Scalable total synthesis and biological evaluation of haouamine A and its atropisomer

Article abstract of DOI:10.1021/ja903745s

(Chemical Equation Presented) A total synthesis of the complex, bent aromatic ring-containing marine alkaloid haouamine A is achieved through a route in which every step (with the exception of the final deprotection) is performed on a gram-scale. This is

Full text of DOI:10.1021/ja903745s

Published on Web 06/15/2009
Scalable Total Synthesis and Biological Evaluation of Haouamine A and Its
Atropisomer
Noah Z. Burns,† Irina N. Krylova,‡ Rami N. Hannoush,‡ and Phil S. Baran*,†
Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037,
and Department of Protein Engineering, Genentech Inc., 1 DNA Way, South San Francisco, California 94080
Received May 8, 2009; E-mail: pbaran@scripps.edu
Scheme 1. Scalable, Programmed Syntheses of 1 and 2a
Haouamine A1 (1, Figure 1) is a biologically active and
architecturally unique alkaloid whose striking feature is a [7]-aza-
paracyclophane macrocycle containing a highly deformed nonplanar
aromatic ring. Somewhat mysteriously, 1 was discovered to exist
as a mixture of two rapidly interconverting isomers in solution, a
quality attributed to either atropisomerism of the bent arene or
slowed pyramidal inversion at nitrogen. Recent computational work2
supported a theory coupling the latter process with conformational
reorganization of the tetrahydropyridine ring but could not rule out
atropisomerism.3 The total synthesis of 1 from this laboratory4 also
did not answer this question unequivocally due to an inability to
control the atropisomer formed in the low-yielding cyclophane-
forming step, which fortuitously favored the natural planar
stereochemistry.5,6
Figure 1. Synthetic strategy and an unanswered structural question.
Here, we report a scalable and controllable route to 1 and its
atropisomer (2) that features point-to-planar chirality transfer7 via
a one-step, chemoselective cyclohexenone to phenol oxidation to
introduce strain within the macrocycle, a reaction that may find
future use in strained chiral cyclophane synthesis. We also
demonstrate that the strained cyclophane in 1 is crucial for
anticancer activity in PC3 cells.
To program planar chirality within the cyclophane macrocycle
a reduced compound (3, Figure 1) was selected as the bent phenol
precursor. Molecular models suggested that hybridization change
of one of the para carbons of the cyclophane from sp2 to sp3
should significantly reduce the strain present within the mac-
rocycle and thus make for an accessible intermediate5c (numerous
a Reagents and conditions: (a) n-BuLi (1.1 equiv), THF, -78 °C, 10
min; B(OMe)3 (2.0 equiv), -78 to 23 °C, 1 h; H2O; 4 (1.0 equiv),
(PhCN)2PdCl2 (0.1 equiv), Ph3As (0.2 equiv), Ag2O (1.6 equiv), 8:1 THF/
H2O, 23 °C, 18 h, 77%; (b) NaI (10.0 equiv), acetone, 23 °C, 7 h, 96%; (c)
20:1 CH2Cl2/TFA, 5 °C, 24 h; i-Pr2NEt (10.0 equiv), CH3CN (0.002 M),
reflux, 26 h, 79%; (d) LiHMDS (2.0 equiv), LiCl (5.0 equiv), THF, -78 to
0 °C, 20 min; PhSClNt-Bu (1.3 equiv), -78 °C (for 7) or -95 °C (for 8),
1 min, 60% of 9 + 23% 7, or 61% of 10; (e) BBr3 (7.0 equiv), CH2Cl2,
-78 to 5 °C, 20 h, 63% for 1, or 60% for 2.
attempts to form the macrocycle with a preinstalled phenol in
this4 and other5,6 laboratories have all failed). Furthermore, it
was surmised that the point chirality introduced by such a change
in hybridization could be used to select for the planar chirality
present in 1 and 2. Saturated cyclophane 3 could then be traced
† The Scripps Research Institute.
‡ Genentech Inc.
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9172 J. AM. CHEM. SOC. 2009, 131, 9172–9173
10.1021/ja903745s CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
back to the simple functionalized cyclohexenone 4, utilized as
the racemate in this study.8
Ether cleavage then delivered atrop-haouamine A (2) whose solution
isomeric behavior is similar to that of 1. As a further confirmation
of this phenomenon, both intermediates 9 and 10 were found to
converge on cyclophane 11 (Scheme 2), a compound that also exists
as two isomers.
While an initial bioassay was reported along with the isolation
of 1, the large amounts of material that are now made available
with this chemistry have enabled a more thorough investigation
into its bioactivity. Initial results have shown that 1 exhibits high
activity against PC3 human prostate cancer cells with IC50 ) 29 (
2 µM. atrop-Haouamine A (2) also shows high activity (IC50 ) 32
( 3 µM); however, des-methyl 7 and des-methyl 8 (dihydro-1 and
dihydro-2) are much less active (IC50 > 180 µM and IC50 > 75 µM,
respectively) indicating that the presence of the cyclophane is
necessary for activity in PC3 cells. Our findings on the biological
activity of 1 are different than those reported by Zub´ıa and
co-workers.1,9
Thus, a scalable route to haouamine A (1) and atrop-haouamine
A (2) has been developed, allowing for the synthesis of ample
quantities of each. In the case of 1, all steps (with the exception of
the final methyl ether removal) have been conducted on gram-scale.
This synthesis of 1 and 2 has put to rest the question of whether 1
exists as a mixture of atropisomers and was enabled by the
development of a method for the chemoselective aromatization of
cyclohexenones that allows for point-to-planar chirality transfer;
application of this strategy to other chiral strained cyclophane-
containing natural products is underway. As a result of this work,
the haouamine material supply is no longer an issue, and extensive
biological studies (including determination of the mechanism of
action of 1) are taking place and will be reported shortly.
To commence this plan, racemic bromo-indeno-tetrahydropyri-
dine 54a (Scheme 1) was cross-coupled with racemic tosyloxy-
iodocyclohexenone 49 in a one-step procedure involving lithium-
halogen exchange, quenching with B(OMe)3, addition of water, and
direct transfer of the resulting boronic acid to 4 in the presence of
palladium. The product 6 (77% yield on gram-scale) was isolated
as an inseparable mixture of diastereomers that was converted to a
mixture of primary iodides in high yield. N-Boc deprotection and
heating of the unpurified amine-TFA salt (after removal of excess
TFA) in dilute acetonitrile with Hu¨nig’s base then delivered
macrocycles 7 and 8 (79% combined yield, gram-scale) as a 1.45:1
readily separable mixture (this ratio implies a small amount of
selectivity in either the macrocyclization or the previous coupling
step). Interestingly, each of these compounds was found to exist
as two isomers in solution providing early evidence that 1 is not a
mixture of atropisomers. X-ray crystallographic analysis of both 7
and 8 identified their correspondence to haouamine A (1) and atrop-
haouamine A (2), respectively.
Efforts were then focused toward the key aromatization step.
Initial attempts to oxidize the silyl dienol ether of 7 and 8 with
palladium10 or MnO2 saw competitive oxidation of the indeno-
11
tetrahydropyridine core. Several other attempts to oxidize the
dienolate or cyclohexenone directly also failed.9 Taking inspiration
from the use of N-tert-butylbenzenesulfinimidoyl chloride by
Mukaiyama12 to introduce R,ꢀ-unsaturation to ketones in one step,
it was discovered that treatment of the lithium dienolates of 7 and
8 with this reagent rapidly affected the desired oxidation to deliver
the bent phenol macrocycles 9 and 10 as isomeric mixtures in
respective 60% and 61% yield (23% of the starting material could
be recovered in the reaction of 7 to 9). This represents the first use
of such a reagent to generate aromatic systems, and it should find
future applicability to do so particularly in strained systems of this
type due to its high oxidation potential and the possibility to
introduce asymmetry into the starting cyclohexenone.8 A low
reaction temperature (-78 °C for 7 and -95 °C for 8), the addition
of lithium chloride, and a very short (1 min) reaction time were
necessary to prevent subsequent reaction of the phenol product with
the reagent. This transformation has proved to be highly practical
and scalable, as it has been conducted on 1.05 g of 7 and 600 mg
of 8 with no yield diminishment. BBr3-mediated removal of the
methyl ethers in 9 then delivered haouamine A (1) in 63% yield.
As a testament to the practicality of this route, its utilization has
allowed for the production of over 550 mg of (()-1 to date.
Syntheses of enantiopure 1 and 2 (ca. 10 mg) have also been
accomplished from enantiopure 5.4b,9
Acknowledgment. We thank Drs. D.-H. Huang and L. Paster-
nack for NMR spectroscopic assistance and Dr. G. Siuzdak and
Dr. Arnold L. Rheingold (UCSD) for mass spectrometric and X-ray
crystallographic assistance, respectively. Funding for this work was
provided by Bristol-Myers Squibb, the NIH/NCI (CA134785), the
Skaggs Institute for Chemical Biology, and the ARCS Foundation
(predoctoral fellowship to N.Z.B.).
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.
References
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(4) Racemic: (a) Baran, P. S.; Burns, N. Z. J. Am. Chem. Soc. 2006, 128,
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Scheme 2. Reductive Convergence of 9 and 10 to 11
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(9) See Supporting Information for details.
X-ray crystallographic analysis of 10 secured its identity as the
atropisomer of 9. Interestingly, the isomer of 10 in this crystal
structure displayed an inversion at nitrogen and an alternative
tetrahydropyridine conformation as compared to the crystal structure
of 11 providing physical substantiation to computational results.2
(10) Bierling, B.; Kirschke, K.; Oberender, H.; Schulz, M. J. Prakt. Chem. 1972,
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