c© 2006 Institute of Chemistry, Slovak Academy of SciencesDOI: 10.2478/s11696-006-0009-9

Furan-Containing Thiacyanine Analogs and their AntimicrobialActivity

aI. KORTIŠOVÁ, bP. FOLTÍNOVÁ, and aJ. HALGAŠ

aDepartment of Chemistry, Faculty of Natural Sciences, St. Cyril and Methodius University, SK-917 01 Trnava

e-mail: jan.halgas@ucm.sk

bInstitute of Subcellular Biology, Faculty of Natural Sciences, Comenius University, SK-842 15 Bratislava

e-mail: foltinova@m2.fedu.uniba.sk

Received 20 December 2004; Revised 16 November 2005; Accepted 21 November 2005

Condensation of quaternary 2-methylbenzothiazolium bromides with 5-(dialkylamino)furan-2-carbaldehydes afforded a series of new thiacyanine analogs. The compounds exhibit significantactivities against several bacterial, yeast, and mould strains. Derivatives with dipropylamino sub-stituents showed higher inhibitory effects than those with cyclic secondary amino groups. Hence,the biological activity may have been influenced by the lipophilicity of the compounds. The effecton growth and on the plastid system of the unicellular flagellate Euglena gracilis was also studied.

Benzothiazolium salts with an amino function inthe end of a longer chain of conjugated double bondscan be regarded as push-pull systems. Such com-pounds are intensively coloured and are interestingalso for their biological activities. Salts with amino-substituted simple polyene chain have been reportedas anthelmintic agents [1]. Similar benzothiazoliumsalts with a benzene ring in the conjugated systemhave interesting antimicrobial activity [2, 3]. More-over, the compounds of this type have been consid-ered recently also for their potential nonlinear opticalproperties [4].The aim of this work was to synthesize a series of

benzothiazolium salts with a furan ring in the con-jugated system between electron-donating secondaryamino group and positively charged benzothiazoliummoiety. Such structures are closely related with thepreviously reported benzene analogs [2], therefore bi-ological testing of this new series can provide directcomparison of the activities.The compounds were prepared by condensation of

3-alkyl-2-methylbenzothiazolium salts with 5-(dialkyl-amino)furan-2-carbaldehydes (Scheme 1). The start-ing benzothiazolium salts with methyl, ethyl, allyl,or benzyl group in the position 3 were preparedaccording to the known procedures [2, 5] by heat-ing of 2-methylbenzothiazole and the correspondingalkyl halides in nitromethane or acetonitrile. Thealdehydes used in the condensations were preparedalso by reported methods [6] from 5-bromofuran-2-carbaldehyde and secondary amino compounds. Thevinamidinium salts formed in the latter reaction weredecomposed by alkali. The condensations were per-formed by refluxing the reactants in methanol with

N

S

R

O

N

S

R

O

+ CH3

1 X-

+ NR

RCO

H

+ C C

1X-

H

H

NRR

pyridine

Scheme 1

a catalytical amount of pyridine. The reactions werefollowed by TLC (chloroform—methanol, ϕr = 4 : 1)and were finished accordingly. The reaction times var-ied from 90 min to 6 h, the yields were 18—88 %.The particular data for the prepared compounds, aswell as their melting points are given in Table 1. Thestructures of the compounds were confirmed by 1HNMR spectroscopy (Table 2). The values of interac-tion constants J3,4 for two protons of vinylene group(11.4—14.0 Hz) indicate E-configuration at this dou-ble bond, which is typical for this type of condensationproducts. All the compounds are intensively colouredin dark blue or violet. In the UV VIS spectra λmaxvalues were found at 562—570 nm with log {ε} in therange 3.19—4.18 at the concentration 8 × 10−6 moldm−3.The compounds were tested in vitro for their an-

timicrobial activity against gram-positive and gram-negative bacteria, a yeast, a mould, as well as a flagel-late Euglena gracilis (Table 3). Like benzene analogsreported in [2], these furan-containing compounds areequally or even more active against gram-positive bac-

52 Chem. Pap. 60 (1)52—55 (2006)c© 2006 Institute of Chemistry, Slovak Academy of Sciences

THIACYANINE ANALOGS

Table 1. Characterization of the Synthesized Compounds

M.p. Yield

Compound R1 X —NR2 Formula Reaction

Mr ◦C time/min %

I CH3 I N C18H19IN2OS438.33

314—316 240 57

II CH3 I N C19H21IN2OS452.36

280—282 90 60

III CH3 I N O C18H19IN2O2S454.30

285—289 240 88

IV CH3 I —N(CH2CH2CH3)2 C20H25IN2OS468.40

214—217 150 34

V C2H5 I N C19H21IN2OS452.36

273—277 360 54

VI C2H5 I N C20H23IN2OS466.38

269—273 150 53

VII C2H5 I N O C19H21IN2O2S468.36

245—250 180 19

VIII C2H5 I —N(CH2CH2CH3)2 C21H27IN2OS482.43

186—190 120 39

IX CH2CH——CH2 Br N C20H21BrN2OS417.36

234—236 120 29

X CH2CH——CH2 Br N C21H23BrN2OS431.39

188—190 140 18

XI CH2CH——CH2 Br N O C20H21BrN2O2S433.36

243—246 90 64

XII CH2CH——CH2 Br —N(CH2CH2CH3)2 C22H27BrN2OS447.40

189—194 180 22

XIII CH2C6H5 Br N C24H23BrN2OS467.42

221—225 140 44

XIV CH2C6H5 Br N C25H25BrN2OS481.46

185—188 180 65

XV CH2C6H5 Br N O C24H23BrN2O2S483.42

314—316 240 57

teria. For example, the MIC of the compound XIIagainst S. aureus and B. subtilis was 0.4 µg cm−3,while the highest activity of similar benzene analogswas reported to be 2 µg cm−3 [2]. Moreover, the com-pounds IV and XII showed interesting activity – com-parable with that of the standard ampicillin – againstgram-negative E. coli. The activity of the compoundsIX and XII against the yeast strain C. albicans is alsonoteworthy. Two compounds (IV and XII ) that exhib-ited the highest activities in this basic screening weretested further against broader variety of microorgan-isms. As can be seen from Table 4, the two compoundspossess low activity against P. aeruginosa, but veryremarkable activities against all other twelve strains.The effects on the unicellular flagellate Euglena gra-cilis are comparable with those of the benzene analogs,no bleaching effects on the plastid system were ob-served.

EXPERIMENTAL

The melting points were determined on a Koflerblock. The 1H NMR spectra were recorded on a 360MHz instrument Brucker Avance 360 in DMSO-d6 so-lutions with TMS as an internal standard. The UVVIS spectra were recorded on a Hewlett—Packard852A spectrophotometer.The starting 3-alkyl-2-methylbenzothiazolium

salts were prepared using the known procedures andtheir melting points were in accordance with the lit-erature data [2, 5]. Similarly, 5-(dialkylamino)furan-2-carbaldehydes, except the new dipropylamino deriva-tive, had properties described in Ref. [6].

5-(Dipropylamino)furan-2-carbaldehyde

A solution of 5-bromofuran-2-carbaldehyde (2.40

Chem. Pap. 60 (1)52—55 (2006) 53

I. KORTIŠOVÁ, P. FOLTÍNOVÁ, J. HALGAŠ

Table 2. 1H NMR Data of the Synthesized Compounds (δ, J )

N

S

R

O+ C C

1X-

H

H

NRR

12

3

45

67

8

—NR2 N

9 10

N

9 10

11 N O

9 10

—N(9

CH210

CH211

CH3)2

R1 = —12

CH3; —12

CH213

CH3; —12

CH2—13

CH——14

CH2; —12

CH2—C6H5

Compd. H-1 H-2 H-3 H-4 H-5—H8 H-9 H-10 H-11 H-12 H-13 H-14 H-ArJ1,2/Hz J3,4/Hz

I 6.24 7.77 6.44 7.41 7.46—8.04 3.74 2.08 3.924.3 13.6

II 6.43 7.75 6.55 7.39 7.45—8.05 3.78 1.73 1.64 3.944.3 13.7

III 6.33 7.73 6.68 7.54 7.52—8.11 3.83 3.78 3.984.4 13.5

IV 6.41 7.77 6.48 7.82 7.85—8.48 3.52 1.15 0.96 3.524.3 13.5

V 6.22 7.74 6.39 7.37 7.41—8.00 3.70 2.03 4.46 1.334.7 13.5

VI 6.41 7.75 6.50 7.36 7.41—8.00 3.74 1.69 1.53 4.49 1.334.6 13.5

VII 6.31 7.71 6.62 7.50 7.47—8.06 3.78 3.76 4.54 1.344.4 13.0

VIII 6.24 7.85 6.34 7.35 7.50—8.38 3.65 1.41 0.80 4.99 1.114.6 13.7

IX 6.30 7.80 6.38 7.40 7.43—8.04 3.74 2.08 5.28 6.03 5.124.6 13.5

X 6.50 7.79 6.50 7.38 7.44—8.05 3.79 1.73 1.51 5.33 6.00 5.204.6 13.6

XI 6.41 7.77 6.61 7.53 7.48—8.10 3.84 3.81 5.34 6.03 5.234.4 14.0

XII 6.47 7.78 6.41 7.41 7.45—8.32 3.64 1.73 0.95 5.49 6.07 5.154.6 13.6

XIII 6.31 7.36 6.48 7.37 7.39—8.05 3.71 2.08 5.72 7.38—7.414.6 11.4

XIV 6.48 7.76 6.56 7.35 7.39—8.00 3.73 2.51 1.66 5.75 7.33—7.434.8 13.5

XV 6.42 7.55 6.72 7.43 7.48—8.10 3.80 3.64 5.83 7.34—7.474.5 13.5

g; 12.5 mmol) and dipropylamine (2.53 g; 25 mmol)in 5 cm3 of ethanol was heated at 50◦C for 1 h. Theunreacted amine and the solvent were distilled off, theresidue cooled and treated with aqueous 40 % NaOHuntil two layers were formed. The organic layer wasseparated, dissolved in ether, the solution was driedover MgSO4 and evaporated. The bright red oil so-lidified on cooling. Yield 13 %, m.p. = 50—51◦C. 1HNMR: 9.94 (s, 1H, CHO); 7.86 (d, 1H, J3,4 = 4.4 Hz,H-3); 6.52 (d, 1H, H-4); 3.69 (t, 4H, NCH2—); 1.24(m, 4H, NCH2CH2); 0.98 (t, 6H, CH3).

Condensation Products I—XV

A mixture of 3-alkyl-2-methylbenzothiazolium salt

(4 mmol) and 5-(dialkylamino)furan-2-carbaldehydewas heated under reflux in 9 cm3 of methanol witha catalytic amount (2 drops) of pyridine for 90—360min (Table 1). After slow cooling, the solid was filteredoff, washed with 2 cm3 of cool methanol and dried.

Biological Testing

Antimicrobial activities were tested by the stan-dard plate diffusion method using Mueller—Hintonand Sabouraud agar, or by the standard dilutionmethod in Sabouraud medium [7]. The specificationof the used microbial strains is given in the footnoteunder Table 3. The activity against E. gracilis wasmeasured in a liquid Cramer—Myers medium contain-

54 Chem. Pap. 60 (1)52—55 (2006)

THIACYANINE ANALOGS

Table 3. Antimicrobial Activity of Synthesized Compounds

Minimum Inhibitory Concentration (MIC)/(µg cm−3)Compound M.g.a E.g.a

S.a. B.s. E.c. P.v. C.a.

I 2 10 10 250 50 50/10 50/2II 2 10 10 250 50 50/10 10/2III 2 10 10 > 250 50 50/10 50/10IV 2 2 2 50 50 50/10 50/2V 2 10 10 250 50 250/50 50/2VI 2 2 10 250 50 50/10 10/2VII 2 10 10 > 250 50 250/50 50/10VIII 2 50 250 250 250 250/50 250/50IX 2 2 10 50 10 50/10 50/10X 2 2 10 250 50 50/10 50/10XI 2 10 10 250 50 50/10 50/10XII 0.4 0.4 2 10 10 10/2 10/0.4XIII 2 10 10 250 250 250/50 250/50XIV 2 50 50 > 250 250 250/50 250/50XV 2 10 10 > 250 50 50/10 250/50

Ampicillina 0.08/0.04 4/1.5 10/3.8Nystatina 10/2 2/0.4

a) Microbicidal concentration/microbistatic concentration.S.a. – Staphylococcus aureus CCM 3953, B.s. – Bacillus subtilis 18/64, E.c. – Escherichia coli CCM 3955, P.v. – Proteus vulgaris,C.a. – Candida albicans Pn-10, M.g. – Microsporum gypseum, E.g. – Euglena gracilis.

Table 4. Extended Testing of Compounds IV and XII

IC50/(µg cm−3)/MIC/(µg cm−3)Microorganism

Compound IV Compound XII

Staphylococcus aureus CCM 3953 2/5* 0.4/2*Staphylococcus aureus Mau 78/71 2/5* 0.4/2*Staphylococcus epidermidis 1.5/5* 0.5/2*Bacillus subtilis 18/64 2.5/10* 0.5/4*Micrococcus luteus 1.5/4* 0.5/2*Escherichia coli CCM 3988 2/10 2/4Escherichia coli 326/71 2.5/10 2/4Proteus vulgaris 15/30 5/10Pseudomonas aeruginosa CCM 8221 inactive 150/250Candida albicans Pn-10 20/50* 8/15*Candida pseudotropicalis 15/30* 5/10*Microsporum gypseum 10/50* 2/10*Trichophyton rubrum 10/25* 5/20*

*MIC is represented by MMC – minimum microbicidal concentration, i.e. the concentration which completely suppresses microbialgrowth in the subculture (confirmed by subculturing MIC on agar plates).

ing 2.0—250 µg of the tested compound per 1 cm3

of the medium. The cultivation was performed underpermanent illumination for 96 h at 26◦C.

REFERENCES

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3. Buffa, R., Zahradník, P., and Foltínová, P., Collect.Czech. Chem. Commun. 67, 1820 (2002).

4. Hrobárik, P., Zahradník, P., and Fabian, W. M. F.,Phys. Chem. Chem. Phys. 6, 495 (2004).

5. Sališová, M., Prokešová, M., Kubričanová, M., andToma, Š., Chem. Pap. 47, 183 (1993).

6. Nazarova, Z. N. and Pustovarov, V. S., Khim. Getero-tsikl. Soedin. 1969, 586.

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