An expedient synthesis of a functionalized core structure of bielschowskysin

Article abstract of DOI:10.1002/anie.201101360

Photons in action: Cascade sequences, brevity, and efficiency are the hallmarks of the synthesis of a functionalized tricyclo[9.3.0.0]tetradecane ring system of the marine natural product bielschowskysin. The synthesis is achieved in five steps, with the

Full text of DOI:10.1002/anie.201101360

DOI: 10.1002/anie.201101360
Natural Product Synthesis
An Expedient Synthesis of a Functionalized Core Structure of
Bielschowskysin**
K. C. Nicolaou,* Vikrant A. Adsool, and Christopher R. H. Hale
Bielschowskysin (1, Scheme 1a) is a recently discovered
marine natural product that possesses a novel molecular
architecture and impressive biological properties.[1] Isolated
from the Caribbean gorgonian octocoral Pseudopterogorgia
kallos and characterized by spectroscopic and X-ray crystallo-
graphic analysis, bielschowskysin boasts an unprecedented
tricyclo[9.3.0.0]tetradecane ring system (see 2, Scheme 1a)
decorated with a large number of oxygen-containing func-
tional groups and with 11 stereogenic centers.[1] Its biological
properties include antimalarial activity against Plasmodium
falciparum (IC50 = 10 mgmLÀ1) and potent and selective
cytotoxicity against EKVX nonsmall lung cancer cells
(GI50 < 10 nm) and CAKI-1 renal cancer cells (GI50 =
510 nm).[1]
As a consequence of its natural scarcity, the full biological
profile of bielschowskysin remains unexplored, and its
absolute configuration is unknown. These factors leave little
doubt, if any, of the worthiness of bielschowskysin as a
synthetic target, since an endeavor toward its total synthesis
may provide an opportunity to develop new synthetic
strategies and techniques, render the compound readily
available for biological investigations, allow studies of the
structure–activity relationship, and reveal its absolute stereo-
chemistry. Herein, we report our preliminary forays toward
the total synthesis of bielschowskysin that culminated in the
construction of a functionalized tricyclo[9.3.0.0]tetradecane
ring system 3 (Scheme 1b) of the molecule and its 2-epi
enantiomer (2-epi-ent-3). The reported route is notable for its
cascade sequences, brevity, and efficiency.[2]
In contemplating a plan for the total synthesis of
bielschowskysin (1), an intramolecular [2+2] photocycload-
dition[3] similar to a postulated biosynthetic scheme[4] came to
mind. To test this hypothesis, we designed a functionalized
tricyclo[9.3.0.0]tetradecane ring system 3 (Scheme 1b) and its
proposed macrocyclic precursor 4 (Scheme 1b), the latter
generated through a retro [2+2] photocycloaddition reaction,
as shown.
Scheme 2 summarizes the short and enantioselective
route to macrocyclic [2+2] photocycloaddition precursor 4
and its 2-epi enantiomer 2-epi-ent-4. Thus, acyl furan 5 was
reduced enantioselectively with the Noyori catalyst A[5] to
alcohol 6 (84% yield, 92% ee) and then combined with b-
ketoester 7 in the presence of CAN in MeOH to afford a
conjugated ketoester as a mixture of a- and b-methoxy
epimers 8 and 3-epi-8 (58% yield, ca. 1:1 ratio), which were
separated by chromatography.[6] The geometry of the enol
ether bond in these products was tentatively assigned at this
stage on the basis of NMR spectroscopy, and subsequently
proven by X-ray crystallographic analysis of a subsequent
derivative (see below). This highly productive and efficient
process, which rapidly generates structures 8 and 3-epi-8, is
presumed to proceed through the mechanism depicted in
Scheme 3. The importance of exo-enol ether/cyclic ketals (as
in 8 and 3-epi-8) in natural products chemistry and the
challenge of their construction have been recently noted by
Pattenden in a series of elegant studies.[7] This CAN-mediated
Scheme 1. a) Molecular structures of bielschowskysin (1) and its
tricyclo[9.3.0.0]tetradecane ring system (2) and b) functionalized
tricyclo[9.3.0.0]tetradecane ring system (3), and its postulated macro-
cyclic precursor (4).
[*] Prof. Dr. K. C. Nicolaou, C. R. H. Hale
Department of Chemistry and The Skaggs Institute for Chemical
Biology, The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
Fax: (+1)858-784-2469
E-mail: kcn@scripps.edu
Dr. V. A. Adsool
Chemical Synthesis Laboratory@Biopolis
Agency for Science, Technology, and Research (A*STAR)
Singapore 138667 (Singapore)
[**] We thank Dr. D.-H. Huang and Dr. R. Chadha for spectroscopic and
X-ray crystallographic assistance, respectively, and Dr. G. Siuzdak
and Doris Tan for mass spectrometric assistance. Financial support
for this work was provided by A*STAR, Singapore, the Skaggs
Institute for Chemical Research, the National Institutes of Health
(grant AI 055475-09), and the National Science Foundation
(Graduate Research Fellowship to C.R.H.H.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101360.
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Scheme 3. Proposed mechanism for the coupling reaction of 6 and 7
under the influence of CAN.
Scheme 2. Construction of macrocyclic precursor 4. Reagents and
conditions: a) Noyori cat. A (0.025 equiv), HCOONa (10 equiv),
nBu4Cl (0.3 equiv), CH2Cl2/H2O (1:1), 258C, 15 h; then Noyori cat. A
(0.01 equiv), 12 h, 84%, 92% ee; b) 6 (1.0 equiv), 7 (1.2 equiv), CAN
(4.0 equiv), MeOH, 08C, 58% (d.r. ca. 1:1); c) Grubbs I cat. (0.3 equiv
for 8, 0.25 equiv for 3-epi-8), CH2Cl2, reflux, 4 h, 90% for 9 (trans/cis
ca. 15:1), 71% for 3-epi-9 (trans/cis ca. 3:1); d) NaBH4 (6.0 equiv),
THF/H2O (2:1), 08C, 30 min, 83% for 4, 63% for 2-epi-ent-4. CAN=
ammonium cerium nitrate, Cy=cyclohexyl, Ts=toluene-4-sulfonyl.
coupling process provides rapid access to the exo-enol ether
structural motif, as exemplified in Scheme 2.
Treatment of 8 and 3-epi-8 separately with Grubbs I
catalyst[8] resulted in ring closure to furnish the corresponding
macrocyclic hydroxy ketones with the newly generated
double bond predominantly trans (9, 90% yield, trans/cis
ca. 15:1; 3-epi-9, 71% yield, trans/cis ca. 3:1, separated by
chromatography; Scheme 2). X-ray crystallographic analysis
of the 3,5-dinitrobenzoate derivatives of 9 [(+)-9-DNB,
m.p. 196–1978C (Et2O/MeOH/CH2Cl2, 1:1:1); Figure 1][9]
and 3-epi-9 [(+)-3-epi-9-DNB, m.p. 215–2178C (Et2O/
MeOH/CH2Cl2, 1:1:2); Figure 2][9] confirmed their structures,
Figure 1. ORTEP representation of 3,5-dinitrobenzoate (+)-9-DNB
(thermal ellipsoids at 30% probability).
including the geometry of the enol ether double bond, and
that of their precursors (i.e. 8 and 3-epi-8).
Attempts to induce photolytically the desired intramo-
lecular [2+2] cycloaddition with 9 or 3-epi-9 proved unpro-
ductive, with starting material persisting and partial enol
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Figure 2. ORTEP representation of 3,5-dinitrobenzoate (+)-(3-epi-9-
DNB) (thermal ellipsoids at 30% probability).
ether isomerization occurring. This failure was presumably
due to the conjugation of the enol ether olefinic bond with the
two carbonyl groups and/or the presence of the sp2 carbon
atom of the macrocycle carbonyl group, thereby resulting in a
prohibitive strain in the expected polycyclic product. This
carbonyl group was, therefore, reduced within each of the two
methoxy epimers (hydroxy ketones 9 and 3-epi-9) with
NaBH4 to produce, in each case, a single diastereoisomeric
diol (4, 83% yield, and 2-epi-ent-4, 63% yield; Scheme 2).
The configuration of the newly formed hydroxy group in 4
and 2-epi-ent-4 was tentatively assigned at this stage on the
basis of manual molecular models, which indicated a favored
external hydride delivery by the reducing agent (see 9a and 3-
epi-9a; Scheme 2), and was later confirmed through X-ray
crystallographic analysis of downstream intermediates (see
below). Note that an internal hydroxy-directed hydride
delivery in this reduction (i.e. 9 and 3-epi-9 to 4 and 2-epi-
ent-4, respectively) may also explain this stereochemical
outcome, as seen by manual molecular modeling studies.
Pleasingly, irradiation of a benzene or chloroform solution
of macrocycle 4 with UV light (450 Watt Hanovia, > 254 nm
or Rayonet, > 254 nm) for 48 h resulted in the formation of
tetracycle 3 (90% yield) as a single diastereoisomer. The
structure of compound 3 was unambiguously proven by X-ray
crystallographic analysis of its racemic bis-3,5-dinitroben-
zoate ester [(Æ )-3-bDNB, m.p. 203–2058C (Et2O/CH2Cl2,
slow diffusion); Figure 3].[9] The [2+2] photocycloaddition
of 4 is presumed to proceed through a radical mechanism and
the intermediacy of its transient enol ether geometrical
isomer 4a, as shown in Scheme 4. Thus, photoexcitation of
the chromophore of substrate 4 under the influence of light
Figure 3. ORTEP representation of (Æ)-3-bDNB (thermal ellipsoids at
30% probability).
closure, facilitated by the newly acquired rigidity and
proximity, to afford the observed tetracyclic product 3.
Indeed, NMR spectroscopic monitoring of the reaction
reveals initial formation of the enol ether isomer of 4,
namely compound 4a, which dissipates with time to the final
product, presumably through radical species 10a and 11a.
Interestingly, isolated geometrical isomer 4a only partially
reverts back to the original geometrical isomer 4 under the
irradiation conditions, which suggests the equilibrium
between 10 and 10a lies far to the right, in favor of 10a.
The alternative pathway of diradical 10 to 12 through 11 is
apparently shut, most likely because of unfavorable geo-
metrical constraints imposed by strain in the macrocycle.
Macrocycle 2-epi-ent-4 was also converted into a [9.3.0.0]
tetracyclic core structure (2-epi-ent-3, 88% yield; Scheme 4)
by employing the same photoirradiation conditions (see the
Supporting Information). Although no olefin isomerization of
2-epi-ent-4 (as with 4) was observed by NMR spectroscopic or
TLC analysis, the transient existence of the corresponding
geometrical isomer may be inferred by examination of the
structure of the product 2-epi-ent-3. The structure of 2-epi-ent-
3 was confirmed by X-ray crystallographic analysis of its
racemic bis-4-methoxybenzoate derivative [(Æ )-(2-epi-ent-3-
bMB), m.p. 207–2098C (MeOH/CH2Cl2, 5:1); Figure 4].[9]
Racemic bis-dinitrobenzoate (Æ )-3-bDNB and racemic bis-
methoxybenzoate (Æ )-(2-epi-ent-3-bMB) were prepared
from racemic alcohol (Æ )-6 (obtained by Grignard addition
to furfural). Interestingly, their enantiomeric counterparts
obtained from enantiopure 3 and 2-epi-ent-3, respectively, did
not yield suitable crystals for X-ray crystallographic analysis.
It should be noted that the asymmetric creation of the first
stereocenter of the molecule in step one (i.e. in 6) allows the
À
may furnish diradical 10, which apparently undergoes C C
bond rotation to afford the less-strained diradical 10a. The
latter species suffers, according to the rule of fives,[10] facile
and regioselective ring closure to afford cyclopentane inter-
mediate diradical 11a, which then undergoes a second ring
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Communications
Figure 4. ORTEP representation of (Æ)-(2-epi-ent-3-bMB) (thermal
ellipsoids at 30% probability).
[1] J. Marrero, A. D. Rodrꢀguez, P. Baran, R. G. Raptis, J. A.
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[2] For previous model studies toward bielschowskysin, see a) B.
Doroh, G. A. Sulikowski, Org. Lett. 2006, 8, 903; b) R. Miao,
S. G. Gramani, M. J. Lear, Tetrahedron Lett. 2009, 50, 1731.
[3] T. Bach, J. P. Hehn, Angew. Chem. 2011, 123, 1032; Angew.
Chem. Int. Ed. 2011, 50, 1000.
[4] P. A. Roethle, D. Trauner, Nat. Prod. Rep. 2008, 25, 298.
[5] a) A. Fujii, S. Hashiguchi, N. Uematsu, T. Ikariya, R. Noyori, J.
Am. Chem. Soc. 1996, 118, 2521; b) L. Ferrie, S. Reymond, P.
Capdevielle, J. Cossy, Org. Lett. 2007, 9, 2461.
Scheme 4. Photocycloaddition of macrocyclic precursor 4 and 2-epi-
[6] For a precedent of this reaction involving a simple furan and a
malonate diester, see a) L. M. Weinstock, E. Corley, N. L.
Abramson, A. O. King, S. Karady, Heterocycles 1988, 27, 2627;
b) J.-M. Simone, F. Loiseau, D. Carcache, P. Bobal, J. Jeanneret-
Gris, R. Neier, Monatsh. Chem. 2007, 138, 131; for related
electrochemically induced reactions, see c) H. Wu, K. D. Moel-
ler, Org. Lett. 2007, 9, 4599; d) J. Mihelcic, K. D. Moeller, J. Am.
Chem. Soc. 2003, 125, 36; e) J. Mihelcic, K. D. Moeller, J. Am.
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[8] a) P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs, Angew.
Chem. 1995, 107, 2179; Angew. Chem. Int. Ed. Engl. 1995, 34,
2039; b) P. Schwab, R. H. Grubbs, J. W. Ziller, J. Am. Chem. Soc.
1996, 118, 100.
[9] CCDC 813043 [(+)-9-DNB], 813041 [(+)-(3-epi-9-DNB)],
813042 [(Æ )-3-bDNB], and 814212 [(Æ )-(2-epi-ent-3-bMB)]
contain 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.
ent-4. Conditions: a) C6H6 or CHCl3, ambient temperature, hn (Hanovia
450 Watt, >254 nm or Rayonet, >254 nm), 48 h for 3, 90%; 16 h for
2-epi-ent-3, 88%.
construction of a complex structure (i.e. 3) containing the
[9.3.0.0] ring framework, seven stereogenic centers, and five
functional groups from two simple starting materials (i.e.
acylfuran olefin 5 and b-ketoester olefin 7).
The described route provides a five-step enantioselective
entry into the novel carbocyclic [9.3.0.0] core structure of
bielschowskysin (from simple building blocks 6 and 7), which
bears substantial functionality that may endow it with the
potential to serve as a scaffold for building further molecular
complexity and possibly imparting biological activity. Such
studies may prove useful in the total synthesis of the natural
product and its mimics for biological investigations.
Received: February 23, 2011
Published online: April 26, 2011
[10] a) R. S. H. Liu, G. S. Hammond, J. Am. Chem. Soc. 1967, 89,
4936; b) R. Srinivasan, K. H. Carlough, J. Am. Chem. Soc. 1967,
89, 4932.
Keywords: antimalarial agents · cytotoxic agents ·
.
natural products · photocycloaddition
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