Synthesis of Pyranobenzopyran Derivatives as a Potent Antibacterial Agent

강한 항균제인 Pyranobenzopyran 유도체의 합성

INTRODUCTION

Pyranobenzopyrans and its derivatives are reported to possess various biological activities1-10 such as antibacterial, antifungal, CNS depressant, antiviral, ulcer inhibitor etc. Suksdorfin and DCK which contain pyranobenzopyran moiety are known to be potent anti -HIV agents.3 Similarly suksdorfin inhibited HIV-1 replication in H9 lymphocytes with an in vitro EC50 value of 1.3 μm and a therapeutic index of value of ˃40, while DCK showed extremely potent inhibitory activity against HIV-1.Pyranobenzopyrans are also gaining importance in laser dyes.11 The biological importance of pyranobenzopyran impressed us to synthesize new heterocyclic compounds which contain pyranobenzopyran moieties. All the synthesized compounds were screened for their antibacterial activity.

 

RESULTS AND DISCUSSION

In order to synthesize ethyl-2H,5H-2,5-dioxopyrano[3,2-c] benzopyran-3-oate. 2a-d, the Knoevengeal condensation of 3-formyl-4-hydroxy-2H(1)-benzopyran-2-one12 1a-d with diethyl malonate was carried out in the presence of piperidine to afford the above compound in quantitative yield. With an interest to carry out Michael addition, ethyl-2H,5H-2,5-dioxopyrano[3,2-c] benzopyran-3-oate 2a-d and ethyl aceto acetate were heated in presence of sodium methoxide to give ethyl 2,4-dihydroxy-5H, 12H-5,12-dioxo [2] benzo pyrano [4,3-c] [1] benzopyran-1-oate 3a-d. Solution of 3a-d in NaOH gives coloration with FeCl3. 3a-d on Pechmann condensation with ethyl aceto acetate gives 2-acetyl-1,6-dihydroxy-3H,7H,14H-3,7,14-trioxo-pyrano[2/,3/,5,6] [2] benzo pyrano [3,2-c] benzopyran 4a-d. Michael addition followed by cyclisation of acetyl acetone with ethyl-2H,5H-2,5-dioxo-pyrano[3,2-c] benzopyran-3-oate 2a-d in the presence of sodium methoxide in boiling absolute methanol afforded 1-acetyl-2-methyl-4H,5H,12H,4,5,12-trioxo-dipyrano [3,4-d;3/,2/-c] benzopyran 5a-d. The structures of the compounds 2a-d to 5a-d were confirmed on the basis of spectral and analytical data. These synthetic reactions are summarized in Scheme 1.

Table 1.Characterization data of compounds 2a-d, 3a-d, 4a-d, 5a-d

Scheme 1.

All the above synthesized compounds were screened in vitro for their antimicrobial activity against variety of bacterial strains. Gram negative strains of bacteria used were S. typhi and E. coli while gram positive bacterial strain used was S. aureus. The minimum inhibition concentration (MIC) was determined using Tube Dilution technique according to standard procedure13 (Table 2). The standard drugs used for comparison were ciprofloxacin, cloxacillin and gentamycin. By visualizing the antimicrobial data it could be observed that many of the compounds possess significant antibacterial activity.

Table 2.=Not active up to 150μg/mL. Std: Ciprofloxacin (5 μg/mL); Cloxacillin (10μg/mL); Gentamycin (5 μg/mL).

 

CONCLUSION

The antibacterial activity of the compounds 2-5(a-d) was compared and it was found that amongst them compound 3-5(a-d) showed significant activity against S.aureus, S. typhi and E. coli. due to fused pyranonebenzopyranone ring system. Compound 3b and 3d with methyl substitution at C-8 and C-10 showed significant antibacterial activities against S. aureus. Compounds 4a-4d showed comparable antibacterial activity against S. aureus and E. coli. Compound 4d having methyl group at C-12 position showed very significant activity against E. coli. The compound 5d having methyl group at C-10 position showed higher activity against S. aureus and S. typhi.

 

EXPERIMENTAL

Melting points were taken in open capillaries and are uncorrected. The IR spectra were recorded on a Perkin-Elmer 257 spectrometer using KBr discs. 1H NMR and 13C NMR spectras in DMSO-d6 were recorded on VXR-300 MHz using TMS as internal standard and mass spectra were recorded on Shimadzu GC-MS. The homogeneity of the compounds was described by TLC on silica gel plates. The spots are developed in iodine chamber.

Ethyl-2H, 5H-2, 5-dioxo-pyrano [3, 2-c] benzopyran-3-oate 2a-d:

A mixture of 3-formyl-4-hydroxy-2H(1)-benzopyran-2-one 1a-d (0.01 mol), diethyl malonate (0.01 mole) and piperidine (0.5 mL) was heated on water bath for 2 hr. The mixture was left at room temperature for 3 hr. and then it was poured into ice-cold water and acidified to get solid. The solid was filtered, washed with sodium bicarbonate and finally with water, dried and recrystallized from ethanol to get ethyl-2H,5H-2,5-dioxopyrano[3,2-c] benzopyran-3-oate. 2a-d.

Ethyl 2, 4-dihydroxy-5H, 12H- 5, 12-dioxo [2] benzo pyrano [4, 3-c] [1] benzopyran- 1-oate 3a-d:

A mixture of ethyl-2H, 5H-2, 5-dioxopyrano [3, 2-c] benzopyran-3-oate 2a-d (0.01 mol), ethyl aceto acetate (0.01 mol) and sodium methoxide (0.01 mol) was heated at 170 ℃ for 8 hr. The reaction mixture was cooled, triturated with cold hydrochloric acid. The solid obtained was filtered, washed with water, dried and recrystallized from methanol to get ethyl 2,4-dihydroxy-5H,12H-5,12-dioxo[2] benzo pyrano [4,3-c] [1] benzopyran-1-oate 3a-d.

2-Acetyl-1,6-dihydroxy-3H, 7H, 14H, 3, 7,14-trioxo-pyrano [2/,3/, 5,6] [2] benzo pyrano [3,2-c] benzopyran. 4 a-d.

Ethyl 2, 4-dihydroxy-5H, 12H, 5, 12-dioxo [2] benzo pyrano [4,3-c] [1] benzopyran 1-oate 3a-d (0.01 mol) and ethyl aceto acetate (0.01 mol) were taken in methanol (10 mL), to it piperidine (1.5 mL) was added and was refluxed for 8hr. The methanol was recovered in rotaevaporator. The reaction mixture was then decomposed into crushed ice and neutralized with dilute HCl to afforded solid product. The solid obtained was filtered, washed with water, dried and recrystallized from methanol to get 2-acetyl- 1,6-dihydroxy-3H,7H,14H,3,7,14-trioxo-pyrano[2/,3/,5,6] [2] benzo pyrano [3,2-c] benzopyran 4a-d.

1-Acetyl-2-methyl-4H,5H,12H,4,5,12-trioxo-dipyrano [3,4-d;3/,2/-c]benzopyran 5a-d.

To a solution of sodium methoxide (0.01 mol) in absolute methanol (10 mL), acetyl acetone (0.01 mol) was added and the solution was refluxed for 20 min. then ethyl-2H,5H-2,5-dioxopyrano[3,2-c] benzopyran-3-oate 2a-d (0.01 mol) was added and the reaction mixture was refluxed for 5 hr. The methanol was recovered in rota evaporator. The reaction mixture was then decomposed into crushed ice and neutralized with dilute HCl to get the solid product. The solid obtained was filtered, washed with water, dried and recrystallized from ethanol to get 1-acetyl-2-methyl-4H,5H,12H,4,5,12-trioxo-dipyrano[3,4-d;3/,2/-c] benzopyran. 5a-d.

2b. IR (KBr): 1719, 1614, 1527, 1429, 1375, 1297, 1209, 1043, 814, etc cm-1.

1H NMR DMSO-d6 (δ-ppm): 1.25 (t, 3H, CH3), 2.19 (s, 3H, CH3), 3.9 (q, 2H, -OCH2), 7.1 (s, 1H, C4-H), 7.3 (s, 1H, C10-H), 7.59 (d, 1H, C8-H, J = 7.0 Hz), 7.8 (d, 1H, C7-H, J = 7.0 Hz).

Mass m/z (relative intensity, %): 300(26) (M+), 272(10), 244(13), 228(7), 200(9), 175(23), 172(17), 149(23), 144(19), 116(37), 91(100), 77(26) etc.

3b. IR (KBr): 3420, 2924, 2363, 1731, 1659, 1620, 1447, 1384, 1315, 1230, 1175, 1057, 906, 872, 815, 789, etc. cm-1.

1H NMR DMSO-d6 (δ-ppm): 1.22 (t, 3H, CH3), 2.1 (s, 3H, CH3), 4.25 (q, 2H, -OCH2), 5.6 (s, 1H, -OH, D2O exchangeable), 6.7 (s, 1H, C3-H), 7.05 (s, 1H, C7-H), 7.8 (d, 1H, C9-H, J = 7.5 Hz), 8.05 (d, 1H, C10-H, J = 7.5 Hz), 9.5 (s, 1H, -OH, D2O exchangeable).

Mass m/z (relative intensity, %): 382(24) (M+) 354(10), 342(9),336(6), 326(14), 308(12), 298(7), 280(9), 252(8), 224(11), 209(23), 158(19), 150(28), 134(6), 106(34), 91(66), 84(47), 83(39), 77(100), 68(51), 57(55) etc.

4b. IR (KBr): 3433, 2944, 1723, 1621, 1577, 1494, 1426,1374, 1296, 1209, 1118, 1045, 817, etc. cm-1

1H NMR CDCl3 (δ-ppm): 2.0 (s, 3H, CH3), 2.1 (s, 3H, CH3), 5.8 (s, 1H, -OH, D2O exchangeable), 6.8 (s, 1H, C5-H), 7.1 (s, 1H, C9-H), 7.4 (d, 1H, C11-H, J = 7.0 Hz), 8.1 (d, 1H, C12-H, J = 7.0 Hz), 9.8 (s, 1H, -OH, D2O exchangeable).

Mass m/z (relative intensity, %): 420 (31) (M+) 404(14), 378(10), 350(17), 336(13), 322(10), 308(6), 294(18), 280(14), 252(12),237(20), 200(15), 174(35), 134(64), 106(100), 91(62), 77(79) etc.

5b. IR (KBr): 2925, 2357, 1731, 1658, 1620, 1448, 1385, 1315, 1228, 1176, 1057, 788 etc. cm-1.

1H NMR DMSO-d6 (δ-ppm): 2.05 (s, 3H, CH3), 2.15 (s, 6H, 2 CH3), 6.9 (s, 1H, C7-H), 7.39 (d, 1H, C9-H, J = 7.5 Hz), 7.9 (d, 1H, C10-H, J = 7.5 Hz).

Mass m/z (relative intensity, %): 352(19) (M+), 338(8), 310(10), 299(5), 282(6), 254(6), 249(10), 226(13), 211(23), 196(27), 178(20), 149(30), 137(60), 121(23), 105(41), 91(100), 77(67), 51(53), etc.