Synthesis and antimicrobial activity of novel thiazolidinones

A novel synthesis of thiazolidine-2-thione and thiazolidin-2-one derivatives is described with the iodo-cyclothiocarbamation reaction as the key step for the heterocyclic ring formation. This new method has been applied to the synthesis of thiazolidinones as bioisosteric analogs of Linezolid 2. Antimicrobial properties of two new thiazole derivatives are reported.


Introduction
2][3][4] One of the early oxazolidinones studied in detail was DuP-721 (1) 5 (Figure 1) exhibiting a broad spectrum of antibacterial activity including activity against drug-resistant Gram positive bacteria as well as several anaerobes and Mycobacterium tuberculosis.However, further development of DuP-721 (1) was discontinued because of safety concerns in animal models.Continued efforts led to the identification of Linezolid (2) 4,6 (Figure 1) with excellent activity against ever increasing Methicillin Resistant Staphylococcus aureus (MRSA) and with a better safety profile; Linezolid was approved for clinical use in 2001.However, adverse effects are its potential for inhibition of monoamine oxidase and myelosuppression.Practically all oxazolidinones studied so far have shown the potential for myelosuppression.6b Therefore, it has been an objective to identify a molecule with not only a wider spectrum of antibacterial activities but also providing a better safety profile.So far, most synthetic efforts have been directed at modifying substituents in the aromatic ring of 1 and 2. 7 Recently, changes at the 5-methylene group have also been reported. 8However, these efforts have not led to a 'better Linezolid'.
We attempted subtle and bioisosteric changes in the pharmacophore oxazolidinone in order to study the impact of these changes on desirable antibacterial activity and also adverse toxicities.Therefore, we considered the modification of the oxazolidinone ring by replacing the ring oxygen by the bulkier sulfur atom leading to thiazolidines; 9 thiazolidinethione 3 and thiazolidinone 4 (Figure 2) were chosen as targets.Seneci et al. 10 have reported the synthesis of ring-modified oxazolidinone analogs of 1 by introducing as nitrogen, sulfur and phosphorus atoms.Introduction of sulfur was expected to bring in a bulky polarisable atom, thus altering geometry, polarisability, stability, lipophilicity and steric and electronic characteristics of the molecule.It was anticipated that the thiazolidine ring improves the activity of the lead molecules.However, the strategy for the preparation of the corresponding thiazolidine-2-ones, based on thiirane ring opening by a carbamate, proved unsuccessful. 10Seneci et al. succeeded in synthesizing oxazolidine-2-thione 5 by the hydrolysis of the oxazolidinone ring of 1 followed by ring closure with thiophosgene (Scheme 1).Compound 5 exhibited reduced antibacterial activity (MIC against MRSA was 64-128 µg/mL).A general strategy for the synthesis of thiazolidine rings was required that could lead to this new class of compounds.There is a report 11 on the iodo-cyclisation of N-allyland S-allylthioureas for the synthesis of dihydrothiazoles (Scheme 2, Eq. 1 and 2); the synthesis of achiral oxazolidinones utilizes an iodo-cyclocarbamation reaction. 12By the latter reaction an N-allylcarbamate 6 is cyclized using iodine through the intermediacy of an iodonium species 7 leading to oxazolidinones 8 (Scheme 2, Eq. 3).Our strategy to synthesize the thiazolidinones was to employ a hitherto unknown iodo-cyclothiocarbamation reaction as the key step involving a dithiocarbamate in place of a carbamate for the iodo-cyclisation reaction.The use of this method would lead to the synthesis of racemic thiazolidinones.Only the S enantiomer of the antibacterial oxazolidin-2-ones is active, the R enantiomer is completely inactive, and we expected that the biological activity of the thiazolidinone racemate would be reduced by 50%.

Synthesis
Retrosynthetic analysis (Scheme 3) identifies 2-iodomethylthiazolidine-2-thione as key intermediate, which can conceivably be obtained by an iodo-cyclisation reaction of an N-allyldithiocarbamate.This dithiocarbamate could be synthesized by N-allylation of the corresponding thiourethane that is available by the S-alkylation of the triethylammonium dithiocarbamate derived from a primary amine.The thiazolidine-2-thione could then be converted into the desired 5-(acetylaminomethyl)thiazolidine-2-thiones and thiazolidin-2-ones.Condensation of aniline (9a) with carbon disulfide in the presence of triethylamine gave the corresponding dithiocarbamate salt 10a; subsequent alkylation with benzyl bromide furnished the corresponding S-benzyl derivative 11a (Scheme 3); N-allylation of 11a using sodium hydride and allyl bromide in the presence of tetrabutylammonium iodide gave the N-allyl derivative 12a.
The key reaction of iodo-cyclothiocarbamation went smoothly and was carried out successfully using iodine in acetonitrile to give the thiazolidin-2-thione 13a.

Scheme 5
The thioacetate analog 23 of Linezolid 2 was synthesized from mesylate 22 15 and potassium thioacetate (Scheme 6).Antibacterial and antifungal primary screening was performed using agar diffusion assay.Antibacterial MIC was performed by NCCLS agar dilution method (NCCLS M7 A5) 16 and antifungal MIC was performed by NCCLS microbroth dilution method [M 27 (A) 17 and M 38 (P) 18 ].The activity profiles of the thiazolidines (RS)-3, (RS)-4, 13-20 and oxazole 23 against the bacterial strains tested were found to be inferior compared to the oxazolidinones 1 and 2. We observed that there is almost complete loss of antibacterial activity of the thio compounds when compared to the oxygen analogs.Compounds which were inactive in primary screening showed MIC of >16 µg/mL against the bacteria tested.However, the activity profile against the fungi used for screening was found to be a little encouraging.The thiazolidinones (RS)-3 and 19a showed mild antifungal activity (Table 1).

Summary
We have shown that the cyclothiocarbamation reaction can be used to synthesize thiazolidinones.Contrary to our expectations, the thiazolidinone analogues were found to be inactive as antibacterials.The present study adds to the importance of the oxazolidinone as the pharmacophore as changes in the ring renders the compounds inactive.Given the precedence that thiazolidinone compounds have hitherto been tested for antifungal activity, 9 future modifications of compounds 3 and 19a may lead to a new generation of inhibitors.

Benzyl phenyldithiocarbamate (11a).
To the dithiocarbamic acid salt 10a (5.40 g, 20 mmol) in acetonitrile (20 mL) was added benzyl bromide (3.76 g, 22 mmol), and the mixture was stirred at room temperature overnight.The solvent was evaporated under reduced pressure, the residue was diluted with ethyl acetate (50 mL) and washed with water (20 mL).The organic layer was separated, washed with brine and dried (Na 2 SO 4 ).Evaporation of the solvent followed by purification of the residue over a silica gel column using ethyl acetate/hexane (20:80) as eluent furnished 11a (4.4 g, 85%) as a sticky yellow solid.R f = 0.7 (ethyl acetate/hexane 20:80). 1

Benzyl N-allyl-N-phenyldithiocarbamate (12a).
To a solution of the dithiocarbamate 11a (3 g, 11.6 mmol) in dry tetrahydrofuran (50 mL), cooled to 0 °C, was added sodium hydride (50% dispersion in oil, 668 mg, 13.9 mmol), and the mixture was stirred for 30 min.Then, allyl bromide (1.68 g, 13.9 mmol) and tetrabutylammonium iodide (428 mg, 1.16 mmol) were added, and the reaction mixture was gradually warmed to room temperature and stirred overnight.The reaction mixture was concentrated to one-third of its volume, diluted with ethyl acetate (50 mL) and washed with water.The aqueous layer was extracted with ethyl acetate (2 × 10 mL) and the combined organic extracts were washed with brine and dried (Na 2 SO 4 ).The solvent was removed under reduced pressure, and the residue was chromatographed over a silica gel column using ethyl acetate/hexane (10-90:15-85) as eluent to furnish 12a (3 g, 87%) as an oil.R f = 0.72 (ethyl acetate/hexane 20:80).