Vol 2, No 12 (2013)

(1)

AN EFFICIENT SYNTHESIS AND BIOLOGICAL EVALUATION

OF SPIRO[ACENAPHTHYLENE-1,2′-PYRROLIDINE]

DERIVATIVES AS POTENT ANTI-INFECTIVE AGENTS

Anshu Dandia

[a]

**_{*, Sukhbeer Kumari}**

[b]

_{and Pragya Soni}

[a]

Keywords: Spiro[acenaphthylene-1,2’-pyrrolidines], Knoevenagel adduct, trifluoroethanol, antimalarial and antimicrobial activities. A series of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives (4a-j) were synthesized by highly efficient one-pot three-component reaction of acenaphthenequinone, sarcosine and Knoevenagel adduct in 2,2,2-trifluoroethanol as a reusable green solvent. All the synthesized compounds were screened for their ‘in vitro’ antimalarial activity against the growth of Plasmodiumfalciparum, the malaria causing parasite. Some of them showed antimalarial activity with IC50 values as low as 0.003 and 0.005 mg mL-1. The compounds were evaluated for their antibacterial activity against Escherichiacoli, Staphylococcusaureus, Pseudomonasaeruginosa and Streptococcuspyogenes and for antifungal activity against Candida albicans, Aspergillusniger, and Aspergillusclavatus. Some of the compounds exhibited excellent antibacterial activity and compounds 4c and 4e demonstrated significant antifungal activities than the standard drugs.

Corresponding Authors Tel: +91-9414073436 Fax: 0091-141-2523637

E-Mail: [email protected]

[a] Department of Chemistry, University of Rajasthan, Jaipur, India

[b] Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India

Introduction

Despite the enormous progress in medicinal chemistry, infectious diseases remain a biggest threat to society and have provided new challenges to researchers worldwide.1 Among these, malaria and microbial infections are the two most widespread and lethal infectious diseases in the world.2-4_{Approximately 40 % of the world’s population is} affected by malaria.5_{Each of these diseases cause about 2} million deaths worldwide every year.6,7_{The development} and spread of multi-drug-resistant (MDR) and extensively drug resistant (XDR) microorganisms have stimulated research efforts globally. Therefore, the molecular manipulation of promising lead compounds is still a major line of approach to develop new drugs.8,9_{So, the discovery} of novel and potent antimicrobial agents is the best way to overcome microbial resistance and develop effective therapies.10

Plasmodium falciparum is the parasite responsible for most malaria cases (80%), which often prove fatal.11_The burden of malaria has not declined, partly because the parasites have become resistant to the available standard antimalarial drugs such as quinine and chloroquine.12-14_The alarming spread of drug resistant strains of P. falciparum underscores the urgency and continuous need for the discovery of new therapeutics.15

Considerable attention has been focused on spiro compounds in particular to spiropyrrolidines, due to their interesting biological activities.16_{They have been found to} possess antimicrobial, antitumor, antibiotic, anticonvulsant, potential antileukaemic, local anaesthetic and antiviral activities. Furthermore, they also act as inhibitors of human NK-I receptor activity.17,18

In context of this program, we have previously reported the synthesis of spiropyrrolidine derivatives in a variety of solvents.19 _{A literature survey revealed that a number of} methods have been reported for the synthesis of spiropyrrolidines,20_{but all these methods often involve the} use of toxic solvents such as methylene chloride,21a acetonitrile,21b_{and dioxane}21c_{or refluxing in} petroleum-based solvents such as toluene22a _{and benzene.}22b

Therefore, the search continues for a better solvent for the synthesis of spiropyrrolidines in terms of operational simplicity, reusability, economic viability, and greater selectivity.

In recent years, fluorinated alcohols (RFOH) have made a display of their unique properties to be as solvents, co-solvents and additives in the organic synthesis.23_{The ready} availability of fluorinated alcohols have initiated a boom in their applications in the past decade.24_{Reactions in} fluorinated alcohols are generally selective and do not result in by-product formation. Also, isolation of the products is simple and fluorinated alcohols can be easily recovered by distillation.25

(2)

O O

H

N COOH

R COOR'

NC

+ +

1

2

3

Reflux

TFE N_OCH3

H NC

R'OOC R

4

20-30 min

O O

H O

NH H3C

COOH

N O NC

R'OOC H

R

N CH₃ O H NC R'OOC

R

1 ₂

T. S.

N O

7

4

N

O

CH3 C O

OH CH₂CF₃

-CO2

5

N

O C

O

O H O

CH2CF3

6

CF3CH2OH

CN

R'OOC

R H Antimicrobial and antimalarial activities of the prepared

compounds have been investigated, and the results showed that some of the synthesized compounds have broad spectrum of antimicrobial and antimalarial activities.

Result and discussion

Chemistry

We have explored the improved methodology for the synthesis of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives using trifluoroethanol. Utilization of our improved method reduced the reaction time with a concomitant increase in the yield and recovery of the solvent etc.

In order to develop a general, practicable and an environmentally benign method for the synthesis of spiro azaheterocycles, the reaction between acenaphthenequinone (1), sarcosine (2) and Knoevenagel adduct (3) was investigated (Scheme 1) in different solvents and the results are listed in Table 1. It is noteworthy to mention that the polar solvents afforded better yield than the nonpolar ones and the best result was obtained in TFE.

Scheme 1. Synthesis of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives 4a-j

Table 1. Synthetic results of 4a under different reaction conditions Entry Solvent Temp.,

o_C

Time, min

Yielda_, %

1 Ethanol Reflux 58 80

2 Aqueous methanol Reflux 42 83

3 Methanol Reflux 120 76

4 Dichloromethane Reflux 270 48

5 Acetonitrile Reflux 180 68

6 THF Reflux 270 56

7 1,4-dioxane Reflux 270 61

8 [bmim]BF4 Reflux 65 82

9 TFE Reflux 20 94

a_{Isolated yield}

With the optimized conditions in hand, we probed the scope of the reaction. As evidenced by the results in Table 2, different Knoevenagel adducts smoothly reacted, producing the corresponding spiro[acenaphthylene-1,2′-pyrrolidine] derivatives in generally good to excellent yields. All the products are known compounds and were characterized by comparing the IR, 1_{H NMR, and}13_{C NMR spectroscopic} data and their melting points with the literature values.

Table 2. Synthetic results of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives 4a-j

Product R R′ Time, min

Yielda,

%

Mpb₍o_C)

4a 4-F Me 20 94 196-198

4b 4-Cl Me 25 92 208-210

4c 4-Br Me 22 93 198-200

4d 4-CH3 Me 28 93 178-180

4e 4-OCH3 Me 30 92 146-148

4f 4-F Et 20 93 202-204

4g 4-Cl Et 23 92 170-172

4h 4-Br Et 30 93 162-164

4i 4-CH3 Et 25 94 180-182

4j 4-OCH3 Et 30 91 168-170 a _{Isolated yield,}b _{Melting points of compounds are consistent}

with reported values19b

Scheme 2. Proposed mechanism for synthesis of spiro derivatives One of the major advantages of this protocol is the isolation and purification of the products, which has been achieved by simple filtration and crystallization of the crude products. After the reaction, TFE can be easily separated (by distillation) and reused four times without decrease in its activity. In this process, TFE act as Bronsted acid (pKa = 12.4 for TFE) and enhance the electophilic character of the carbonyl groups is by high hydrogen bond ability of the trifluoroethanol (CF3CH2OH), which facilitated the ‘in situ’ generation of azomethine ylide 7 by the reaction of acenaphthenequinone 1, sarcosine 2. Subsequent 1,3-dipolar cycloaddition reaction of dipolarophile 3 (Knoevenagel adduct) and azomethine ylide 7 afford spiro derivatives 4

(3)

Biology

Malaria and microbial infections severely influence the health and economies of the poor countries, therefore low-cost of antimalarial and antimicrobial drugs are equally important alongwith the view point of efficacy and safety. Spiro[acenaphthylene-1,2′-pyrrolidine] derivatives (4a–j) were tested for their in vitro anti-bacterial activity against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pyogenus strains (Table 3). The MICs of synthesized compounds were carried out by broth micro dilution method as described by Rattan.28 _Their anti-fungal activity was tested against the Candida albicans, Aspergillus niger, and Aspergillus clavatus strains (Table 4). In all cases, the minimal inhibitory concentration (MIC) values (the highest dilution showing at least 99% inhibition) were compared with standard drugs. The in vitro antimalarial (Table 5) assay was carried out in 96 well microtitre plates according to the micro assay protocol of Rieckmann and co-workers with minor modifications. Parasites were cultured with human erythrocytes (blood group O+_{) at 5% haematocrit in RPMI 1640 supplemented} with 10% human plasma as previously described.29 _All synthesized compounds were screened for their biological activities.

Antibacterial activity

Table 3. Minimum inhibitory concentrations of

spiro[acenaphthylene-1,2′-pyrrolidine] derivatives against E. coli, P. aeruginosa, S. aureus, and S. pyogenus microbial strains.

No _{Antibacterial activity (MIC}a_{, μg mL}-1₎ E. Coli

MTCC442

P. Aeruginosa

MTCC 441

S. Aureus

MTCC96

S. Pyogenus

MTCC443

4a ₁₂₅ ₁₂₅ _62.5 ₅₀₀

4b ₁₀₀ ₁₅₀ ₁₂₅ ₂₅₀ 4c 250 125 250 125

4d ₁₂₅ ₂₅₀ ₅₀₀ ₁₀₀

4e ₂₅₀ ₅₀₀ ₂₅ _12.5

4f 100 25 50 6.25

4g ₂₅₀ ₅₀₀ ₁₀₀ ₂₅₀ 4h ₂₀₀ ₁₂₅ ₁₀₀ ₂₅₀

4i _62.5 ₁₀₀ ₂₀₀ ₂₀₀

4j ₁₂₅ ₂₀₀ ₂₅₀ ₁₅₀ Ab ₁₀₀ _-- ₂₅₀ ₁₀₀

Gb _0.05 ₁ _0.25 _0.5

Cb ₅₀ ₅₀ ₅₀ ₅₀

Cb ₂₅ ₂₅ ₅₀ ₅₀

Nb ₁₀ ₁₀ ₁₀ ₁₀

a_{MIC = Minimum inhibitory} _{concentration, the lowest}

concentration of the compound which inhibits the growth of the bacterium by at least 99%. A-Amoxycillin; G-Gentamycin; C-Chloramphenicol; C-Ciprofloxacin; N-Norfloxacin;b_{Standard drug.}

As shown in Table 3, some of the synthesized compounds showed excellent activity against the tested microbial strains, i.e., 4f (MIC = 6.25 μg mL-1_{) and}_4e_{(MIC = 12.5 μg mL}-1₎ against S. Pyogenus, compounds 4e and 4f (MIC = 25 μg mL-1_{) against}_{S. Aureus}_and_{P. Aeruginosa}_{respectively.} Compound 4f (MIC = 50 μg mL-1_{) showed good activity} against S. Aureus, compounds 4a and 4i (MIC = 62.5 μg mL-1_{) against} _{S. Aureus and E. Coli} _{respectively,} compounds 4b (MIC = 125 μg mL-1₎_4g_{(MIC = 100 μg mL} -1_),_4h_{(MIC = 100 μg mL}-1_{) and}_4i_{(MIC = 200 μg mL}-1₎ against S. Aureus. Equivalent inhibitory activity as compared to Ampicillin was shown by compounds 4b (MIC = 100 μg mL-1_{) and}_4f_{(MIC = 100 μg mL}-1_{) against E. Coli,} compounds 4c (MIC = 250 μg mL-1_{) and}_4j_{(MIC = 250 μg} mL-1_{) against S. Aureus, compound}_4d_{(MIC = 100 μg mL}-1₎ against S. Pyogenus. Compounds 4a, 4d and 4j (MIC = 125 μg mL-1_{), showed moderate activity against} _{E. Coli,} compounds 4c (MIC = 125 μg mL-1_{), and}_4j_{(MIC = 150 μg} mL-1_{), against S. Pyogenus. Rest of the compounds showed} poor inhibitory activity against the tested microbial strains.

Antifungal Activity

spiro[acenaphthylene-1,2′-pyrrolidine] derivatives for C. albicans, A. niger, and A. clavatus fungal strains.

a_{MIC=Minimum inhibitory concentration, the lowest concentration}

of the compound which inhibits the growth of the fungus by at

least 99%. N-Nystatin; Gr-Greseofulvin; b _{Standard drug.}

From Table 4, some of the synthesized compounds showed significant activity against the tested fungal strains, i.e., 4e (MIC = 200 μg mL-1_{) and}_4c_{(MIC = 250 μg mL}-1₎ against C. Albicans. Equivalent inhibitory activity as compared to Greseofulvin was shown by compounds 4a, 4b,

4d, 4g and 4h (MIC = 500 μg mL-1_{), against}_{C. Albicans.} On the other hand, rest of the compounds showed poor MIC values compared to the reference compounds Nystatin and Greseofulvin.

No. Antifungal Activity (MICa_{μg mL}-1₎

C. Albicans A. Niger A. Clavatus

MTCC 227 MTCC 282 MTCC 1323 4a 500 >1000 >1000

4b ₅₀₀ ₂₅₀ ₂₅₀ 4c 250 500 500

4d ₅₀₀ _>1000 _>1000 4e 200 250 500

4f _>1000 ₁₀₀₀ ₁₀₀₀

4g 500 500 1000

4h ₅₀₀ ₅₀₀ ₁₀₀₀

4i 1000 250 500

4j ₁₀₀₀ ₅₀₀ ₁₀₀₀

Nb ₁₀₀ ₁₀₀ ₁₀₀

(4)

Antimalarial activity

spiro[acenaphthylene-1,2′-pyrrolidine] derivatives for P. falciparum

a_{MIC = Minimum inhibitory concentration, the lowest}

concentration of the compound which inhibits the growth of the Plasmodiumfalciparum by at least 99%. b_{Standard drug.}

From Table 5, it was clearly concluded that compounds

4b (MIC = 0.003 µg mL-1_),_4e_{(MIC = 0.005 µg mL}-1_{) and}

4c (MIC = 0.012 µg mL-1_{) showed excellent activity as} compared to both of the standard drugs against Plasmodium falciparum. Compounds 4a (MIC = 0.023 µg mL-1_),_4d (MIC = 0.025 µg mL-1_),_4f_{(MIC = 0.057 µg mL}-1_),_4h (MIC = 0.098 µg mL-1_{) and}_4i_{(MIC = 0.15 µg mL}-1_), showed good activity as compared to the quinine. Equivalent inhibitory activity as compared to quinine was shown by compound 4g (MIC = 0.26 µg mL-1_{). Compound}

4j showed poor activity as compared to both of the standard drugs.

Experimental

Melting points were determined on a Toshniwal apparatus. The purity of compounds was checked on thin layers of silica gel in various non-aqueous solvent systems, for e.g., benzene: ethylacetate (8:2). IR spectra (KBr) were recorded on a Magna FT IR–550 spectrophotometer and 1_{H NMR and} 13_{C NMR spectra were recorded on Brukar Avance II 400}

MHz spectrometer using DMSO-d6 at 400 MHz and 100

MHz, respectively and Brukar DRX-300 using DMSO-d6 at 300.15 MHz and 75.47MHz. TMS was used as internal reference. Mass spectrum of representative compound was recorded on JEOL SX-102 spectrometer at 70 eV. Elemental microanalyses were carried out on a Carlo-Elba 1108 CHN analyzer.

General procedure for synthesis of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives (4a-j)

An equimolar mixture of acenaphthenequinone 1 (1 mmol), sarcosine 2 (1 mmol) and Knoevenagel adduct 3 (1 mmol) in 2,2,2-trifluoroethanol was refluxed for appropriate time (20-30 min). The progress of the reaction is mentioned by TLC. After completion of the reaction, the corresponding solid products 4 was obtained through simple filtration and

washed with TFE to furnish pure spiro[acenaphthylene-1,2′-pyrrolidine] derivatives. The TFE was distilled off (to recover for the next run).

Spectra of selected compounds

Compound 4a. Methyl

3′-cyano-4′-(4-fluorophenyl)-1′- methyl-2-oxo-2H-spiro[acenaphthylene-1,2′-pyrrolidine]-3′-carboxylate

Mp: 196-198 o_{C. IR (KBr, ν}

max, cm-1): 2232 (CN), 1754 (CO), 1732 (CO). 1_{H NMR (400 MHz, DMSO-d}

6) δ: 1.91 (s, 3H, CH3), 2.98 (s, 3H, OCH3), 3.62 (t, J = 9.2 Hz, 1H, CH), 3.76 (t, J = 8.8 Hz, 1H, CH), 4.83 (t, J = 9.6 Hz, 1H, CH), 7.32-8.13 (m, 10H, ArH). 13_{C NMR (100 MHz, DMSO-d}

6) δ: 35.6, 46.2, 53.8, 59.4, 62.8, 81.8, 115.7, 121.1, 124.2, 125.3, 128.1, 128.6, 128.2, 130.3, 131.7, 132.1, 133.0, 133.4, 135.6, 140.3, 160.1, 165.3, 202.2. MS m/z: 415 [M + H]+_. Anal. Calcd for C25H19FN2O3: C, 72.45; H, 4.62; N, 6.76. Found: C, 72.60; H, 4.53; N, 6.61.

Compound 4g. Ethyl 3′-cyano-4′-(4-chlorophenyl)-1′- methyl-2-oxo-2H-spiro[acenaphthylene-1,2′-pyrrolidine]-3′-carboxylate

Mp: 170-172 o_{C. IR (KBr, ν}

6) δ: 0.19 (t, j = 7.2 Hz, 3H, CH3), 1.89 (s, 3H, CH3), 3.36-3.43 (m, 2H, CH2), 3.52 (t, J = 8.8 Hz, 1H, CH), 3.71 (t, J = 8.8 Hz, 1H, CH), 4.81 (t, J = 9.2 Hz, 1H, CH), 7.11-8.09 (m, 10H, ArH). 13_{C NMR (100 MHz, DMSO-d}

6) δ: 12.9, 34.1, 47.3, 57.2, 61.2, 62.7, 79.8, 115.3, 122.1, 122.3, 125.1, 127.4, 130.5, 131.1, 131.7, 132.5, 132.8, 132.9, 134.6, 141.3, 164.8, 200.9. MS m/z: 445 [M + H]+_{. Anal. Calcd for C}

26H21ClN2O3: C, 70.19; H, 4.76; N, 6.30. Found: C, 70.28; H, 4.82; N, 6.22.

Compound 4i Ethyl 3′-cyano-1′-methyl-2-oxo-4′-p-tolyl-2H-spiro[acenaphthylene-1,2′-pyrrolidine]-3′-carboxylate

Mp: 180-182 o_{C. IR (KBr, ν}

6) δ: 0.13 (t, J = 7.6 Hz, 3H, CH3), 1.93 (s, 3H, CH3), 2.08 (s, 3H, CH3), 3.32-3.39 (m, 2H, CH2), 3.59 (t, J = 9.2 Hz, 1H, CH), 3.76 (t, J = 9.2 Hz, 1H, CH), 4.79 (t, J = 8.4 Hz, 1H, CH), 7.26-8.15 (m, 10H, ArH). 13_{C NMR (100 MHz, DMSO-d}

6) δ: 9.3, 12.7, 34.1, 45.2, 57.9, 61.3, 62.1, 80.0, 115.2, 122.1, 122.8, 127.5, 127.9, 128.1, 130.2, 130.6, 131.4, 131.6, 132.8, 132.9, 133.3, 137.1, 141.2, 164.9, 201.1. MS m/z: 425 [M + H]+_{. Anal.} Calcd for C27H24N2O3: C, 76.39; H, 5.70; N, 6.60. Found: C, 76.63; H, 5.89; N, 6.61.

Conclusion

The present investigation describes an efficient access to highly functionalized spiropyrrolidines via one-pot three-component reaction of acenaphthenequinone, sarcosine and Knoevenagel adduct in trifluoroethanol which are in excellent yields without use of any catalyst and additive. This method bestowed with merits like avoiding the use of any base, metal or Lewis acid catalyst, ease of product isolation/purification by non-aqueous work-up, high chemoselectivity, no side reaction, low costs and simplicity in process and handling and environmentally benign nature. These advantages of TFE made this process very useful for

Antimalarial activity (MICa_{µg mL}-1₎

Compound Mean IC50 Values

4a 0.023 µg mL-1

4b 0.003 µg mL-1

4c 0.012 µg mL-1

4d 0.025 µg mL-1

4e 0.005 µg mL-1

4f 0.057 µg mL-1

4g 0.26 µg mL-1

4h 0.098 µg mL-1

4i 0.15 µg mL-1

4j 0.42 µg mL-1

Chloroquineb _{0.020 µg mL}-1

(5)

the synthesis of spiro[acenaphthylene-1,2′-pyrrolidine] derivatives. Synthesized compounds were tested for their ‘in vitro’ antimicrobial and antimalarial activity. Promising biological results were obtained. Most of the compounds showed better antimalarial activity.

Acknowledgements

Financial assistance from the CSIR (02(0143)/13/EMR-II), New Delhi is gratefully acknowledged. One of the author (SK) is thankful to CSIR-JRF for research fellowship. We are also thankful to the Central Drug Research Institute (CDRI), Lucknow and SAIF Chandigarh for the spectral analyses and Microcare Laboratory, Surat, Gujarat, India for antimicrobial and antimalarial screening of the compounds reported herein.

References

1_{Salathe, M., Kazandjieva, M., Lee, J. W., Levis, P., Jones, M. W.,} Feldman, J. H., PNAS2010, 107, 22020-22025.

2_{Kaur, K., Jain, M., Reddy, R. P., Jain, R.,}_{Eur. J Med. Chem.}₂₀₁₀_, 45, 3245-3264.

3_{Shah, N. M., Patel, M. P., Patel, R. G.,}_{Eur. J Med Chem}_.₂₀₁₂_,₅₄_, 239-247.

4_{Gogoi, S., Shekarrao, K., Duarah, A., Bora, T. C., Gogoi, S.,} Boruah, R. C., Steroids2012, 77, 1438–1445.

5_{Torok, D. S., Ziffer, H.,}_{J. Med. Chem.}₁₉₉₅_,₃₈_{, 5045–5050.} 6_{Hurtley, S., Ash, C., Roberts, L.,}_Science₂₀₁₀_,₃₂₈_{, 841.} 7_{Woodford, N.,}_{Expert Opin. Investig. Drugs}₂₀₀₃_,₁₂_{, 117.} 8_{Rawal, R. K., Phabhakar, Y. S., Kati, S. B., De Clercq, E.,}_Bioorg.

Med. Chem.2005, 13, 6771–6776.

9_{Bonde, C. G., Gaikwad, N. J.,}_{Bioorg. Med. Chem.} ₂₀₀₄_,₁₂_, 2151–2161.

10_{Mungra, D. C., Patel, M. P., Rajani, D. P., Patel, R. G.,}_{Eur. J.} Med. Chem.2011, 46, 4192-4200.

11_{Baird, J. K.,}_{N. Engl. J. Med}_.₂₀₀₅_,₃₅₂_{, 1565-1577.}

12_{Winstanley, P. A., Ward, S. A., Snow, R. W.,}_{Microbes Infect}_.

2002, 4, 157–164..

13_{Biagini, G. A., O’Neill, P. M., Nzila, A., Ward, S. A., Bray, P. G.,} Trends Parasitol. 2003, 19, 479-487.

14_{Kremsner, P. G., Krishna, S.,}_Lancet₂₀₀₄_,₃₆₄_{, 285-294.} 15_{Madapa, S., Tusi, Z., Sridhar, D., Kumar, A., Siddiqi, M. I.,}

Srivastava, K., Rizvi, A., Tripathi, R., Puri, S. K., Shiva Keshava, G. B., Shukla, P. K., Batra, S., Bioorg. Med. Chem.

2009, 17, 203–221.

16_{(a) Augustine, T., Kanakam, C. C., Vithiya, S. M., Ramkumar, V.,} Tetrahedron Lett. 2009, 50, 5906–5909; (b) Karthikeyan, K., Kumar, R. S., Muralidharan, D., Perumal, P. T., Tetrahedron Lett.2009, 50, 7175-7179.

17_{(a) Basavaiah, D., Reddy, R. K.,}_{Org. Lett.}₂₀₀₇_{, 9, 57-60; (b)} Kumar, R. R., Perumal, S., Senthilkumar, P., Yogeeswari, P., Sriram, D., Eur. J. Med. Chem.2009, 44, 3821-3829. 18_{(a) Abou-Gharbia, M. A., Doukas, P. H.,}_Heterocycles₁₉₇₉_,₁₂_,

637-640; (b) Kornet, M. J., Thio, A. P., J. Med. Chem. 1976, 19, 892-898; (c) Rosenmond, P., Hosseini-Mrerscht, M., Bub, C., Liebigs Ann. Chem.1994, 2, 151-154.

19_{(a) Dandia, A., Jain, A. K., Laxkar, A. K., Bhati, D. S.,} Tetrahedron2013, 69, 2062-2069; (b)Dandia, A., Jain, A. K., Laxkar, A., K., Bhati, D. S., Tetrahedron Lett. 2013, 54,

3180-3184; (c) Dandia, A., Jain, A. K., Sharma, S., TetrahedronLett. 2012, 53, 5859–5863; (d) Dandia, A., Jain, A. K., Bhati, D. S., TetrahedronLett.2011, 52, 5333–5337; (e) Dandia, A., Jain A. K., Laxkar, A. K., RSC Adv.2013, 3, 8422-8430.

20_{(a) Periyasami, G., Raghunathan, R., Surendiran, G., Mathivanan,} N., Eur. J. Med. Chem. 2009, 44, 959–966; (b) Huang, Z., Zhao, Q., Chen, G., Wang, H., Lin, W., Xu, L., Liu, H., Wang, J., Shi, D., Wang, Y., Molecules 2012, 17, 12704-12717; (c) Prasanna, R., Purushothaman, S., Suresh, M., Raghunathan, R., TetrahedronLett. 2011, 52, 792–797; (d) Kumar, R. R., Perumal, S., Senthilkumar, P., Yogeeswari, P., Sriram, D., Tetrahedron 2008, 64, 2962-2971; (e) Ghandi, M., Yari, A., Jamal S., Rezaei, T., Taheri, A., Tetrahedron Lett.2009, 50, 4724–4726.

21_{(a) Galliford, C. V., Martenson, J. S., Stern, C., Scheidt, K. A.,} Chem. Commun. 2007, 631; (b) Fresneda, P. M., Castan˜eda,

M., Sanz, M. A., Bautista, D., Molina, P., Tetrahedron2007, 63, 1849–1856; (c) Li, X., Li, Z., Zheng, A., Li, G., Yu, X., Yi, P., J. Heterocycl. Chem. 2011, 48, 836.

22_{(a) Jayashankaran, J., Durga R., Manian, R. S., Sivaguru M.,} Raghunathan, R., TetrahedronLett. 2006, 47, 5535–5538; (b) Kumar, R. S., Rajesh, S. M., Perumal, S., Yogeeswari, P., Sriram, D., Tetrahedron: Asymmetry2010, 21, 1315–1327. 23_{(a) Dubrovina, N., Boener, A.,}_Synthesis ₂₀₀₇_,₁₉_{, 2925–2943;}

(b) Begue, J. P., Delpon, D. B., Crousse, B., Synlett 2004, 18–29; (c) Khaksar, S.; Heydari, A., Tajbakhs, M., Vahdat, S. M., J. Fluorine Chem. 2010, 131, 1377–1381; (d) Khaksar, S. Mohammadzadeh, S., J. Fluorine Chem. 2012, 135, 87–90; 24_{(a) Povey, J., Smales, C., Hassard, S., Howard, M. J.,}_{J. Struct.}

Biol. 2007, 157, 329–338; (b) Buck, M., Quart. Rev. Biophysics1998, 31, 297–355; (c) Eberson, L., Hartshorn, M. P., Perssona, O., Radnera, F., Chem. Commun. 1996, 2105– 2112; (d) Be´gue´, J.-P., Bonnet-Delpon, D., Crousse, B., Synlett 2004, 18–29; (e) Uneyama, K., Organofluorine Chem., Blackwell Publishing Ltd, Oxford, 2006.

25_{Khaksar, S., Yaghoobi, M.,}_{J. Fluorine Chem.}₂₀₁₂_,₁₄₂_{, 41–44.} 26_{(a) Dandia, A., Laxkar, A. K., Singh, R.,}_{Tetrahedron Lett.}₂₀₁₂_,

53, 3012-3017 (b) Dandia, A., Jain A. K., Sharma, S., RSC Adv. 2013, 3, 2924-2934; (c) Dandia, A., Parewa, V., Jain, A. K., Rathore, K. S., Green Chem. 2011,13, 2135-2145; (d) Dandia, A., Parewa, V., Gupta, S. L., Rathore, K. S., J. Molecular Catal. A: Chemical2013, 373, 61-71; (e) Dandia, A., Singh, R., Joshi, J., Kumari, S., Eur. Chem. Bull.2013, 2, 683-686; (f) Dandia, A., Singh, R., Joshi, J., Maheshwari, S., Eur. Chem. Bull.2013, 2, 825-829; (g) Dandia, A., Gupta, S. L., Bhaskaran, S., Eur. Chem. Bull.2013, 2, 836-841. 27_{(a) Dandia, A., Singh R., Khaturia, S.,}_{Bioorg. Med. Chem}_.₂₀₀₆_,

14, 1303–1308; (b) Dandia, A., Singh, R., Khaturia, S., J. Fluorine Chem. 2007, 128, 524–529; (c) Dandia, A., Upreti, M., Rani, B., Pant, U. C., Gupta, I. J., J. Fluorine Chem.

1998, 91, 171-174; (d) Dandia, A., Arya, K., Sati, M., Sarawgi, P., J. Fluorine Chem. 2004, 125, 1273-1277; (e) Dandia, A., Arya, K., Bioorg. Med. Chem. Lett. 2007, 17, 3298-3304; (f) Dandia, A., Singh, R., Bhaskaran, S., Ultrason. Sonochem.2011, 18, 1113–1117; (g) Dandia, A., Singh, R., Sarawgi, P., J. Fluorine Chem. 2004, 125, 1835-1840; (h) Dandia, A., Singh, R., Khaturia, S., Mérienne, C., Morgant, G., Loupy, A., Bioorg. Med. Chem.2006, 14, 2409-2417.

28_{Rattan, A.,}_{Antimicrobials in Laboratory Medicine}_{. Churchill B.I.,} Livingstone, New Delhi, 2000, 85-108.

29_{Trager, W., Jensen, J. B.,}_Science₁₉₇₆_,₁₉₃_{, 673.}