Total synthesis of anibamine, a novel natural product as a chemokine receptor CCR5 antagonist

Article abstract of DOI:10.1021/ol070748n

The total synthesis of anibamine, the first and only natural product known as a chemokine receptor OCR5 antagonist, is reported herein. Anibamine was synthesized from acetylacetone and cyanoacetamide in 10 steps.

Full text of DOI:10.1021/ol070748n

ORGANIC
LETTERS
2007
Vol. 9, No. 10
2043-2046
Total Synthesis of Anibamine, a Novel
Natural Product as a Chemokine
Receptor CCR5 Antagonist
Guo Li,† Karen Watson,‡ Robert W. Buckheit,‡ and Yan Zhang*,†
Department of Medicinal Chemistry, Virginia Commonwealth UniVersity,
410 North 12th Street, P.O. Box 980540, Richmond, Virginia 23298-0540, and
ImQuest BioSciences, Inc., 7340 ExecutiVe Way, Suite R, Frederick, Maryland 21704
yzhang2@Vcu.edu
Received March 27, 2007
ABSTRACT
The total synthesis of anibamine, the first and only natural product known as a chemokine receptor CCR5 antagonist, is reported herein.
Anibamine was synthesized from acetylacetone and cyanoacetamide in 10 steps.
Anibamine, a novel pyridine quaternary alkaloid recently
isolated from Aniba sp.,1,2 has been found to effectively bind
to the chemokine receptor CCR5 with an IC50 at 1 µM in
competition with 125I-gp120, a HIV viral envelop protein
binding to CCR5 with high affinity.1 The chemokine receptor
CCR5, a G-protein-coupled receptor, has been identified as
an essential co-receptor for HIV virus entry to host cells.3
Therefore, an antagonist of CCR5 receptor that would inhibit
the cellular entry of human immunodeficiency virus type Ι
(HIV-1) provides a new therapy choice for the treatment of
HIV infection.4
throughput screening, such as, SCH-D,5 TAK220,6 and
UK4278577 (Figure 1).
Currently, all the known antagonists to CCR5 have been
developed by optimization of the lead compounds from high-
* Address correspondence to this author.
† Virginia Commonwealth University.
‡ ImQuest BioSciences, Inc.
(1) Jayasuriya, H.; Herath, K. B.; Ondeyka, J. G.; Polishook, J. D.; Bills,
G. F.; Dombrowski, A. W.; Springer, M. S.; Siciliano, S.; Malkowitz, L.;
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Singh, S. B. J. Nat. Prod. 2004, 67, 1036-1038.
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Shoemaker, R. H. J. Nat. Prod. 2004, 67, 1732-1735.
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Farber, C. M.; Saragosti, S.; Lapoume´roulie, C.; Cognaux, J.; Forceille,
C.; Muyldermans, G.; Verhofstede, C.; Burtonboy, G.; Georges, M.; Imai,
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G.; Parmentier, M. Nature 1996, 382, 722-725. (b) Littman, D. R. Cell
1998, 93, 677-680.
Figure 1. Anibamine and some known CCR5 antagonists
Anibamine is the first and the only natural product that
has been identified as a CCR5 antagonist with high affinity.
(4) (a) Reeves, J. D.; Piefer, A. J. Drugs 2005, 65, 1747-1766. (b) Este,
J. A. Curr. Med. Chem. 2003, 10, 1617-1632. (c) Schwarz, M. K.; Wells,
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J. 2006, 25, 447-456.
10.1021/ol070748n CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/21/2007

Clearly, anibamine possesses a novel structural skeleton
compared with all other known CCR5 antagonists.5 Thus the
chemical synthesis of anibamine and its analogues may lead
to a new class of AIDS therapeutic agents. Herein is the
first report of the total synthesis of anibamine.
Scheme 2. The Chemical Synthesis of Intermediate 3
As shown below, anibamine has a unique structural
skeleton, in which a pyridine quaternary ion is embodied by
a fused five-member ring. Such a skeleton can also be called
the 2,3-dihydro-1H-indolizinium ion. On the pyridine ring,
two ten-carbon aliphatic chains are attached at positions 3
and 5 via cis double bonds. In addition, both positions 2
and 4 are occupied by methyl groups. The retrosynthetic
analysis of anibamine is illustrated in Scheme 1.
Scheme 1. The Retrosynthetic Analysis of Anibamine
procedure of Haley et al.,12 the condensation reaction between
acetylacetone and cyanoacetamide gave compound 7, which
was brominated with NBS in TFA and H2SO4 to give 8.13
Next, the nucleophilic cyanization of compound 8 was
achieved by refluxing with cuprous cyanide in DMF to give
3.13 Here, the total yield was 46% over 3 steps. The first
route is one step longer than the second one, yet is more
convenient for large-scale synthesis. Since two cyano groups
can be introduced subsequently in the second route, it may
provide the possibility of attaching two different side chains
on positions 3 and 5. This will be critical to anibamine
analogue synthesis in the future.
The two ten-carbon side chains can be introduced by
Wittig reactions between the Wittig reagent of 1-bromo-
nonane and the 3,5-dialdehyde intermediate 2. Intermediate
2 can be prepared by the reduction of the 3,5-dicyano
intermediate. The fused five-member-ring system can be
achieved by introducing a three-carbon side chain at position
2, followed by the cyclization reaction to form the indoli-
zinium ring. Apparently the substituted pyridine 3 is the most
critical intermediate since it possesses all the potential
functional groups.
The bromination14 of 3 with TBAB/P2O5 led to the
2-bromo-substituted intermediate 9 (Scheme 3). The pal-
Scheme 3. The Synthesis of Intermediate 11
Two synthetic routes were explored to prepare the
intermediate 3, as shown in Scheme 2. In the first route, the
condensation reaction between acetylacetone and N,N-
dimethylformide dimethyl acetal provided compound 4.8
Then 4 was refluxed with hydroamine hydrochloride in
methanol to give compound 5.9 Next 5 was coupled with
aniline10 to provide compound 6. Finally the cyclization
reaction between 6 and malononitrile11 led to the intermediate
3 in an overall 30% yield. In the second route, using the
(5) Palani, A.; Tagat, J. R. J. Med. Chem. 2006, 49, 2851-2857.
(6) Imamura, S.; Ichikawa, T.; Nishikawa, Y.; Kanzaki, N.; Takashima,
K.; Niwa, S.; Iizawa, Y.; Baba, M.; Sugihara, Y. J. Med. Chem. 2006, 49,
2784-2793.
ladium-catalyzed alkynylation reaction of compound 9 with
protected propargyl alcohol gave compound 10.15,16
(7) Wood, A.; Armour, D. Prog. Med. Chem. 2005, 43, 239-271.
(8) Schenone, P.; Mosti, L.; Menozzi, G. J. Heterocycl. Chem. 1982,
19, 1355-1361.
(12) Haley, C. A. C.; Maitland, M. J. Chem. Soc. 1951, 3155-3174.
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Sathyamurthy, N. J. Org. Chem. 2003, 68, 3446-3453.
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2001, 42, 4849-4851.
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Heterocycl. Chem. 1986, 23, 1035-1038.
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2044
Org. Lett., Vol. 9, No. 10, 2007

To our delight, the hydrogenation of compound 10 under
Pd/C in methanol at room temperature gave intermediate 11
in quantitative yield. After that, several different reducing
agents were applied to prepare intermediate 2. These results
are summarized in Table 1.
dialdehyde 2 can be used in this step without further
purification after the DIBAL-H reduction reaction, and the
combined yield was 71% over two steps from compound
11.
All four possible isomers were observed as products of
this reaction (Scheme 4). Various chromatographic conditions
Table 1. Preparation of Intermediate 2
Scheme 4. The Wittig Reaction
yield
entry
reduction condition
product
(%)
1
2
3
4
5
6
7
Raney-Ni, NaH2PO2, Py/AcOH/H2O no reaction
Raney-Ni, NaH2PO2, AcOH/H2O
Raney-Ni, 5 bar of H2, AcOH
Raney-Ni, 1 bar of H2, AcOH
Raney-Ni, 75% HCOOH aq
DIBAL-H, THF, 0 °C to reflux
DIBAL-H, toluene, 0 °C
2′
2′
2′
2′′
70
95
90
13
no reaction
2
77
With use of Raney-Ni17,18 as the reduction reagent in
different solvent systems, the 3,5-diaminomethyl-substituted
product (2′) was the only one except for the procedure with
HCOOH as solvent, which led to compound 2′′ in 13% yield.
When DIBAL-H was used as the reduction reagent in
toluene, the desired product (2) was obtained in 77%
separation yield,19 in contrast to the unsuccessful reaction
in THF. Our explanation of this result is that the activity of
the electrophilic reduction reagent DIBAL-H in toluene is
higher than that in the comparatively more polar solvent
THF.20,21
To introduce the aliphatic side chains onto positions 3 and
5, the Wittig reaction was explored under different basic
conditions, such as NaH/toluene,22 NaH/DMSO,23 LHMDS,24
and n-BuLi.25 No reaction was observed when NaH was used
as the base in toluene. A trace amount of the target product
was observed while most of the starting material decomposed
under NaH/DMSO and n-BuLi conditions. Finally, interme-
diate 12 was obtained as the major product when LHMDS
was adopted as the base. We noticed that the crude
have been applied yet no successful separation of any one
of these four isomers has occurred. Therefore, after primary
purification, the mixture was used for the next step. By
calculating the ratios of the integrates of the methyl group
at position 6 on the pyridine ring in the 1H NMR spectrum,
the composition of these four isomers in the mixture were
determined, and compound 12 was characterized as the major
one.
Interestingly, the most common PMB deprotection reagent,
DDQ, was proved inefficient in both CH2Cl2/H2O26,27 and
CH2Cl2/ buffer (pH 7) system28 for compound 12. In fact,
the PMB group was removed quickly with reasonable yield
under acidic conditions to give compound 13.29
The five-member-ring closure was achieved by treating
compound 13 with methanesulfonyl chloride and triethyl-
amine at room temperature.30,31 The crude product was
purified by preparative HPLC with the previously reported
condition.1 Both anibamine (1) and its (11E,22E) isomer were
isolated as the trifluoroacetic acid salts. The spectral proper-
ties of anibamine were compared with those in the literature
and no significant differences were observed.
(17) Rapold, T.; Senn, M. U.S. Patent, 5,384,403, 1995.
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Tillyer, R.; Frey, L.; Heid, R.; Xu, F.; Foster, B.; Li, J.; Reamer, R.; Volante,
R.; Grabowski, E. J. J.; Dolling, U. H.; Reider, P. J. J. Org. Chem. 1999,
64, 9658-9667.
Scheme 5. The Synthesis of Anibamine
(20) Bajwa, N.; Jennings, M. P. J. Org. Chem. 2006, 71, 3646-3649.
(21) Paquette, L. A.; Zhao, M. J. Am. Chem. Soc. 1993, 115, 355-356.
(22) Wightman, R. H.; Cresp, T. M.; Franz, S. J. Am. Chem. Soc. 1976,
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1448. (b) Heathcock, C. H.; Finkelstein, B. L; Jarvi, E. T.; Radel, P. A.;
Hadley, C. R. J. Org. Chem. 1988, 53, 1922-1942.
The anti-HIV activity of anibamine and its (11E,22E)
isomer was evaluated. In an assay that determines inhibition
Org. Lett., Vol. 9, No. 10, 2007
2045

of HIV-1BaL attachment to GHOST R5 cells, anibamine
showed an EC50 at 0.6 µM and its (11E,22E) isomer did at
0.8 µM.32
In summary, anibamine was synthesized from acetyl-
acetone and cyanoacetamide in 10 steps with 7.9% overall
yield, or from acetylacetone and N,N-dimethylformide di-
methyl acetal in 11 steps with 5.1% overall yield. To our
knowledge, this is the first report of the total synthesis of
anibamine. The above synthetic routes also offer the op-
portunity to prepare anibamine analogues for further bio-
logical evaluation and SAR study for their anti-HIV
activity.
(26) Oikawa, Y.; Yoshioka, T.;Yonemitsu, O. Tetrahedron Lett. 1982,
23, 885-888.
Acknowledgment. The project was partially supported
by DHHS AI069975.
(27) Horita, K.; Yoshioka, T.; Tanaka, T.; Oikawa, Y.; Yonemitsu, O.
Tetrahedron 1986, 42, 3021-3028.
(28) Yu, W. S.; Zhang, Y.; Jin, Z. D. Org. Lett. 2001, 3, 1447-1450.
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61, 7719-7726.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(30) Ciufolini, M. A.; Roschangar, F. J. Am. Chem. Soc. 1996, 118,
12082-12089.
(31) Bates, R. W.; Boonsombat, J. J. Chem. Soc., Perkin Trans. 2001,
1, 654-656.
(32) Dextran sulfate were provided as a relevant positive control
compound for the individual assays, with anti-HIV activity of EC50 at 14.5
µM.
OL070748N
2046
Org. Lett., Vol. 9, No. 10, 2007