low as 0.3 mol % while retaining full
conversion. Theoretically, this number can be reduced even
further if the reaction vessel could be more strictly purged of
moisture and oxygen. We suggest that to attain full conversion
a sufficient quantity of the rhodium catalyst must be
introduced to compensate for catalyst deactivation by
exogenous moisture in the reaction vessel. Upon reaction
completion and analysis of the products, the rhodium catalyst
was found to have promoted partial lactonization (∼40%),
owing to its strong Lewis acid properties. This result was easily
accommodated by direct conversion of the mixture solely to
the desired chiral Weinreb amide 10 using standard
conditions12 in 81% yield over two steps.
Conversion of the primary alcohol 10 to the corresponding
iodide 11 via an Appel reaction13 provided the precursor for a
Ni-catalyzed coupling reaction recently described by Weix.14 In
the presence of (bpy)NiI2 (bpy = bipyridyl) and Mn0, primary
iodide 11 is readily coupled to commercially available (E)-2-
bromo-2-butene to provide amide 12 in high yield (91%). Use
of undistilled commercial TMSCl was found to be important
for reproducibly high conversion of 11 to 12. We surmise that
traces of HCl in the TMSCl serve to activate the manganese
metal surface. Use of freshly distilled TMSCl led, as expected,
to lower conversion to 12 after the indicated reaction time.
Reduction of the Weinreb amide 12 with DIBAL-H in ether at
−78 °C then provided aldehyde 4 in nearly quantitative yield
(Scheme 2).
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DOI: 10.1021/acs.orglett.8b02222
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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making available the necessary quantities of 1 to hopefully
realize the full potential of (−)-rasfonin (1) as a novel cancer
therapeutic agent.
Scheme 3. Completion of Pyranone 2
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02222.
Experimental procedures (PDF)
Spectral data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rkb@rkbmac.chem.rochester.edu.
ORCID
acid 3 with alcohol 2 proceeded without remark, and a final
treatment with HF (48% in water) in acetonitrile8 afforded
(−)-rasfonin (1) i whose spectroscopic data were in complete
agreement with those of authentic (−)-rasfonin and our
previously prepared synthetic sample in nearly quantittive
yield (Scheme 4).1
Robert K. Boeckman, Jr.: 0000-0001-7230-4642
Notes
The authors declare the following competing financial
interest(s): (−)-Rasfonin generated via the outlined synthesis
is currently undergoing biological profiling at the Max-Planck
Institute of Molecular Physiology in Dortmund Germany by
Dr. Herbert Waldmann and his group.
Scheme 4. Completion of (−)-Rasfonin (1)
ACKNOWLEDGMENTS
■
The authors would like to thank Dr. Douglas Tusch, Dr.
Daniel Austin, and Ali Lambright of the Department of
Chemistry at the University of Rochester for their initial efforts
toward the synthesis of the northern hemisphere.
REFERENCES
■
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DOI: 10.1021/acs.orglett.8b02222
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