Synthesis of (+)-Cortistatin A

Article abstract of DOI:10.1021/ja8023466

Cortistatin A is a marine steroid with highly selective and perhaps mechanistically unique antiangiogenic activity. Herein we report a synthesis of this natural product by way of cortistatinone , an intermediate ideally suited for investigating the key pharmacophore of the cortistatin family. The synthesis begins with a terrestrial steroid and traverses a route to cortistatin A through the discovery of unique chemical reactivity. Specifically, we demonstrate the first example of a directed, geminal C-H bisoxidation, a new fragmentation cascade to access expanded B-ring steroid systems, a chemoselective cyclization to install the hallmark oxabicycle of the cortistatin family, and a remarkably selective hydrogenation reaction, which should find extensive use in future syntheses of the cortistatins and designed analogues. The synthesis displays a level of brevity, efficiency, and practicality that will be crucial in evaluating the medicinal potential of this fascinating class of marine steroids. Copyright

Full text of DOI:10.1021/ja8023466

Published on Web 05/14/2008
Synthesis of (+)-Cortistatin A
Ryan A. Shenvi, Carlos A. Guerrero, Jun Shi, Chuang-Chuang Li, and Phil S. Baran*
Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California
92037
Received March 31, 2008; E-mail: pbaran@scripps.edu
Steroids have historically elicited attention from the chemical
sciences owing to their utility in living systems, as well as their
intrinsic and diverse beauty.1 The cortistatin family (Figure 1, 1-7
and others),2 a collection of unusual, marine 9-(10,19)-abeo-
androstane steroids, is certainly no exception; aside from challenging
stereochemistry and an odd bricolage of functional groups, the
salient feature of these sponge metabolites is, inescapably, their
biological activity. Cortistatin A, the most potent member of the
small family, inhibits the proliferation of human umbilical vein
endothelial cells (HUVECs, IC50 ) 1.8 nM), evidently with no
general toxicity toward either healthy or cancerous cell lines
(IC50(testing /IC50(HUVECs) g 3300).2a From initial pharmacological
cells)
studies, binding appears to occur reversibly, but to an unknown
target, inhibiting the phosphorylation of an unidentified 110 kDa
protein, and implying a pathway that may be unique to known
antiangiogenesis compounds.2d
Compelled by the pharmacological potential of the cortistatins,2d,3
together with the unanswered questions surrounding their biological
activity,4 our group embarked on a synthesis of cortistatin A (1),
aiming for a concise route from inexpensive, commercially available
materials and the opportunity to develop new chemistry as the
occasion arose. A crucial target structure became the cortistatin A
ketonic core, (+)-cortistatinone (8, Figure 1), which was anticipated
to allow for straightforward elaboration to the natural product, as
well as divergence to other family members. A semisynthetic route
to these marine steroids was deemed an acceptable strategy largely
due to the economy of using prednisone, which at $1.20/g possesses
70% of the carbon atoms and the corresponding, enantiopure
chirality of 1.
Prednisone could be converted to the known steroid core 9
(Scheme 1) in a short two-step sequence (see Supporting Informa-
tion) and in 92% overall yield after recrystallization. The C1,C2
trans-vicinal diol was targeted through the intermediacy of R-dis-
posed epoxide 10, which was installed using tert-butyl hydroper-
oxide instead of the precedented, but nonscalable, dimethyldioxirane
(DMDO) procedure.5 Reductive amination of the unsaturated ketone
proceeded uneventfully, and refluxing the crude reaction mixture
with ethyl formate generated formamide 10 in good yield. The
epoxide, however, proved intractable to a number of standard
procedures for nucleophilic addition. Under acidic aqueous condi-
tions, the nascent diol underwent facile cyclization onto the C11
ketone, followed by dehydration to yield an unproductive dihy-
drofuran, which itself is a rare motif among steroids.6 Conversely,
basic aqueous conditions led to undesired cleavage of the formyl
moiety, a group which would figure prominently as a methyl
surrogate of the target molecule. Eventually, it was found that tetra-
n-butylammonium acetate (TBAA) in refluxing benzene opened the
epoxide at C2, producing the trans-hydroxy acetate 11 (see X-ray
11′).
Figure 1. Isoquinoline-bearing cortistatins A-D (1-4) and J-L (5-7)
(cortistatins E-H bear pyridines or piperidines and exhibit markedly
diminished potency), and a general retrosynthetic strategy to target a steroid
skeleton, namely, the inexpensive, terrestrial steroid, prednisone.
intermediate 11 by (1) Mukaiyama hydration7 of the trisubstituted
olefin, (2) reaction of the amidodiol with trimethyl orthoformate,
and (3) solvolysis of the C2 acetate. Notably, reacting epoxyfor-
mamide 10 under identical conditions for Mukaiyama hydration
gave a 5:1 stereoisomeric mixture of tertiary alcohols, disfavoring
the desired C5 R-stereochemistry.
During preliminary reconnaissance in accessing the cortistatin
core, the most difficult functionality to secure turned out to be the
C19 methine oxidation state,8 suggesting the importance of its
installation early in the sequence. Unfortunately, existing methods
for such a transformation (angular methylfaldehyde oxidation state)
are reported to give generally low yields9 and, more importantly,
proved completely ineffectual in our system. Consequently, a new
process was conceived to access a dibrominated 19-carbon, utilizing
in situ generated acetoxy hypobromite (AcOBr).10 Success was
realized by significantly lowering the reaction temperature and
extending the reaction time, resulting in an iterative, double methyl
activation (Figure 2, 17f18f19), while suppressing SN2 attack
of the alcohol on the σ*C-Br orbital of the monobromide (17f20;
possibly SH2 attack of the transient O-centered radical). To the best
of our knowledge, this is the first example of an alcohol-directed,
geminal dihalogenation of an unactiVated hydrocarbon. The
selectivity for dibromination (57%) over mono- or tribromination
well surpasses what would be expected with only the governance
After extensive experimentation, it was found that the key
orthoamide 12 (see X-ray 12) could be synthesized in one pot from
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J. AM. CHEM. SOC. 2008, 130, 7241–7243 7241

C O M M U N I C A T I O N S
Scheme 1. Synthesis of the (+)-Cortistatin A Core: Cortistatinone (8)a
a Reagents and conditions: (a) t-BuO2H (2 equiv), DBU (2 equiv), THF, 23 °C, 72 h, 82%; (b) NH4OAc (15 equiv), Na(BH3)CN (1.2 equiv), MeOH,
THF, 23 °C,18 h; then HCO2Et (74 equiv), Et3N (11 equiv), 54 °C, 12 h, 73%; (c) TBAA (5 equiv), Co(acac)2 (0.2 equiv), PhH, 90 °C, 24 h, 48%, 76%
brsm; (d) Co(acac)2 (0.2 equiv), PhSiH3 (2.2 equiv), O2, THF, HC(OMe)3, 23 °C, 12 h; then TsOH•H2O (3 equiv), rt, 2 h; then K2CO3 (5 equiv), MeOH,
6 h, 65%; (e) PhI(OAc)2 (5 equiv), Br2 (5 equiv), CH2Cl2, -30 °C, 10 h; then TMSCl (5 equiv), imidazole (5 equiv), 0 °C, 15 min, 57%; (f) DBU (2 equiv),
LiCl (10 equiv), THF, 23 °C, 24 h, 85%; (g) SmI2 (2.2 equiv), 1:9 DMPU:THF, 23 °C, 5 min; then TBCHD (1.1 equiv), 23 °C, 5 h; (h) LiBr (20 equiv),
Li2CO3 (20 equiv), DMF, 80 °C, 1 h (65% over 2 steps); (i) AlH3 (0.5 M in THF, 5 equiv), THF, 23 °C, 1 h; then K2CO3 (4 equiv), MeOH, 23 °C, 12 h;
then Ac2O (20 equiv), Et3N (40 equiv), DMAP (0.1 equiv), CH2Cl2, 23 °C, 3 h, 89%; (j) MgBr2•Et2O (1.1 equiv), 2,6-(t-Bu)2Py (2.1 equiv), PhH, 80 °C,
1 h; PPTS (5 equiv), butanone: H2O (1:1), 90 °C, 2 h; then K2CO3 (10 equiv), 23 °C, 5 h (82%).
of statistics,11 which would produce the dihalide in a maximum
yield of 27% (see Supporting Information). Use of the well-
precedented PhI(OAc)2/I2 conditions for monoiodination12 resulted
in competitive bicyclic ether (20) formation, likely due to a much
larger coefficient of the σ*C-I orbital.
The unstable dibromo alcohol was capped with a trimethylsilyl
group to prevent intramolecular etherification. Alkylation of the
9,11-enolate with the proximal dibromomethyl proceeded with DBU
and lithium chloride to give one bromocyclopropane diastereomer
13 (Scheme 1), whose configuration was confirmed by X-ray
diffraction of alcohol 13′.
After extensive experimentation, a cascade sequence (see Figure
2 for a possible mechanism) was developed to achieve isomerization
of bromocyclopropane 13 to cycloheptyl R-bromoketone 14 in high
yield, via radical opening of the three-membered ring (21f22)
extrusion of bromine radical (22f23), and trapping of the extended
enolate (23f14) with 2,4,4,6-tetrabromo-2,5-cyclohexadienone
(TBCHD).13,14 The major diastereomer of the reaction is likely the
R-disposed allylic, C9 bromide 14; although spectroscopy could
not assign this stereochemistry unambiguously, ensuing reactions
and stereoelectronic considerations suggest this configuration.15
Thus, the oxidation state deliberately embedded in the 19-methyl
dibromide 19 translated smoothly into the olefinic C19 methine of
the cortistatin core. Without the carefully placed bromine atom in
the cyclopropane ring of 13, this fragmentation led to a cycloheptyl
ketone that could not be brought to the desired oxidation state.
Elimination of the R-bromide 14 with lithium carbonate delivered
the cross-conjugated dienone (see X-ray 16, Scheme 1). Under the
action of alane, the heteroadamantane A ring core was reductively
unmasked to reveal the entire A ring of cortistatin A; in situ
Figure 2. Key transformations en route to (+)-cortistatin A and mechanistic
analyses.
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C O M M U N I C A T I O N S
Scheme 2. Completing the Synthesis of (+)-Cortistatin Aa
valuable in probing the crucial2d heterocyclic domain of the
cortistatins; targeting this intermediate should also facilitate future
syntheses. Efforts are now underway to establish the antiangiogenic
mode of action of cortistatin A and analogues thereof.
Acknowledgment. Dedicated to Professor E. J. Corey on the
occasion of his 80th birthday. Dr. Neil Shenvi is gratefully
acknowledged for helpful discussions. Dr. Arnold Rheingold, Dr.
Antonio DiPasquale, Dr. Matthew Belousoff, and Dr. Raj Chadha
are acknowledged for X-ray crystallographic assistance. We thank
Dr. D.-H. Huang and Dr. L. Pasternack for NMR spectroscopic
assistance, and Dr. G. Siuzdak for assistance with mass spectros-
copy. Financial support for this work was provided by Amgen, the
Beckman Foundation, Bristol-Myers Squibb, DuPont, Merck,
Roche, the Searle Scholarship Fund, the Department of Defense
(predoctoral fellowship to R.A.S.), and the NIH (predoctoral
fellowship to C.A.G.).
a Reagents and conditions: (a) N2H4 (10 equiv), Et3N (10 equiv), EtOH,
50 °C, 6 h, I2 (2 equiv), Et3N (3 equiv), THF, 23 °C, 5 min; (b)
7-(trimethylstannyl)-isoquinoline (4 equiv), Pd(PPh3)4 (0.5 equiv), CuCl (10
equiv), LiCl (10 equiv), DMSO, 23 °C, 10 min, 53% (over 2 steps); (c)
Raney Ni (88 wt equiv), i-PrOH, H2O, 50 °C, 1 h, 50% (ca. 100% brsm).
desilylation and triacetylation delivered dimethylaminotriacetate 15
as a diastereomeric mixture at C11. Acetylation of the C11 alcohol
served to activate the 8,9-olefin toward conjugate displacement (see
24, Figure 2), which was achieved upon heating with MgBr2•Et2O
in benzene, delivering the bridging bicyclic ether of the cortistatin
core 25. Mild deketalization using pyridinium p-toluenesulfonate,
followed by solvolytic removal of the A-ring acetates delivered
(+)-cortistatinone (8), whose proton and carbon NMR spectra bore
a satisfying similarity to the reported spectra for cortistatin A.
To complete the synthesis, a challenging task lay ahead:
appending the requisite ꢀ-disposed C17 isoquinoline in the presence
of a tertiary amine, a vicinal diol, and two olefins; these objectives
were achieved by the sequence delineated in Scheme 2. First,
cortistatinone (8) was treated with hydrazine to generate an
intermediate C17 hydrazone, which was not isolated, but rather
immediately subjected to iodine and triethylamine to form an
alkenyl iodide,16 sparing both the C3 tertiary amine and the internal
diene. Second, Stille coupling17 delivered the isoquinoline 26
without incident; it is projected that this reaction should be amenable
to the installation of numerous heterocyclic side chains. Finally, in
a consummate example of chemoselective reduction, the benzylic
olefin was reduced with Raney nickel in water and isopropanol to
yield synthetic (+)-cortistatin 1, whose spectral characteristics were
identical to those reported in the literature.
Thus, (+)-cortistatin A (1), the most biologically active member
of the marine-derived cortistatin family, was synthesized from the
inexpensive terrestrial steroid prednisone (ca. 3% overall yield,
unoptimized), which is commercially available in multikilogram
quantities. Certain aspects of this synthesis carry important lessons
in chemical reactivity, selectivity, and synthesis strategy, including
(1) the four-step sequence (9f12) to install all requisite A ring
stereochemistry; (2) a newly invented alcohol-directed, dibromi-
nation reaction (12f19); (3) an isohypsic (oxidation-state conserv-
ing) cascade (13f14) to access the 9-(10,19)-abeo-androstane
skeleton; (4) an olefin-sparing, heteroadamantane fragmentation to
differentiate the tethered aminodiol; (5) a mild SN′ cyclization to
close the final ring of the cortistatin skeleton (15f25); and (6) a
selective benzylic hydrogenation to facilitate the transformation of
(+)-cortistatinone to (+)-cortistatin A (8f1). Access to 8 will prove
Supporting Information Available: Detailed experimental proce-
dures, copies of all spectral data, and full characterization. This material
is available free of charge via the Internet at http://pubs.acs.org.
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