A novel strategy to assemble the β-diketo acid pharmacophore of HIV integrase inhibitors on purine nucleobase scaffolds

Article abstract of DOI:10.1021/jo701336r

(Chemical Equation Presented) Claisen condensation, the key step in constructing the pharmacophore of aryl β-diketo acids (DKA) as integrase inhibitors, fails in certain cases of highly electron-deficient heterocycles such as purines. A general synthetic

Full text of DOI:10.1021/jo701336r

Among the molecules with anti-HIV integrase inhibitory
activity,10,11 DKA-containing inhibitors have emerged as sig-
nificant potential anti-HIV drug candidates.12 The compounds
S-1360 with a diketotriazole bioisostere of the DKA pharma-
cophore,13 L-731,988 with a pyrrole scaffold,14,15 and one of
our compounds in which the DKA is constructed on a
pyrimidine nucleobase scaffold16 are among examples of potent
inhibitors of integrase of this class of compounds (Figure 1).
A Novel Strategy to Assemble the â-Diketo Acid
Pharmacophore of HIV Integrase Inhibitors on
Purine Nucleobase Scaffolds
Vinod Uchil, Byung Seo, and Vasu Nair*
The Center for Drug DiscoVery and the Department of
Pharmaceutical and Biomedical Sciences, UniVersity of
Georgia, Athens, Georgia 30602
Vnair@rx.uga.edu
ReceiVed June 21, 2007
FIGURE 1. Structues of three â-diketo compounds that are inhibitors
of HIV-1 integrase.
However, there are some serious limitations in the synthetic
methodology required to produce a structurally diverse family
of new diketo acids that are of interest as HIV integrase
inhibitors. This synthetic limitation is particularly problematic
in certain cases for scaffolds that are highly electron deficient.17
Our initial attempts to introduce the DKA moiety on the 6- or
8-positions of purine scaffolds utilized the available literature
strategy, i.e., a cross-Claisen condensation involving a purinyl
ketone and an alkyloxalate. Literature methods show that the
oxalylation of aryl ketones with dimethyl-, diethyl-, or tert-
butyloxalates in the presence of NaH, NaOEt, or NaOMe affords
the precursor of DKA, which, on alkaline or acidic hydrolysis,
furnishes the desired DKA. The yields for the cross-Claisen
condensations are in the range of 10-95%, depending on the
structure of the substrate and the reaction conditions (base,
solvent, reaction temperature, and time).18-21 However, all of
these literature methods met with complete failure when applied
Claisen condensation, the key step in constructing the
pharmacophore of aryl â-diketo acids (DKA) as integrase
inhibitors, fails in certain cases of highly electron-deficient
heterocycles such as purines. A general synthetic strategy
to assemble the DKA motif on the purine scaffold has been
accomplished. The synthetic sequence entails a palladium-
catalyzed cross-coupling, a C-acylation involving a tandem
addition/elimination reaction, and a novel ferric ion-catalyzed
selective hydrolysis of an enolic ether in the presence of a
carboxylic acid ester.
The enzymes of the pol gene of the human immunodeficiency
virus (HIV) have been identified as important viral targets for
the discovery and development of anti-HIV therapeutic agents.1-3
HIV-1 integrase is one of the three enzymes of the pol gene
that is critical for viral replication. It catalyzes biochemical steps
that involve endonucleolytic cleavage (3′-processing) of the viral
DNA in the cytoplasm and strand transfer (integration) of the
tailored viral DNA into host cell DNA in the nucleus. While
inhibitors of the two other key enzymes of the pol gene, reverse
transcriptase and protease, have led to a number of clinically
approved agents that have had an enormous impact in the
treatment of HIV infection,4-6 research efforts on inhibitors of
HIV integrase have not resulted in a single FDA-approved
drug.7-9 Because integrase is essential for HIV replication and
has no human counterpart, it remains a significant target for
the discovery of new anti-HIV agents.
(7) Pommier, Y.; Johnson, A. A.; Marchand, C. Nat. ReV. Drug DiscoVery
2005, 4, 236.
(8) Nair, V. Frontiers Med. Chem. 2005, 2, 3.
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(10) Nair, V.; Chi, G. ReV. Med. Virol. 2007, 17, 277.
(11) Maurin, C.; Bailly, F.; Cotelle, P. Curr. Med. Chem. 2003, 10, 1795.
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M. D. Science 2000, 287, 646.
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S.; Young, S. D.; Clark, D. L.; Medina, J. C. PCT, WO 62513, 1999.
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Cragoe, E. J., Jr.; Streeter, K. B.; Schwam, H.; Michelson, S. R.; Patchett,
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(19) Pais, G. C. G.; Zhang, X.; Marchand, C.; Neamati, N.; Cowansage,
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Y.; Burke, T. R. J. Med. Chem. 2002, 45, 3184.
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Schrier, R. W., Baxter, J. D., Dzau, V. J., Fauci, A. S., Eds.; Mosby: St.
Louis, MO, 2000; Vol. 45, pp 1-40.
(5) Nair, V.; St. Clair, M. H.; Reardon, J. E.; Krasny, H. C.; Hazen, R.
J.; Paff, M. T.; Boone, L. R.; Tisdale, M.; Najera, I.; Dornsife, R. E.; Averett,
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10.1021/jo701336r CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/05/2007
J. Org. Chem. 2007, 72, 8577-8579
8577

TABLE 1. Preparation of Vinyl Ethyl Ethers via Stille Coupling
SCHEME 1. Representation of the General Synthetic
Sequence for Assembling the DKA Pharmacophore
to certain positions of purine or other electron-deficient
heterocyclic scaffolds. In this Note, we report a new and
reproducible synthetic strategy to assemble the DKA moiety
on electron-deficient heterocyclic systems (Scheme 1).
Appropriately functionalized haloheterocycles (1a-e), pre-
pared by a previously developed methodology,22 were converted
to the vinyl ethyl ethers (2a-e) by a Stille palladium-catalyzed
cross-coupling23,24 in 47-97% yield (Table 1). C-acylation25
of the vinyl ether intermediates, 2a-e, with methylchlorooxoac-
etate in the presence of pyridine at 0 °C gave the keto esters,
3a-e, in 31-79% yield (Table 2). This synthetic step proceeds
through a tandem addition-elimination mechanism.26 Selective
hydrolysis of the enolic ether functionality in 3a-e turned out
to be unexpectedly challenging. The hydrolysis favored the
ester functionality and then completely stopped at the carboxy-
lic acid stage under all of the conditions that were attempt-
ed.27,28
Because of the failure of the hydrolysis of 3a-e, a totally
new method that could effect selective enol ether hydrolysis
under mild conditions was sought. Although ferric chloride has
been demonstrated to promote deprotection chemistry,29-31 its
use as a hydrolytic agent for the cleavage of enol ethers has
not been described. Thus, this report represents the first use of
ferric chloride hexahydrate (FeCl3‚6H2O) as an agent for the
selective cleavage of enol ethers in the presence of an ester
functionality. The DKA pharmacophores, 4a-e, were produced
in 42-91% yield (Table 3). As the solubility of FeCl3‚6H2O in
the reaction solvents, dichloromethane or dichloroethane, is low,
it is possible that this reagent plays a catalytic role, and the
hydrolysis step is preceded by metal complexation.32 Liberation
of the free enols, 4a-e, from the iron complex was ac-
complished by treatment with 1 N HCl. The enols required
purification by anion exchange (Sephadex) chromatography as
they failed to elute from conventional columns due to their
strong binding with silica gel and alumina. Finally, hydrolysis
of the ester functionality of 4a-e was easily accomplished under
basic conditions (NaOH, MeOH).17
In summary, we have developed an efficient synthetic
approach to assemble the DKA motif at defined positions of
highly electron-deficient heterocycles such as purines. The
method is applicable to a wide range of other electron-deficient
heterocyclic scaffolds for which the Claisen condensation may
be inefficient. A further advantage of this versatile approach,
over the traditional Claisen condensation, is its degree of
selectivity in tolerating functional groups such as additional ester
units, which are susceptible to participation in cross-Claisen or
Dieckmann condensations to produce undesired side products.
The strategy described is of particular significance for providing
a synthetic pathway for a diverse class of new HIV integrase
inhibitors, which may also have biological potential as inhibitors
of the replication of hepatitis C virus (HCV).33
(22) Nair, V.; Young, D. A.; De Silvia, R. J. Org. Chem. 1987, 52, 1344.
(23) Nair, V.; Turner, G. A.; Buenger, G. S.; Chamberlain, S. D. J. Org.
Chem. 1988, 53, 3051.
(24) Gundersen, L.-L.; Bakkestuen, A. K.; Aasen, J. A.; Øverås, H.; Rise,
F. Tetrahedron 1994, 50, 9743.
(25) Tietze, L-F.; Meier, H.; Foss, E. Synthesis 1988, 274.
(26) March, J. AdVanced Organic Chemistry, Reactions, Mechanisms and
Structures, 4th ed.; John Wiley & Sons: New York, 1999; p 599.
(27) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic
Synthesis, 3rd ed.; John Wiley & Sons: New York, 1999.
(28) March, J. AdVanced Organic Chemistry, Reactions, Mechanisms and
Structures, 4th ed.; John Wiley & Sons: New York, 1999; p 375.
(29) Fadel, A.; Yefsah, R.; Salaun, J. Synthesis 1987, 37.
(30) Ikemoto, N.; Kim, O. K.; Lo, L-C.; Satyanarayana, V.; Chang, M.;
Nakanishi, K. Tetrahedron Lett. 1992, 33, 4295.
(31) Kim, K. S.; Song, Y. H.; Lee, B. H.; Hahn, C. S. J. Org. Chem.
1986, 51, 404.
(32) Sen, S. E.; Roach, S. L.; Boggs, J. K.; Ewing, G. J.; Magrath, J. J.
Org. Chem. 1997, 62, 6684.
(33) Summa, V.; Petrocchi, A.; Pace, P.; Matassa, V. G.; De Francesco,
R.; Altamura, S.; Tomei, L.; Koch, U.; Neuner, P. J. Med. Chem. 2004,
47, 14.
8578 J. Org. Chem., Vol. 72, No. 22, 2007

TABLE 2. Tandem Addition-Elimination To Produce Keto Esters
TABLE 3. Selective Hydrolysis of Enol Ether Functionality
Experimental Section
dissolved in EtOAc (50 mL) and washed with 1 N HCl (50 mL)
followed by water (50 mL). The organic layer was dried over
anhydrous Na2SO4 and concentrated to give a yellow residue, which
was purified by anion exchange chromatography (DEAE sephadex
resin, CH3CN:H2O, 1:1) and crystallized.
General Procedure for Stille Coupling. A mixture of halopurine
1a-e (2.8 mmol), bis(triphenylphosphine)palladium(II) chloride
(0.02 mmol), and ethoxyvinyl(tributyl)tin (3.8 mmol) in dry DMF
(50 mL) was heated under N2 at 75 °C for an appropriate time.
DMF was distilled off and the resulting residue was dissolved in
EtOAc (50 mL) and filtered through a plug of celite. The solvent
was removed and the residue obtained was purified by flash
chromatography on silica gel to give the desired product.
General Procedure for C-Acylation. To a stirred solution of
ethoxy vinyl ether 2a-e (0.7 mmol) and pyridine (1.4 mmol) in
dry chloroform (10 mL) at 0 °C was added, under dropwise
conditions, methyl chlorooxoacetate (1.4 mmol) in dry chloroform
(5 mL). The reaction mixture was allowed to stand in the refrigerator
at 0 °C for an appropriate period of time and then washed with
water and dried over anhydrous sodium sulfate. Removal of CHCl3
gave a yellowish syrup from which pure product was isolated by
column chromatography on silica gel.
Acknowledgment. This project was supported by Grant No.
RO1 AI 43181 from the National Institutes of Health (NIAID).
The contents of this paper are solely the responsibility of the
authors and do not necessarily represent the official views of
the NIH.
Note Added after ASAP Publication. The reagent in step
2 of Scheme 1 and the equation in Table 2 was incorrect in the
version published ASAP October 5, 2007; the corrected version
was published ASAP October 11, 2007.
Supporting Information Available: 1H NMR, 13C NMR, and
HRMS data and physical constants of all the compounds synthe-
sized. This material is available free of charge via the Internet at
http://pubs.acs.org.
General Procedure for Selective Hydrolysis of Enol Ether.
To a solution of ethoxy keto ester 3a-e (0.5 mmol) in dichlo-
romethane or dichloroethane (30 mL) was added FeCl3‚6H2O (0.9
mmol) and the reaction mixture was stirred at reflux for the
appropriate time. The solvents were removed and the residue
JO701336R
J. Org. Chem, Vol. 72, No. 22, 2007 8579