A synthesis of the ABC tricyclic core of the clionastatins serves to corroborate their proposed structures

Article abstract of DOI:10.1021/ol500265v

A synthesis of the ABC tricyclic ring system of the clionastatins, an unusual pair of highly chlorinated androstane steroids, has been accomplished. This work provides strong support for the original structural proposal. An unexpected substrate-dependent reversal in alkene chlorination diastereoselectivity was critical to success. This approach should be amenable to an eventual enantioselective synthesis of the natural products themselves.

Full text of DOI:10.1021/ol500265v

Letter
pubs.acs.org/OrgLett
A Synthesis of the ABC Tricyclic Core of the Clionastatins Serves To
Corroborate Their Proposed Structures
Samuel S. Tartakoff and Christopher D. Vanderwal*
Department of Chemistry, University of California, 1102 Natural Sciences II, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: A synthesis of the ABC tricyclic ring system of
the clionastatins, an unusual pair of highly chlorinated
androstane steroids, has been accomplished. This work
provides strong support for the original structural proposal.
An unexpected substrate-dependent reversal in alkene
chlorination diastereoselectivity was critical to success. This
approach should be amenable to an eventual enantioselective
synthesis of the natural products themselves.
he steroid family of metabolites comprises an enormous
number of diverse structures, largely conforming to the
tetracyclic 6−6−6−5 ring system. They are among the most
studied natural products, with well understood biosynthetic
pathways1 and many successful laboratory syntheses.2 Clionas-
tatins A and B (1 and 2, Figure 1), isolated from the
in the stereoselective synthesis of polychlorinated natural
products4 led us to undertake a synthesis.
T
When considering a retrosynthetic approach to the
clionastatins, several obvious challenges needed to be
addressed. Both the C1 and C19 chlorides are neopentylic,
presumably making late-stage introduction by substitution
difficult. This issue is further complicated by the fact that the
C1 and C2 chlorides are both pseudoequatorial (both in
energy-minimized structures, Figure 1, and according to
coupling constant data3), and dichlorination reactions of
cyclohexenes tend to favor the formation of diaxial products.5
Finally, the high degree of unsaturation found in 1 and 2
prevents rapid synthesis by biomimetic π-cyclization cascades6
and adds significant strain to the ring system. Of special note is
the tetrasubstituted Δ8,9-enone which, while not unknown, is
difficult to install via standard Saegusa−Ito or other similar
oxidative conditions.7
Because of the aforementioned challenges, and also to
corroborate the unusual proposed structure, we first decided to
target a truncated tricyclic system (3, Scheme 1) containing
much of the complexity found in the natural products. A late-
stage elimination and oxidation of the B-ring enone would take
us back to intermediate 4, the dichlorination product of 5.
Redox manipulations separate 5 from Diels−Alder product 6,
and the precursors of this cycloaddition are simple, readily
available materials. This route was particularly attractive
because, in addition to addressing several of the expected
challenges outlined above, our model diene 9 could later be
replaced by an enantioenriched Hajos−Parrish ketone-derived
diene, introducing the D-ring with the functionality needed to
complete the total syntheses of 1 and 2.
Figure 1. Clionastatins A and B, the ABC tricyclic ring system, and an
energy-minimized structure of clionastatin A.
Mediterranean burrowing sponge Cliona nigricans in 2004,3
possess several features that distinguish them from any
previously isolated steroid. In spite of their well-known
androstane steroidal framework, they each have a 3,5,8(9)-16-
tetraen-7,15-dione oxidation pattern not found in any other
natural or synthetic steroids. Furthermore, they are the first
polyhalogenated steroids observed in nature and remain one of
the few chlorinated steroids where the chlorine atoms are not
integral to chlorohydrins. Fattorusso and co-workers assigned
the relative stereochemistry by ROESY correlations but were
not able to obtain an X-ray crystal structure from the small
amount of amorphous solid isolated. The novel structures of 1
and 2, their potent cytotoxicity, and our long-standing interest
Known cyclohexenone 7 is made by the Baylis−Hillman
reaction of formaldehyde with 4-(tert-butyldimethylsilyloxy)-
cyclohexenone, which can be obtained enantioselectively.8
Received: January 24, 2014
Published: February 26, 2014
© 2014 American Chemical Society
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Letter
agreement with the observations of Danishefsky,11 namely that
systems of this type usually proceed through an endo transition
state, with the diene approaching from the sterically more
hindered face.
Scheme 1. Retrosynthetic Plan for ABC Ring System 3
Attempts to oxidize the resultant enol ether to the desired
enone 10 by a variety of one-step methods failed to give more
than modest yields. However, treatment of the Diels−Alder
cycloadduct with NBS, followed by heating the crude mixture
to reflux in DMF in the presence of LiCl and Li2CO3, generated
the desired enone in 50% yield over three steps. This procedure
was modified from the published conditions12 because the use
of LiBr instead of LiCl resulted in a mixture of C19 chlorinated
and brominated product. Reduction of the A-ring ketone using
NaBH4 afforded alcohol 11 in 85% yield as a single
diastereomer, in agreement with the observations of
Mechoulam for similar steroidal systems.13
Dehydration of the alcohol to form alkene 5 proved
problematic because formation of the intermediate triflate
using either Tf2O/Et3N or PhNTf2/LDA resulted in complex
mixtures, with isolated yields of desired product ranging
between 10 and 50%. Attempts to form the mesylate or tosylate
under a variety of conditions were unsuccessful under a variety
of conditions, presumably because of the sterically encumbered
nature of the alcohol. Treatment with Martin sulfurane14
resulted in efficient and reproducible dehydration, but because
of the expense associated with that reagent, SOCl2 and pyridine
were most often used, affording alkene 5 in 40−60% yield.
Dichlorination of 5 with Et4NCl3 produced dichloride 12 as
the major component of a 4:1 diastereomeric mixture. NOESY
experiments indicated the preferential formation of the
undesired stereoisomer. Efforts to alter the dichlorination
diastereoselectivity by varying concentration, solvent, and
temperature were unproductive. Other dichlorinating agents,
including PhICl2 and SO2Cl2, either failed to cause reaction or
resulted in a complex mixture of products. Unfortunately, the
desired diastereomer could not be isolated cleanly from the
Allylic alcohol 7 was converted to chloride 8 in good yield, and
silyloxydiene 9 is a known compound.9 Thermal Diels−Alder
cycloadditions with α-substituted cyclic enones are known to be
challenging,10 but Corey and co-workers have shown that Lewis
acid activation can result in rapid, stereoselective reactions.7
Indeed, when diene 9 was added to a mixture of dienophile 8
and 2 equiv of Et2AlCl, cycloadduct 6 was formed in nearly
quantitative yield. The cycloadduct could be obtained in >10:1
diastereoselectivity when the reaction was conducted at −78 °C
for 12 h; however, these reactions did not reach completion. As
a result, the Diels−Alder reaction was usually performed over
15 min at 0 °C, resulting in a 4:1 dr, with greater overall yield
than that obtained from the lower temperature, more selective
protocol. The reaction appears to be entirely endo-selective,
with the major diastereomer as shown in Scheme 2, and with
the minor isomer epimeric at C5, C9, and C10 (using C4 as the
point of reference). The observed configuration, which was
confirmed by NOE correlations at a later stage, was in
Scheme 2. Synthesis of the Unnatural and Natural Diastereomers of the Clionastatin ABC Ring System (14 and 3, Respectively)
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Letter
dichlorination reaction product mixture (4/12). However, a
small amount of the undesired diastereomer could be purified,
and conversion of enone 12 to the TES enol ether followed by
sequential in situ treatment with PhSeBr and m-CPBA resulted
in dienone 13. Cleavage of the silyl ether of 15 followed by
elimination using excess Tf2O in pyridine/CH2Cl2 produced
the C1,C2 epimeric core (14) in 50% yield over three steps.
Not surprisingly, the dichlorination of 5 had led predom-
inantly to pseudodiaxial dichloride 12 and would not suffice to
obtain a significant quantity of the desired pseudodiequatorial
diastereomer 4. To evaluate dichlorination in a different
context, deprotection of 5 was conducted with TBAF to afford
the somewhat unstable free alcohol 15. Unexpectedly, this
alkene could be dichlorinated with Et4NCl3 to afford a mixture
favoring the desired diastereomer 17, albeit in moderate yields.
Ratios of up to 5:1 in favor of 17 have been observed, but the
best compromise with respect to diastereoselectivity and yield
arises from the specific conditions shown. The major
complication in this reaction is the competitive oxidation to a
C4 ketone that apparently facilitates decomposition.
have ruled out a significant change in conformational
preferences because the H NMR resonances attributed to
the A-ring protons of these two substrates are virtually
1
superimposable. Analysis of coupling constants in the A-ring
3
(especially J4−5 = 3.6 Hz) suggests that the hydroxy/silyloxy
groups preferentially occupy pseudoequatorial positions in the
ground state. Furthermore, a pseudoaxial disposition of these
groups would enforce a boatlike A-ring conformation, with
attendant flagpole interactions between the C3 proton and the
angular chloromethyl group. The pseudoequatorial positioning
of these groups militates against any explanations based on
steric differences for the change in dichlorination selectivity
between 5 and 15. It would be premature to comment any
further on this phenomenon at this point, especially given the
complexity of chlorination mechanisms. Studies to understand
this result are ongoing.
The diastereomeric mixture of 16/17 could be carried
through a sequence similar to that used to convert 12 to 14,
and desired ABC tricyclic model system 3 was obtained
uneventfully. Purification to diastereomeric purity at any single
stage was difficult, but complete elimination of the undesired
stereoisomer was reliably achieved over the course of the four-
step sequence.
The reason for the dramatic alteration in dichlorination
stereoselectivity between substrates 5 and 15 is not clear. We
The relative configurations shown for diastereomeric
dichlorination products 12 and 4 are supported by the NOE
correlations presented in Figure 2. Access to both diaster-
eomers further secures these assignments. The NOE
correlation between protons on C2 and C19 in the final
products, 3, which is replaced by a correlation between C1 and
C19 in diastereomer 14, further improves our confidence.
1H NMR spectra of the diastereomeric tricyclic products 14
and 3 were compared with the published NMR spectra for
clionastatin B (for simplicity, a spectrum of a mixture is shown
in Figure 3; purified spectra for each can be found in the
Supporting Information). Clearly, the C1 and C2 protons of
the unnatural diastereomer 14 do not correlate well with the
authentic spectra, likely owing to their deshielded, equatorial
orientation. However, while the splitting pattern for the model
system 3 differs slightly from the spectrum of the natural
Figure 2. Key NOE correlations that support the relative stereo-
chemistry put forth for compounds 12, 4, 14, and 3.
1
Figure 3. Comparison of H NMR spectra of clionastatin B with a mixture of diastereomeric clionastatin ABC tricycles strongly corroborates the
original structural and stereochemical assignment (top spectrum reprinted from the Supporting Information of ref 3. Copyright 2004, American
3
3
3
Chemical Society). J1,2(1) = 9.0 Hz, J1,2(3) = 8.6 Hz, J1,2(14) = 0 Hz (singlet).
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(8) Arthurs, C. L.; Lingley, K. F.; Piacenti, M.; Stratford, I. J.; Tatic,
T.; Whitehead, R. C.; Wind, N. S. Tetrahedron Lett. 2008, 49, 2410−
2413.
products, the chemical shifts of the key resonances from the A
and B rings match nearly perfectly.
We have synthesized the highly chlorinated ABC tricyclic
ring system of clionastatins A and B in 12 steps from known,
readily available enone 7. We have validated the key Diels−
Alder disconnection to this highly oxidized and chlorinated
steroid and addressed the installation of all three chlorides
found in clionastatin A, which lays the groundwork for a
convergent synthesis of the natural products themselves.
Important aspects of this work include (1) the diastereose-
lectivity of the Diels−Alder cycloaddition of chiral cyclo-
hexenone 7, which should ultimately permit an enantioselective
synthesis of the clionastatins starting from enantioenriched 7,
(2) the remarkable reversal of dichlorination diastereoselectiv-
ity upon cleavage of the silyl ether in 5, and (3) the confidence
gained in Fattorusso’s original structural and stereochemical
assignments of clionastatins A and B. Clearly, the fortuitous
diastereoselective dichlorination reaction of homoallylic alcohol
15 could not have been predicted at the outset, and this work
underscores the empirical nature of the chemical synthesis of
unusual natural products. Work toward the enantioselective
syntheses of 1 and 2 is currently underway and will be reported
in the future.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectral data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cdv@uci.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NIH for research support via grant GM-086483
and the NSF for support via a Graduate Research Fellowship to
S.S.T. We are grateful to Hung V. Pham (UCLA) for providing
the minimized structure of clionastatin A in Figure 1 and to Dr.
Allen Hong and Sean Feng (UCI) for assisting in the synthesis
of early intermediates.
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■
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