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Synthesis and Antimicrobial Activity of Dimethyl (2-cyanophenylamino) (Substituted Aryl) Methyl Phosphonates
Synthesis and Antimicrobial Activity of Dimethyl (2-cyanophenylamino) (Substituted Aryl) Methyl Phosphonates
Journal of the Korean Chemical Society. 2008. Dec, 52(6): 657-662
Copyright © 2008, The Korean Chemical Society
  • Received : June 21, 2008
  • Published : December 20, 2008
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About the Authors
G. Chandra Sekhar Reddy
B. Siva Kumar
A. Uma Ravi Sankar
M. Veera Narayana Reddy
C. Suresh Reddy

Abstract
A new class of α-aminophosphonic acid esters ( 4a-l ) have been synthesized by three component one-pot reaction with equimolar quantities of 2-amino benzonitrile ( 1 ), dimethylphosphite ( 3 ) and various aldehydes ( 2a-l ) in dry toluene at reflux conditions via Kabachnic-Fields reaction in high yields (74-85%).The structure of title compounds has been established by elemental, IR, 1 H, 13 C, 31 P and Mass spectral analysis. They were found to possess significant antimicrobial activity.
Keywords
INTRODUCTION
Due to numerous important applications of organophosphorus compounds a detailed survey of literature has been made to get an over view on the present status of organophosphorus compounds and their chemistry. Considerable interest has been focused on the synthesis of α-substituted phosphonic acids since they are structural analogous of naturally occurring α-amino acid in biological systems. Among the α-functionalised phosphonic acids, α-amino phosphonic acid derivatives are gaining interest in medicinal chemistry. 1 The use of α-amino alkyl phosphonates as enzyme inhibitors, 2 antibiotics and pharmacological agents, 3 herbicides, 4 heptants of catalytic antibiotics 5 and inhibitors of EPSP synthase, 6 HIV protease, 7 renin, 8 PTPases 9 are well documented.
RESULTS AND DISCUSSION
A new class of a-aminophosphonic acid esters ( 4a-l ) was conveniently synthesized by three component one-pot reaction of equimolar quantities of 2-amino benzonitrile ( 1 ), dimethylphosphite ( 3 ) and various aldehydes ( 2a-l ) in dry toluene at reflux conditions via Kabachnic-Fields reaction for 3-4 hours. Progress of the reaction was monitored by TLC analysis at different intervals and the product were purified by column chromatography using ethyleacetate:hexane (1:3) as step grade mixtures as eluents. Due to presence of the nitrile group at ortho position in conjugation to the aromatic amino group in 2-amino benzonitrile, the π-electron density increases due to resonance on the substrate and thus renders the -NH 2 group of the aromatic amine more nucleophilic. This factor facilitates its nucleophilic addition to the carbonyl carbon of the aldehyde and subsequently yields of the products increases. Another purpose of taking -CN group is that the products may get reduced/oxidized/hydrolysed to CH 2 -NH 2 /CH 2 OH by enzymatic reduction/hydrolysis and increases its solubility in the bio-medium and subsequently increases its biological activity. This assumption is proved by high yields of dimethyl (2-cyanophenylamino)(substituted aryl) methyl phosphonates ( 4a-l ) and their increased antimicrobial activity. These results explain the purpose of introducing a -CN function in the aromatic group of the products.
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Mass Spectral data of compounds4a-f
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Mass Spectral data of compounds 4a-f
The IR spectra of title compounds ( 4a-l ) showed absorption bands at 3310-3410 cm -1 (N-H), 10 1230-1251 cm -1 (P=O), 11 - 15 1014-1031 cm -1 (P-O-C), 16 735-766 cm -1 (P-C aliphatic ) 17 and 2216-2221 cm -1 (C≡N) 18 stretching frequencies.
Aromatic protons of the two benzene rings of the title compounds ( 4a-l ) showed a complex multiplet at d 6.44-8.15. 19 P-C-H protons of 4a-l appeared as doublet of doublet in the region δ 4.75-5.38 ( 2 J P-H = 16.0-17.8, 3 J H-H =9.9-10.4 Hz) due to its coupling with the phosphorus and neighboring N-H proton. The N-H proton exhibited a triplet in the range of δ 5.42-5.52 due to coupling with neighbouring proton and phosphorus. The methoxy protons of the dimethylphosphite moiety resonated as a two distinct doublets in the range d 3.65-3.81 (d, 3 J C-H = 10.7-12.0 Hz) 20 , 21 showing their non-equivalence.
The 13 C NMR spectral data of 4a-l showed characteristic absorption peaks for aromatic carbons. The carbon chemical shift of methoxyl carbon of P-O-CH 3 resonated as a doublet at 54.2-54.3 ppm (d, J = 6.5-7.4 Hz). 22 Methyne carbon, attached to nitrogen and phosphorus, appears as a doublet at 53.0-56.8 ppm (d, 2 J = 7.4 -8.1 Hz).
The 31 P NMR signal appeared as a singlet in the range 21.11-22.96 ppm in all the compounds. 23
The FAB-Mass spectra of 4a-d ( 1 ) agreed with the proposed structures. The fragmentation pathway of 4b is rationalysed as typical example of this series ( 2 ). 24 , 25
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CONCLUSION
A new class of a-aminophosphonic acid esters with moderate antimicrobial activity were conveniently synthesized in good yields in uncatalysed one-pot three component Kabachnic-Fields reaction.
EXPERIMENTAL
The melting points were determined in open capillary tubes on a Mel-Temp apparatus and were uncorrected. IR spectra (ν max in cm -1 ) were recorded in KBr pellets on Perkin Elmer 1000 unit. The 1 H, 13 C & 31 P NMR spectra were recorded on Varian Gemini 300 and Varian AMX 400 MHz NMR spectrometer operating at 300 & 400 MHz for 1 H, 75.46 & 100.57 MHz for 13 C and 121.7 MHz for 31 P. All compounds were dissolved in CDCl 3 and chemical shifts were referenced to TMS ( 1 H & 13 C) and 85% H 3 PO 4 ( 31 P). Micro analytical data were obtained from Central Drug Research Institute, Lucknow, India.
ANTIMICROBIAL ACTIVITY
Schrader-Clark 26 proposed that organophosphorus compounds containing the general structure (A) may have significant biological activity.
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All organophosphorus compounds are inherently good phosphorylating agents of enzymes by virtue of the group P-XYZ in the general structure (A). Slight variation in structure can have very dramatic effects on the efficiency of organophosphorus compounds in bio-activity. These chemically and biologically variable parameters which are hard to estimate are involved in deciding “structure-activity” relationship of these compounds.
Antimicrobial activity of compounds (4a-l)
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aReference compounds
Compounds 4a-l were screened for their antibacterial activity ( 2 ) against Staphylococcus aureus (gram positive) and Escherichia coli (gram negative) by the disc-diffusion method in Mueller-Hinton agar medium, at various concentrations (250, 500 mg/disc) in dimethyl formamide (DMF). These solutions were added to each filter disc and DMF was used as control. The plates were incubated at 35 ℃ and examined for zone of inhibition around each disc after 12 h. The results were compared with the activity of the standard antibiotic Penicillin (250 mg/disc). Their antifungal activity 27 were evaluated against Curvularia lunata and Fusarium oxysporium at different concentrations (250 & 500 mg/disc). Griseofulvin was used as the reference compound. Fungal cultures were grown on potato dextrose broth at 25 ℃ and finally spore suspension was adjusted to10 5 spore/mL. Most of the compounds showed moderate activity against both bacteria and fungi.
- General Procedures for the Synthesis of [(2-cyano-phenylamino)-(3,4-dimethoxy-phenyl)-methyl]-phosphonic acid dimethyl ester (4a)
To a stirred solution of 2-amino benzonitrile ( 1 ) (2.36 g, 0.02 mole), 3-chloro benzaldehyde ( 2a ) (1.23 mL, 0.02 mole) in dry toluene were added dimethyl phosphite ( 3 ) (1.35 mL, 0.02 mole), in dry toluene (30 mL) at room temperature. After the addition has been completed the temperature of the reaction was raised to 40-50 ℃ and maintained for four hours. Progress of the reaction was evaluated by running TLC (silica gel) at different intervals using ethyl acetate and hexane (1:3 by volume) as a mobile phase. After the completion of reaction solvent was removed under reduced pressure in a rotary evaporator and obtained crude product was washed repeatedly with petroleum ether, water and purified by column chromatography on 60-120 mesh silica gel using ethyl acetate:hexane (1:3) as eluent to afford the pure [(3-Chloro-phenyl)-(2-cyano-phenylamino)-methyl]-phosphonic acid dimethyl ester ( 4a ), yield 5.96 g (~85%), mp. 161-163 ℃.
Other compounds ( 4b-l ) were prepared by using the same procedure and were characterized with IR, 1 H-NMR, 13 C-NMR, 31 P-NMR and Mass spectral studies.
- The representative analytical data for [(3-Chlorophenyl)-(2-cyano-phenylamino)-methyl]-phosphonic acid dimethyl ester (4a)
Pale-yellow solid; Yield: 85%; mp 161-163 ℃; Molecular formula: C 16 H 16 ClN 2 O 3 P; Elemental analysis: Carbon 54.78 found (54.79 cal ); Hydrogen 4.58 found (4.57 cal ); Nitrogen 8.00 found (7.99 cal ); IR (KBr) (ν max cm -1 ): 3382 (N-H), 2215 (C≡N), 1242 (P=O), 1020 (P-O-C), 754 (P-C aliph .); 1 H-NMR (δ ppm): 6.45- 7.47 (m, 8H arom ), 4.77-4.87 (dd, J = 6.0Hz, 1 H, CH at Arom.ring), 5.48 (t, 1 H on Nitrogen), 3.69 (d, J = 9.0 Hz, 3H, OCH 3 at Phosphorus), 3.77 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus); 13 C NMR (δ ppm): 98.1-148.3 -C arom , 117.1 -C nitrile , 56.2 at Nitrogen, 54.2 -OCH 3 at Phosphorus; 31 P NMR (δ ppm): 21.32; MS (EI, 70 eV): m/z (%) = 350 (38, M +• ), 241(100).
[(2-cyano-phenylamino)-(3,4-dimethoxy-phenyl)-methyl]-phosphonic acid dimethyl ester ( 4b ) Pale-yellow solid; Yield: 83%; mp 111-113 ℃; Molecular formula: C 18 H 21 N 2 O 5 P; Elemental analysis: Carbon 57.44 found (57.45 cal ); Hydrogen 5.59 found (5.58 cal ); Nitrogen 7.44 found (7.45 cal ); IR (KBr) (ν max cm -1 ): 3404 (N-H), 2216 (C≡N), 1251 (P=O), 1018 (P-O-C), 755 (P-C aliph .); 1 H-NMR (δ ppm): 6.55-8.13 (m, 7H arom ), 4.77-4.87 (dd, J = 6.0Hz, 1 H, CH at Arom.ring), 5.48 (t, 1 H on Nitrogen), 3.65 (d, J = 12. Hz, 3H, OCH 3 at Phosphorus), 3.75 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus), 3.89 (d, J = 9.0 Hz, 6H, OCH 3 at Arom.ring); 13 C NMR (δ ppm): 97.5- 149.3 -C arom , 119.9 -C nitrile , 54.0 at Nitrogen, 54.3 -OCH 3 at Phosphorus, 55.6 at Arom.ring; 31 P NMR (δ ppm): 22.29; MS (EI, 70 eV): m/z (%) = 376 (18, M +• ), 259 (100).
[(2-cyano-phenylamino)-(4-dimethylamino-phenyl)-methyl]-phosphonic acid dimethyl ester ( 4c ) Dark-yellow solid; Yield: 78%; mp 151-153 ℃; Molecular formula: C 18 H 22 N 3 O 3 P; Elemental analysis: Carbon 60.16 found (60.17 cal ); Hydrogen 6.15 found (6.13 cal ); Nitrogen 11.69 found (11.70 cal ); IR (KBr) (ν max cm -1 ): 3356 (N-H), 2218 (C≡N), 1236 (P=O), 1031 (P-O-C), 766 (P-C aliph .); 1 H-NMR (d ppm): 6.54-8.12 (m, 8H arom ), 4.81-4.95 (dd, J = 6.0Hz, 1 H, CH at Arom.ring), 5.48 (t, 1 H on Nitrogen), 3.65 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus), 3.75 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus); 13 C NMR (δ ppm): 97.5-148.6 -C arom , 120.1 -C nitrile , 53.0 at Nitrogen, 54.3 -OCH 3 at Phosphorus, 44.5 at Arom.ring; 31 P NMR (δ ppm): 21.25; MS (EI, 70 eV): m/z (%) = 359 (17, M +• ), 250 (100).
[(2-cyano-phenylamino)-(2-hydroxy-phenyl)-methyl]-phosphonic acid dimethyl ester ( 4d ) Offwhite solid; Yield: 79%; mp 136-138 ℃; Molecular formula: C 16 H 17 N 2 O 4 P; Elemental analysis: Carbon 57.82 found (57.83 cal ); Hydrogen 5.14 found (5.12 cal ); Nitrogen 8.42 found (8.41 cal ); IR (KBr) (ν max cm -1 ): 3400 (N-H), 2215 (CN), 1250 (P=O), 1015 (P-O-C), 760 (P-C aliph .); 1 H-NMR (d ppm): 6.56-7.44 (m, 8H arom ), 5.25-5.35 (dd, J = 6.0Hz, 1 H, CH at Arom. ring), 5.45 (t, 1 H on Nitrogen), 3.73 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus), 3.79 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus); 13 C NMR (δ ppm): 97.4-156.3 -C arom , 119.0 -C nitrile , 53.8 at Nitrogen, 54.2 -OCH 3 at Phosphorus; 31 P NMR (δ ppm): 22.90; MS (EI, 70 eV): m/z (%) = 332 (15, M +• ), 223 (100).
[(2-cyano-phenylamino)-(4-hydroxy-phenyl)-methyl]-phosphonic acid dimethyl ester ( 4e ) Paleyellow solid; Yield: 81%; mp 137-139 ℃; Molecular formula: C 16 H 17 N 2 O 4 P; Elemental analysis: Carbon 57.82 found (57.83 cal ); Hydrogen 5.14 found (5.12 cal ); Nitrogen 8.42 found (8.41 cal ); IR (KBr) (ν max cm -1 ): 3395 (N-H), 2216 (C≡N), 1250 (P=O), 1021 (P-O-C), 759 (P-C aliph .); 1 H-NMR (d ppm): 6.55-8.12 (m, 8H arom ), 5.25-5.35 (dd, J = 6.0Hz, 1 H, CH at Arom. ring), 5.45 (t, 1 H on Nitrogen), 3.69 (d, J = 9.0 Hz, 3H, OCH 3 at Phosphorus), 3.77 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus); 13 C NMR (δ ppm): 97.4-148.5 -C arom , 113.2 -C nitrile , 56.8 at Nitrogen, 54.3 -OCH 3 at Phosphorus; 31 P NMR (δ ppm): 22.85; MS (EI, 70 eV): m/z (%) = 332 (15, M +• ), 223 (100).
[(2-cyano-phenylamino)-phenyl-methyl]-phosphonic acid dimethyl ester ( 4f ) Pale-yellow solid; Yield: 80%; mp 127-130 ℃; Molecular formula: C 16 H 16 ClN 2 O 3 P; Elemental analysis: Carbon 54.77 found (54.79 cal ); Hydrogen 4.56 found (4.57 cal ); Nitrogen 8.01 found (7.99 cal ); IR (KBr) (ν max cm -1 ): 3310 (N-H), 2220 (C≡N), 1230 (P=O), 1017 (P-O-C), 735 (P-C aliph .); 1 H-NMR (d ppm): 6.44-7.46 (m, 9H arom ), 4.78-4.89 (dd, J = 6.0Hz, 1 H, CH at Arom.ring), 5.47 (t, 1 H at Nitrogen), 3.70 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus), 3.78 (d, J = 12.0 Hz, 3H, OCH 3 at Phosphorus); 13 C NMR (δ ppm): 96.3-145.6 -C arom , 118.1 -C nitrile , 56.5 at Nitrogen, 54.7 -OCH 3 at Phosphorus; 31 P NMR (δ ppm): 20.85; MS (EI, 70 eV): m/z (%) = 316 (20, M +• ), 199 (100).
Acknowledgements
The authors thank Prof. C. Devendranath Reddy, Department of Chemistry, Sri Venkateswara University, Tirupati for his advice. One of the authors CSR thanks BRNS (2006/37/39/BRNS/2292), Mumbai, India for providing financial assistance.
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