Enantioselective Synthesis of Both Epimers at C-21 in the Proposed Structure of Cytotoxic Macrolide Callyspongiolide

Article abstract of DOI:10.1021/acs.orglett.6b01523

Both epimers at C-21 in the proposed structure of (+)-callyspongiolide have been synthesized in a convergent and enantioselective manner. The 14-membered macrolide with a sensitive C2-C3 cis-olefin functionality was installed by a Yamaguchi macrolactoniza

Full text of DOI:10.1021/acs.orglett.6b01523

Letter
pubs.acs.org/OrgLett
Enantioselective Synthesis of Both Epimers at C‑21 in the Proposed
Structure of Cytotoxic Macrolide Callyspongiolide
Arun K. Ghosh* and Luke A. Kassekert
Department of Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana
47907, United States
S
* Supporting Information
ABSTRACT: Both epimers at C-21 in the proposed structure of (+)-call-
yspongiolide have been synthesized in a convergent and enantioselective manner.
The 14-membered macrolide with a sensitive C2−C3 cis-olefin functionality was
installed by a Yamaguchi macrolactonization of hydroxyl alkynoic acid followed
by hydrogenation over Lindlar’s catalyst. The C5 methyl stereocenter was
constructed by a ring-closing olefin metathesis followed by addition of methyl
cuprate to an α,β-unsaturated δ-lactone. Other key reactions are chiral Corey−
Bakshi−Shibata (CBS) reduction and Sonogashira coupling to conjoin the macrocyclic core and side chain.
arine sponges of the genus Callyspongia have proven to be
rich sources of natural products that display a wide
M
spectrum of biological activity.1−4 Earlier this year, Shin and co-
workers isolated callyazepin, a nitrogenous macrocycle display-
ing moderate cytotoxicity against K562 and A549 cell lines.5
Proksch and co-workers recently tested a methanolic extract of
the marine sponge Callyspongia sp., and biological results
revealed complete inhibition of the murine lymphoma cell line
L5178Y.6 The structure and relative stereochemistry was
determined using NMR and HRMS analysis, yet the sole C21
stereocenter located on the diene-ynic side chain was left
unassigned. More interesting was the discovery that against
human Jurkat J16 T and Ramos B lymphocytes, callyspongiolide
was highly cytotoxic with IC50 values of 70 and 60 nM,
respectively. With the addition of Q-VD-OPH, apoptosis
persisted, suggesting a mode of cell death that does not involve
a caspase pathway. Furthermore, increased levels of hypodiploid
nuclei were observed at EC50 levels of 80 nM for Jurkat J16 T and
50 nM for Ramos B lymphocytes. As part of our continuing
interest in the synthesis of bioactive natural products, and taking
into consideration the novel structure of this macrolide, we
sought to establish a convergent and enantioselective synthesis of
callyspongiolide to aid in structural determination, and to
expedite further biological studies. Herein, we report our
preliminary investigation that has led to a convergent and highly
stereoselective synthesis of the proposed structure of both
epimers at C-21 of (+)-callyspongiolide.
Our retrosynthetic analysis is shown in Figure 1. We planned a
late-stage Sonogashira coupling of vinyl iodide 2 with either
enantiomer of enyne 3 to provide access to both epimers of the
natural product. The cis-macrolactone would be constructed in a
similar manner as our reported synthesis of Laulimalide7−9 by
macrolactonization of a hydroxyl alkynoic acid followed by
hydrogenation. The seco acid 4 could potentially be obtained via
the ring-opening of lactone 5. Olefin 5 would be formed through
a Julia−Kocienski olefination10,11 using an aldehyde obtained
from known12 allyl alcohol 6, with the sulfone derived from
Figure 1. Retrosynthetic analysis of callyspongiolide.
known13 diol 7. For the synthesis of enyne 3, Wittig olefination
with the aldehyde derived from olefin 8 would provide 3. We
Received: May 27, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01523
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Letter
planned a CBS reduction14,15 of the corresponding ketone
derived from 8 to provide access to both enantiomers.
Scheme 2. Julia−Kocienski Olefination
Our synthesis of the macrocyclic core began from the known12
chiral alcohol 6. As shown in Scheme 1, treatment of allyl alcohol
Scheme 1. Synthesis of Aldehyde 14
Scheme 3. Synthesis of Vinyl Iodide 2
6 with acryloyl chloride and Et3N gave diene 11, which was
subjected to ring closing metathesis using the Grubbs II catalyst
to give α,β-unsaturated lactone 12. Lactone 12 was subjected to
1,4-addition using Me2CuLi2I to provide 13 stereoselectively.
The methyl group added from the bottom face of the molecule to
provide lactone 13 in 95% yield as a single diastereomer by 1H
NMR analysis. Removal of the tert-butyldiphenylsilyl (TBDPS)
ether with tetrabutylammonium fluoride (TBAF) afforded the
corresponding alcohol, which upon oxidation using Dess−
Martin periodinane (DMP) in the presence of NaHCO3
furnished aldehyde 14 in 77% yield over two steps.
Our elaboration of the C10−C11 trans-olefin is shown in
Scheme 2. Protected diol 713 was readily prepared from chiral
epoxide 15 as described by Crimmins and co-workers.13 Selective
Mitsunobu displacement with 1-phenyl-1H-tetrazole-5-thiol
(PTSH, 16) followed by protection of the latent secondary
alcohol provided sulfide 17 in 79% yield over two steps.
Oxidation using m-chloroperbenzoic acid (m-CPBA) afforded
sulfone 18. For Julia−Kocienski olefination, sulfone 18 was
deprotonated using LiHMDS in DMF at −60 °C. Addition of
aldehyde 14 then provided the desired olefin 5 in 74% yield with
a trans/cis ratio of 34:1 as determined by 1H NMR analysis.
Our synthesis of macrolactone and its conversion to vinyl
iodide 2 is shown in Scheme 3. Treatment of lactone 5 with N,O-
dimethylhydroxylamine hydrochloride and isopropylmagnesium
chloride (iPrMgCl) in THF at −20 °C provided the
corresponding Weinreb amide. Protection of the resulting
secondary alcohol initially proved troublesome, as tert-
butyldimethylsilyl chloride (TBSCl) and imidazole in either
DMF or DCM showed low yields as well as silyl migration
product. However, switching to Et3N as base prevented the
formation of byproduct, and TBS ether 19 was obtained in 85%
yield over two steps. Diisobutylaluminum hydride (DIBAL-H)
B
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reduction followed by treatment with the Ohira-Bestmann
reagent16 and K2CO3 in MeOH gave the homologated alkyne 20.
The terminal acetylene moiety was alkylated by metalation with
n-BuLi in THF at −78 °C followed by treatment with ethyl
chloroformate to give alkynyl ester 21. Selective removal of the
triethylsilyl (TES) ether using pyridinium p-toluenesulfonate
(PPTS) in MeOH at 0 °C followed by LiOH-mediated
hydrolysis provided the corresponding seco acid. Yonemitsu’s
variation17 for macrolactonization proved very effective, whereby
the mixed anhydride is not preformed, but rather the seco acid is
directly added to the cocktail of reagents (2,4,6-trichlorobenzoyl
chloride, DMAP, and DIPEA) to furnish alkynyl lactone 22 in
67% yield over three steps. Reduction of the alkyne moiety
proceeded smoothly using Lindlar’s catalyst poisoned with
quinoline under a hydrogen balloon to provide the cis-lactone 23
in 99% yield. Removal of the TBS ether was ineffective using
TBAF, even at elevated temperatures. Alternatively, employing
HF·pyridine was efficient and afforded the corresponding
alcohol. Reaction of the resulting alcohol with chlorosulfonyl
isocyanate installed the carbamate moiety of 24 in 80% yield over
two steps. Removal of the p-methoxybenzyl (PMB) protecting
group was accomplished using 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) to provide the corresponding alcohol,
which was oxidized using DMP and then subjected to Takai
olefination18 to furnish vinyl iodide 2 in 42% yield over three
steps.
to furnish alcohol (S)-8 in 53% yield over two steps. Chiral
HPLC analysis showed that the alcohol was obtained in 95% ee.
Triethylsilyl trifluoromethanesulfonate (TESOTf) was neces-
sary to protect the resulting hindered alcohol 25, which was
obtained in 96% yield. Ozonolysis of the terminal olefin gave a
mixture of products, including silyl deprotection. However,
Nicolaou’s conditions19 for oxidative cleavage of olefins provided
the corresponding aldehyde, which was then subjected to Wittig
olefination with commercially available phosphonium bromide
26 to give the corresponding TMS-protected enyne in a trans/cis
ratio of 7:1. Global deprotection of all silyl protecting groups was
achieved using TBAF to afford enyne (S)-3 in 53% yield over
three steps. In the same manner, (R)-3 was prepared using the
same chemistry while employing (R)-Me-CBS during the chiral
reduction. For determination of absolute configuration, we
converted alcohol 8 to its benzoate derivative 27 as shown in
Figure 2. X-ray crystallography of the 4-nitrobenzoate derivative
confirmed the stereochemical outcome of CBS reduction.20,21
With the macrocyclic core of the molecule in hand, our
attention was directed toward preparation of the side chain. As
shown in Scheme 4, commercially available 2-bromo-3-
Scheme 4. Synthesis of Enyne (S)-3
Figure 2. Synthesis and ORTEP drawing of 27. Gray = carbon; white =
hydrogen; orange = bromine; blue = nitrogen; red = oxygen; yellow =
silicon.
The synthesis of callyspongiolide is shown in Scheme 5.
Sonogashira coupling of vinyl iodide 2 with enyne (S)-3 gave
callyspongiolide (S)-1 in 79% yield. Similarly, coupling with the
(R)-enantiomer furnished callyspongiolide (R)-1 in 82% yield.
1H (500 MHz) and 13C NMR (125 MHz) spectra of both (S)-1
and (R)-1 epimers are virtually indistinguishable from the spectra
reported for the natural product.6 Further unambiguous
1
assignment was difficult by 800 MHz H NMR. Both specific
rotations of our HPLC-purified22 synthetic callyspongiolide
epimers, however, showed opposite sign of rotation as well as
larger magnitude; synthetic (S)-1, [α]2D0 + 24.5 (c 0.1, MeOH);
synthetic (R)-1, [α]2D0 + 159 (c 0.1, MeOH) compared to that
reported for the natural product, [α]2D0 − 12.5 (c 0.1, MeOH).6
Based upon these results, we presume that the isolated natural
product is the antipode of our synthetic (S)-callyspongiolide 1.
Thus, assignment of the absolute structure of natural
callyspongiolide requires further synthetic and structural studies.
In summary, we have accomplished the enantioselective
synthesis of both epimers at C-21 of the reported structure of
callyspongiolide. The specific rotation of both epimers of our
synthetic callyspongiolide shows opposite sign to the natural
product, and the magnitudes are larger. Our synthesis
constructed the 14-membered macrolide with a sensitive C2−
C3 cis-olefin functionality by Yonemitsu’s variation of Yamaguchi
hydroxybenzaldehyde 9 was protected as the TBS ether. Reverse
prenylation of the resulting aldehyde using 3-methyl-2-
butenylmagnesium chloride furnished alcohol 8 in 98% yield
over two steps. The racemic alcohol was oxidized using
pyridinium chlorochromate (PCC) in the presence of silica gel
in DCM to give the corresponding ketone. The ketone was then
subjected to chiral reduction using Corey’s (S)-Me-CBS catalyst
C
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(3) Youssef, D. T.; van Soest, R. W.; Fusetani, N. J. Nat. Prod. 2003, 66,
861−862.
Scheme 5. Sonogashira Coupling and Callyspongiolide
Synthesis
(4) Buchanan, M. S.; Carroll, A. R.; Addepalli, R.; Avery, V. M.;
Hooper, J. N.; Quinn, R. J. J. Nat. Prod. 2007, 70, 2040−2041.
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macrolactonization followed by hydrogenation over Lindlar’s
catalyst as the key steps. The C5 methyl stereocenter was
constructed by methyl cuprate addition to an α,β-unsaturated δ-
lactone. The synthesis also featured a chiral CBS reduction and
Sonogashira coupling to conjoin the macrocyclic core and its side
chain. The synthesis will provide access to natural callyspongio-
lide as well as a variety of structural derivatives. Further
investigations are in progress.
(20) X-ray crystallographic structural determination was performed in
our X-ray crystallography laboratory by Dr. Matthias Zeller, Department
of Chemistry, Purdue University, West Lafayette, IN.
(21) Complete crystallographic data, in CIF format, have been
deposited with the Cambridge Crystallographic Data Centre. CCDC
1481162 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
(22) HPLC analysis showed the purity of both epimers to be >98%.
Conditions: Daicel Chiralpak IA-3, 4.6 × 250 mm, 3 μm, 15%
isopropanol/hexane, flow = 1.3 mL/min, T = 25 °C, UV = 254 nm,
Rt(S)-1 = 13.6 min, Rt(R)-1 = 12.9 min. Please see Supporting
Information for details.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01523.
■
S
Experimental procedures in addition to 1H- and 13C NMR
spectra for all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: akghosh@purdue.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of this work was provided by the National
Institutes of Health and Purdue University. Dr. Matthias Zeller
(Purdue) is acknowledged for X-ray crystallographic structural
determination.
REFERENCES
■
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1925.
(2) Kobayashi, M.; Higuchi, K.; Murakami, N.; Tajima, H.; Aoki, S.
Tetrahedron Lett. 1997, 38, 2859−2862.
D
DOI: 10.1021/acs.orglett.6b01523
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