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Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors of sirtuinsMohosin Layek, ab Y. Syam Kumar, a Aminul Islam, a Ravikumar Karavarapu, c Amrita Sengupta, c Devyani Halder, c K. Mukkanti b and Manojit Pal * c Received 29th January 2011, Accepted 28th March 2011 DOI: 10.1039/c1md00029b The Pd-mediated alkynylation of N-(3-iodopyridin-2-yl)sulfonamide was investigated in the presence of 2-aminoethanol as a base. The combination of Pd/C–Cu catalysts and 2-aminoethanol facilitated the reaction to proceed via a coupling-cyclization sequence in a single-pot. Unlike earlier Pd-mediated two- step process the present reaction proceeds via a tandem C–C and C–N bond forming reaction affording a direct synthesis of 2-substituted-7-azaindole derivatives. A variety of novel 2-substituted-7-azaindoles were prepared by using this one-pot method. The methodology was explored for a formal synthesis of a Variolin B analogue. When tested in vitro some of the compounds synthesized showed promising sirtuin inhibiting properties in yeast without showing significant cell toxicities. Docking studies using the active molecules were carried out to understand the nature of their interactions with Sir2 protein. 7-Azaindole (or 1H-pyrrolo[2,3-b]pyridine, Fig. 1), a member of azaindole family, is considered as a bioisostere of indole or purine moiety and found to be integral part of many bioactive molecules. 1 Due to their natural occurrences and various physi- cochemical and pharmacological properties this class of compounds have attracted considerable interest. 2,3 For example, Variolins isolated from Antartic sponge Kirkpatrickia varialosa were found to be active against P388 murine leukemia cells. A representative compound Variolin B (Fig. 1) was identified as the most active among them. 4 Recently, an indole based compound (C, Fig. 1) i.e. 6-chloro-2,3,4,9-tetrahydro-1H-carbazole-1-car- boxamide or EX-527 (also known as SEN0014196) that was identified as a potent inhibitor of sirtuin 5a is presently undergoing Phase 1a clinical trial and being developed for the treatment of Huntington’s disease. 5b This prompted us to examine a series of 2-substituted azaindole derivatives as potential inhibitors of sirtuins. We envisaged that due to the presence of an H-bond acceptor at ‘‘N–7’’ of the azaindole ring this class of compounds might show significant interactions with sirtuins. Moreover, replacing the indole ring by an azaindole moiety has resulted in mark improvement in pharmacokinetic properties in several cases earlier. 5c Nevertheless, in spite of structural similarity with indoles the azaindole class has not been previously explored as probable inhibitors of sirtuins. Despite their medicinal value, a straightforward synthesis of azaindoles is not common in the literature partly due to the elec- tron-deficient nature of the key pyridine ring and the strong metal binding affinity of azaindoles. Moreover, the possibility of formation of N-isomers could complicate their synthesis. Never- theless, one of the frequently used strategies for the synthesis of azaindole derivatives involves the construction of a pyrrole ring on a pyridine moiety and like indoles 6–9 palladium catalyzed reactions have been explored as a key synthetic step. 10–16 These include palladium catalyzed annulations of aryl halide with (i) terminal alkynes under Sonogashira conditions 11–13 or (ii) internal alkynes under Larock conditions. 15,16 Despite being quite versa- tile, the synthesis of 2-substituted-7-azaindoles employing terminal alkynes often involves a two step process (Fig. 2) i.e. Sonogashira coupling of 3-halo-2-aminopyridine with terminal Fig. 1 7-Azaindole (A), Variolin B (B) and inhibitor of sirtuin EX-527. a Dr Reddy’s Laboratories Ltd, Bollaram Road, Miyapur, Hyderabad, 500049, India b Chemistry Division, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad, 500072, India c Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad, 500 046, India. E-mail: [email protected]; Fax: +91 40 6657 1581; Tel: +91 40 6657 1500 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c1md00029b 478 | Med. Chem. Commun., 2011, 2, 478–485 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < MedChemComm Cite this: Med. Chem. Commun., 2011, 2, 478 www.rsc.org/medchemcomm CONCISE ARTICLE Published on 19 April 2011. Downloaded on 28/10/2014 18:43:11. View Article Online / Journal Homepage / Table of Contents for this issue

Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors

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Page 1: Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors

Dynamic Article LinksC<MedChemComm

Cite this: Med. Chem. Commun., 2011, 2, 478

www.rsc.org/medchemcomm CONCISE ARTICLE

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Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis:a direct one pot synthesis of 7-azaindoles and their pharmacologicalevaluation as potential inhibitors of sirtuins†

Mohosin Layek,ab Y. Syam Kumar,a Aminul Islam,a Ravikumar Karavarapu,c Amrita Sengupta,c

Devyani Halder,c K. Mukkantib and Manojit Pal*c

Received 29th January 2011, Accepted 28th March 2011

DOI: 10.1039/c1md00029b

The Pd-mediated alkynylation of N-(3-iodopyridin-2-yl)sulfonamide was investigated in the presence

of 2-aminoethanol as a base. The combination of Pd/C–Cu catalysts and 2-aminoethanol facilitated the

reaction to proceed via a coupling-cyclization sequence in a single-pot. Unlike earlier Pd-mediated two-

step process the present reaction proceeds via a tandem C–C and C–N bond forming reaction affording

a direct synthesis of 2-substituted-7-azaindole derivatives. A variety of novel 2-substituted-7-azaindoles

were prepared by using this one-pot method. The methodology was explored for a formal synthesis of

a Variolin B analogue. When tested in vitro some of the compounds synthesized showed promising

sirtuin inhibiting properties in yeast without showing significant cell toxicities. Docking studies using

the active molecules were carried out to understand the nature of their interactions with Sir2 protein.

7-Azaindole (or 1H-pyrrolo[2,3-b]pyridine, Fig. 1), a member of

azaindole family, is considered as a bioisostere of indole or

purine moiety and found to be integral part of many bioactive

molecules.1 Due to their natural occurrences and various physi-

cochemical and pharmacological properties this class of

compounds have attracted considerable interest.2,3 For example,

Variolins isolated from Antartic sponge Kirkpatrickia varialosa

were found to be active against P388 murine leukemia cells. A

representative compound Variolin B (Fig. 1) was identified as the

Fig. 1 7-Azaindole (A), Variolin B (B) and inhibitor of sirtuin EX-527.

aDr Reddy’s Laboratories Ltd, Bollaram Road, Miyapur, Hyderabad,500049, IndiabChemistry Division, Institute of Science and Technology, JNT University,Kukatpally, Hyderabad, 500072, IndiacInstitute of Life Sciences, University of Hyderabad Campus, Gachibowli,Hyderabad, 500 046, India. E-mail: [email protected]; Fax:+91 40 6657 1581; Tel: +91 40 6657 1500

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1md00029b

478 | Med. Chem. Commun., 2011, 2, 478–485

most active among them.4 Recently, an indole based compound

(C, Fig. 1) i.e. 6-chloro-2,3,4,9-tetrahydro-1H-carbazole-1-car-

boxamide or EX-527 (also known as SEN0014196) that was

identified as a potent inhibitor of sirtuin5a is presently undergoing

Phase 1a clinical trial and being developed for the treatment of

Huntington’s disease.5b This prompted us to examine a series

of 2-substituted azaindole derivatives as potential inhibitors of

sirtuins. We envisaged that due to the presence of an H-bond

acceptor at ‘‘N–7’’ of the azaindole ring this class of compounds

might show significant interactions with sirtuins. Moreover,

replacing the indole ring by an azaindole moiety has resulted in

mark improvement in pharmacokinetic properties in several

cases earlier.5c Nevertheless, in spite of structural similarity with

indoles the azaindole class has not been previously explored as

probable inhibitors of sirtuins.

Despite their medicinal value, a straightforward synthesis of

azaindoles is not common in the literature partly due to the elec-

tron-deficient nature of the key pyridine ring and the strongmetal

binding affinity of azaindoles. Moreover, the possibility of

formation of N-isomers could complicate their synthesis. Never-

theless, one of the frequently used strategies for the synthesis of

azaindole derivatives involves the construction of a pyrrole ring

on a pyridine moiety and like indoles6–9 palladium catalyzed

reactions have been explored as a key synthetic step.10–16 These

include palladium catalyzed annulations of aryl halide with (i)

terminal alkynes under Sonogashira conditions11–13 or (ii) internal

alkynes under Larock conditions.15,16 Despite being quite versa-

tile, the synthesis of 2-substituted-7-azaindoles employing

terminal alkynes often involves a two step process (Fig. 2) i.e.

Sonogashira coupling of 3-halo-2-aminopyridine with terminal

This journal is ª The Royal Society of Chemistry 2011

Page 2: Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors

Fig. 2 Previously reported two-step synthesis of 2-substituted-7-azain-

doles.11–13Scheme 2 Preparation of N-(3-iodopyridin-2-yl)substituted

sulfonamide.

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alkynes followed by cyclization of the resulting 3-alkynyl-2-ami-

nopyridine in the presence of (i) a base11a–e e.g. metal hydride i.e.

KH in NMP or metal alkoxide i.e. KOtBu in NMP or DBU in

MeOH or Et3N in DMF; (ii) a Lewis acid12a–d e.g.AuCl3 in EtOH

or CuI in DMF or InBr2 in toluene or (iii) iodine13 in acetonitrile.

However most of these two-step methods suffer from either the

requirement of longer reaction time (up to 72 h) or the use of harsh

reaction conditions. Additionally, requirement of different cata-

lysts or reagents for two individual steps made these approaches

less attractive especially in scale-up synthesis. Some of these

catalysts and reagents are either expensive or not recoverable/

recyclable or their uses cause environmental problems. Thus,

therewas a need for the development ofmore general and effective

method for the synthesis of 2-substituted-7-azaindoles. As part of

our ongoing effort to build a compound library based on azain-

doles17 we also required a simple and straightforward method to

construct the 7-azaindole ring possessing various substituents at

C-2 position. Herein, we report a highly efficient and practical

method for the one pot synthesis of 7-azaindoles (3) from N-(3-

iodopyridin-2-yl)sulfonamide (1, Z ¼ Me or Ph) and terminal

alkynes (2) using 10% Pd/C–PPh3–CuI as a catalyst system

(Scheme1). To thebest of our knowledge, a one-pot synthesis of 2-

substituted-7-azaindole under Pd/C–Cu catalysis has not been

reported earlier.

One of the key reactants i.e. sulfonamide (1) required for our

azaindole synthesis was prepared from 2-amino-3-iodo-pyridine

(4) following a modified literature procedure shown in

Scheme 2.18 Initially, in order to establish the optimum reaction

condition we chose to examine the coupling reaction of

N-(3-iodopyridin-2-yl)benzenesulfonamide (1a) with phenyl-

acetylene (2a) and the corresponding results are summarized in

Table 1. Our present strategy to synthesize 7-azaindole was

originally based on our earlier synthesis of 2-substituted indoles

via a Pd/C-catalyzed coupling-cyclization process in water.19

2-Aminoethanol was found to be an effective base in our

previous study. Accordingly, the reaction of 1 with 2a was

carried out using 10% Pd/C–PPh3–CuI as a catalyst system in

water in the presence of 2-aminoethanol at 80 �C. While the

starting compound (1) disappeared after 1 h (according to TLC)

no desired product however was isolated from the reaction

mixture (entry 1, Table 1). We then examined the use of few

Scheme 1 Pd/C mediated synthesis of 7-azaindole.

This journal is ª The Royal Society of Chemistry 2011

organic solvents e.g. DMF (entry 2, Table 1), 1,4-dioxane

(entry 3, Table 1) and acetonitrile (entry 4, Table 1). While the

desired compound 3a was isolated in these cases acetonitrile

however, was identified as the best solvent in which the reaction

was completed within 4 h affording 3a in 85% yield. The use of

other bases e.g. triethylamine (entry 5, Table 1) and DIPA (entry

6, Table 1) in place of 2-aminoethanol decreased the product

yield. Among the other Pd-catalysts examined (entries 7–9,

Table 1), (PPh3)2PdCl2 was found to be effective (entry 9,

Table 1) but afforded 3a in slightly lower yield. Nevertheless, we

preferred Pd/C because it is cheaper, stable, and easy to handle

and separable from the product. Moreover, the catalyst can be

recycled.20

Having prepared the 7-azaindole derivative 3a successfully we

decided to explore the scope and generality of this one-pot

coupling-cyclization process in the synthesis of other analogues

especially varying the substituent at C-2. Accordingly, a variety

of terminal alkynes were reacted with the sulfonamide 1 (Table 2)

under the optimized conditions as presented earlier (Entry 4 of

Table 1). As evident from Table 2, all the terminal alkynes

participated well in this coupling-cyclization reaction affording

the desired products in moderate to good yields. Various

substituents such as alkyl or aryl groups present in the terminal

alkyne were well tolerated. The alkyl side chain may contain

a primary (entries 2, and 3, Table 2) or secondary alcohol

(entry 4, Table 2). A chloro or cyano group on the alkyl side

chain (entry 5 and 12, Table 2) was also tolerated. All the reac-

tions were generally completed within 2–4 h irrespective of the

nature of substituents present in the terminal alkynes (2a–k)

except the alkyne 2i (entry 9, Table 2). The yields of products

were found to be moderate when terminal alkynes containing

amino acid residue e.g. (S)- phenylglycine methyl ester (2g) or

(S)-leucine methyl ester (2h) were used (entry 7 and 8, Table 2).

The moderate yield of product was also observed when hept-1-

yne was used (entry 11, Table 2) possibly due to the slow evap-

oration of the reactant alkyne under the reaction conditions

employed. Notably, arylalkynes that are known to undergo

spontaneous dimerization under Pd–Cu catalysis were found to

be effective under the present reaction conditions (entry 1 and 10,

Table 2). All the compounds synthesized were well characterized

by spectral and analytical data. Appearance of a singlet in the

region 6.3–6.9 d in the 1H NMR spectra and 104–110 ppm in 13C

NMR spectra of all the compounds synthesized was due to the

hydrogen at C-3 position which indicated the presence of

azaindole ring.

We have shown that Pd/C–Cu catalysis in the presence of 2-

aminoethanol afforded 2-substituted-7-azaindoles in a single-pot

preparation of which required two distinct steps according to the

previously reported methods. A plausible mechanism for the

Pd/C–Cu mediated alkynylation of N-(3-iodopyridin-2-yl)

Med. Chem. Commun., 2011, 2, 478–485 | 479

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Table 1 Effect of solvents, base and catalyst on the reaction of N-(3-iodopyridin-2-yl)benzenesulfonamide (1a) with phenylacetylene (2a)a

Entry Solvent Catalysts Base Time (h) % Yieldb

1. H2O 10%Pd/C-PPh3 2-Aminoethanol 20 02. DMF 10%Pd/C-PPh3 2-Aminoethanol 8 653. 1,4-Dioxane 10%Pd/C-PPh3 2-Aminoethanol 20 504. CH3CN 10%Pd/C-PPh3 2-Aminoethanol 4 855. CH3CN 10%Pd/C-PPh3 Et3N 20 406. CH3CN 10%Pd/C-PPh3 (i-Pr)2NEt 4 707. CH3CN Pd(PPh3)4 2-Aminoethanol 5 40c

8. CH3CN Pd(OAc)2 2-Aminoethanol 6 45c

9. CH3CN (PPh3)2PdCl2 2-Aminoethanol 4 70c

a All reactions were carried out by using 1 (0.832 mmol), 2 (1.247 mmol), 10% Pd/C or other Pd-catalyst (0.025 mmol), PPh3 (0.099 mmol), CuI (0.049mmol) and a base (3.0 equiv) at 80 �C. b Isolated yields. c PPh3 was not used.

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sulfonamide 1 via coupling-cyclization sequence is shown in

Scheme 3. The alkynylation proceeds via generation of an active

Pd(0) species in situ that undergoes oxidative addition with 1 to

give the organo-Pd(II) species E-1. The active Pd(0) species is

generated from the minor portion of the bound palladium (Pd/C)

via a Pd leaching process into the solution.20 The leached Pd then

becomes an active species by interacting with phosphine ligands.

Thus, a dissolved Pd(0)–PPh3 complex is the active species that

actually catalyzes the C–C bond forming reaction in solution.

The catalytic cycle therefore works in solution rather than on the

surface and at the end of the reaction re-precipitation of Pd

occurs on the surface of the charcoal. Once generated, the

organo-Pd(II) species E-1 then facilitates the stepwise formation

of C–C bond via (i) trans organometallation with copper acety-

lide generated in situ from CuI and terminal alkyne followed by

(ii) reductive elimination of Pd(0) to afford the internal alkyne

E-2. The alkyne E-2 thus formed subsequently undergoes Cu-

mediated21a ring closure in an intramolecular fashion to give the

desired product (3). In general, the shorter reaction time (2–4 h

except entry 9, Table 1) required for the preparation of

7-azaindoles in compared to the indole derivatives19 (3–24 h) is

perhaps due to the higher reactivity of 1 aided by the p-electron

deficiency of the pyridine ring. Moreover, the hydrogen bonding

within the acid–base ion pair of 1 and 2-aminoethanol perhaps

increased the reactivity of 1 towards Pd-mediated alkynation

process.21b This also helps in polarizing the Cu-coordinated triple

bond of E-2 thereby facilitating the intramolecular attack by the

sulfonamide anion leading to the azaindole 3. Nevertheless, the

overall reaction mechanism involves (a) generation of actual

catalytic species, (b) the catalytic cycle for C–C bond formation

followed by (c) intramolecular cyclization for C–N bond

formation.

Having prepared a variety of 2-subtituted-7-azaindoles (3), we

envisaged that the core moiety of variolin (7) might be readily

synthesized from the corresponding protected 2-amino-3-iodo

pyridine (1a) using Pd/C–Cu catalyzed coupling-cyclization

process as a key synthetic step (Scheme 4). Accordingly, the

480 | Med. Chem. Commun., 2011, 2, 478–485

alcohol derivative (3b) prepared as above (entry 2, Table 2) was

oxidized to the aldehyde (6) in the presence of MnO2 in chloro-

form. The aldehyde (6) could be converted to the target

compound (7) following a known multistep method.22

We have a long standing interest in bioactive molecules.23 As

part of our ongoing program24 on identification of novel

modulators of sirtuins we tested some of the compounds

synthesized for their sirtuin modulating properties in vitro. The

sirtuins (class III NAD-dependent deacetylases) are being

considered as important targets for cancer therapeutics as they

are shown to be up-regulated in various types of cancer.25 Inhi-

bition of sirtuins allows re-expression of silenced tumor

suppressor genes, leading to reduced growth of cancer cells. Thus

efforts have been devoted for identification of small molecules as

inhibitors of sirtuins.26 In our effort to identify inhibitors of

sirtuins we have used a yeast cell based reporter silencing assay as

a model system for primary screening. Compounds were there-

fore tested at the concentration of 50 mM initially for their ability

to inhibit yeast sirtuin family NAD-dependent histone deacety-

lase (HDAC) Sir 2 protein. Splitomicin, a known inhibitor of

sirtuin, was used as a reference compound in this assay. Various

2-alkyl/aryl substituted azaindoles (3) were tested for their ability

to inhibit Sir2 protein by estimating inhibition of growth of yeast

strain containing Ura3 gene at telomeric locus, in presence of

5-fluoroorotic acid (5-FOA) as described in experimental

procedure.27 Data for three compounds i.e. 3b, 3i and 3j are

presented in Fig. 3. A compound having the sirtuin inhibitory

effect would inhibit the Sir2 protein, and thus the URA3 gene

would be de-repressed resulting the death of the yeast cell in

presence of 5-FOA. A parallel screen was done in absence of 5-

FOA to check the cytotoxicity of the compounds. Among all the

compounds tested 3i and 3j showed significant inhibition i.e. 42%

and 40% respectively in the presence of 5-FOA. In a dose

response study compound 3j showed dose dependent inhibition

across all the doses tested. None of these compounds showed

significant toxic effect as can be seen from yeast growth in the

absence of 5-FOA. The other compounds that showed significant

This journal is ª The Royal Society of Chemistry 2011

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Table 2 Pd/C-mediated synthesis of 2-substituted 7-azaindoles (3)a

Entry Iodide (1); Z ¼ Alkynes (2) R ¼ Products (3) Time (h) %Yieldb

1 Ph 1a –Ph 2a 4.0 85

2 1a –CH2OH 2b 4.0 85

3 1a -(CH2)2OH 2c 3.0 70

4 1a –CH(OH)CH3 2d 3.0 80

5 1a –(CH2)3Cl 2e 3.0 75

6 1a –CH2OC6H4NO2-o 2f 3.0 50

7 1a 4.0 45

8 1a 4.0 50

This journal is ª The Royal Society of Chemistry 2011 Med. Chem. Commun., 2011, 2, 478–485 | 481

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Page 5: Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors

Table 2 (Contd. )

Entry Iodide (1); Z ¼ Alkynes (2) R ¼ Products (3) Time (h) %Yieldb

9 1a –C(CH3)3 2i 12.0 70

10 Me 1b –Ph 2j 2.0 75

11 1a –(CH2)4CH3 2k 2.0 60

12 1a –(CH2)3CN 2l 3.0 72

a All reactions were carried out by using 1 (1.0 equiv), 2 (1.5 equiv), 1 : 4 : 2 ratio of 10% Pd/C: PPh3: CuI and 2-amino ethanol (3.0 equiv) inMeCN at 80�C. b Isolated yields.

Scheme 3 Proposed mechanism for the one-pot synthesis of 2-substituted-7-azaindoles (3) under the catalysis of Pd/C–CuI–PPh3.

482 | Med. Chem. Commun., 2011, 2, 478–485 This journal is ª The Royal Society of Chemistry 2011

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Page 6: Alkynylation of N-(3-iodopyridin-2-yl)sulfonamide under Pd/C–Cu catalysis: a direct one pot synthesis of 7-azaindoles and their pharmacological evaluation as potential inhibitors

Scheme 4 Syntheis of a variolin B analogue.

Fig. 3 Inhibition of Sir2 protein mediated transcriptional silencing at

the telomeric locus in yeast by 3b, 3i and 3j. (A). The growth inhibition of

yeast in presence of 3b, 3i and 3j which is due to inhibition of HDAC

activity of Sir2 protein. (B) Representative % growth inhibitory activity

of the compound 3b, 3i and 3j.

Fig. 4 Docking studies showing H-bond interactions (marked by yellow

dashed line) of amino acid residues of yeast Sir 2 with 3i.

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growth inhibitory activity at the concentration of 50 mM are 3a

(37%) and 3d (30%). The compounds that showed moderate to

low activity include 3c (15%), 3e (12%), 3k (10%) and 3l (10%).

Notably all these compounds contain a linear side chain at C-2.

This journal is ª The Royal Society of Chemistry 2011

Thus the presence of a bulky group at C-2 of the azaindole ring

seems to be crucial for displaying sirtuin inhibitory properties of

this class of compounds. Nevertheless, to understand the nature

of interactions between these compounds and the Sir2 protein

docking simulation studies were carried out using 3i and 3j.

The budding yeast contain five silent information regulator-2

(Sir2) homologues e.g. Sir2-Hist1-4. The yeast Sir2 protein

deacetylates histones H3 and H4 and requires the cofactor

NAD+ for catalytic activity. The X-ray studies28–31 have shown

that a conserved 270 amino acid catalytic domain with variable

N- and C- termini is present in all Sir2 structures. The catalytic

domain consists of a small Zn-binding domain and a large

Rossmann-fold. Based on the interactions with various parts of

the NAD+ cofactor (e.g. adenine, ribose and nicotinamide) the

interface between large and the small subdomain is subdivided

into A (adenine), B (ribose), and C (nicotinamide) pocket. While

a similar interaction has been observed with adenine and ribose

in all sirtuin X-ray structures the interaction with nicotinamide is

less clearly understood. The observed productive and non-

productive conformations of nicotinamide in the crystal struc-

ture indicate the high flexibility of this part of cofactor. Studies

have shown that the acetylated peptide binds in a gap between

the two domains. The aliphatic chain of the acetyllysine residue

inserts into a conserved hydrophobic pocket (where NAD+

binds nearby) and makes extensive van der Waals interactions.

Based on the docking studies we carried out for yeast Sir2 using

compounds 3i and 3j (Fig. 4 and 5) it was observed that these

compounds interact with the nicotinamide or C subpocket. The

X-ray structure of yeast Sir2 was used for an automated ligand

docking using Schrodinger molecular modeling software. Both

the molecules were found to interact with the Asn484 and Cys233

amino acid residues and bind with the similar active site of Sir2.

For example, the ‘‘N–7’’ of the azaindole moiety of both the

molecules formed H-bonding with the Asn484 residue. Similarly,

the SO2 moiety of the N-sulfonyl group formed hydrogen bond

with the ‘‘–SH’’ group of the Cys233 residue (the other

Med. Chem. Commun., 2011, 2, 478–485 | 483

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Fig. 5 Docking studies showing H-bond interactions (marked by yellow

dashed line) of amino acid residues of yeast Sir 2 with 3j.

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commonly found amino acids in the binding region include Lys

501, Hie501, Ser473, Glu504, Ala503). Overall, the binding

energy of 3i (�4.81 Kcal/mol) and 3j (�6.65 Kcal/mol) indicates

that both the molecules interact significantly with yeast Sir2.

In conclusion, Pd-mediated alkynylation of N-(3-iodopyridin-

2-yl)sulfonamide was investigated and the use of a combination

of Pd/C–Cu catalysts and 2-aminoethanol facilitated the

coupling-cyclization sequence to proceed in a single-pot. This

resulted in the development of an direct and straightforward

method for the synthesis of 2-substituted-7-azaindole derivatives

via a tandem C–C and C–N bond formation between

N-(3-iodopyridin-2-yl)sulfonamide and a terminal alkyne. A

number of compounds were prepared for the identification of

novel inhibitors of sirtuins by using this methodology. To the

best of our knowledge this is the first example of Pd/C-mediated

synthesis of 2-substituted-7-azaindole in a single pot. The

methodology was found to be general as it worked with a variety

of terminal alkynes and well tolerated with a range of functional

groups. The methodology is amenable for the synthesis of the

core moiety of variolin and other 7-azaindole based complex

heterocycles of potential pharmacological interest. The meth-

odology therefore has potential to become a practical alternative

to the previously reported methods. All the 2-substituted-7-

azaindoles synthesized were screened for their sirtuins inhibitory

properties in vitro and docking studies were carried out using the

most active compounds to understand the nature of interactions.

Due to the medicinal value of 7-azaindoles the methodology

presented here would find wide applications.

Mr. M. L. thanks Dr D. Kalita and Dr V. Dahanukar for his

encouragement. The authors thank the analytical group of DRL

for spectral data.

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