Synthesis and biological activity of a new class of sulfone linked bis(heterocycles)

Novel sulfone linked bis(heterocycles) having two different heterocyclic rings viz., oxazoline / thiazoline in combination with pyrrole are prepared from E -aroylethenesulfonylacetic acid methyl ester exploiting ester and olefin functionalities. The compounds having pyrrole and thiazoline rings exhibited greater antimicrobial activity.


Introduction
The chemistry of heterocyclic compounds has attracted attention in recent times due to its increasing importance in the field of pharmaceuticals and industries.In fact, the development of simple, elegant and facile methodologies for the synthesis of five membered heterocycles is one of the important aspects in organic synthesis.Among five membered heterocycles, pyrroles, oxazoles and thiazoles have gained attention because of their varied physiological activities.4-Aminopyrrole-2-carboxylate exhibits antibiotic, antiviral and oncolytic activities. 1 Several 2oxazolines have been used as therapeutic agents.Because of their structural relationship to procaine, the aminophenyl-2-oxazolines have local anaesthetic properties and at the same time have a lower toxicity than procaine. 2Besides, the oxazoline and thiazoline rings are important constituents of bioactive natural products and pharmaceuticals. 3The use of oxazoline and thiazoline rings as building blocks in pharmaceutical drug discovery is continually increasing.Hence, it is thought that a worthwhile programme would be to prepare molecules having both pyrrole and oxazole / thiazole rings.Multistep synthetic routes for 3,4-disubstituted pyrroles have been reported in the literature either by coupling of imines and nitroalkanes or using Friedel-Crafts acylation with an electron withdrawing group on the pyrrole nitrogen or 3,4silylated precursors. 4However, these synthetic routes are often complicated and limited to only some substituents.3,4-Disubstitued pyrroles have also been synthesized from Michael acceptors with tosylmethyl isocyanide (TosMIC). 5,6A variety of methods have been reported for the synthesis of oxazolines viz., cyclodehydration of amidoalcohols, 7 condensation of imidate hydrochlorides, 8 carboxylic acids, 9 orthoesters, 10 imino ether hydrochlorides, 11 and nitriles 12 with aminoalcohols.Similarly, syntheses of thiazolines are reported by the coupling of imidate esters with 2-aminothiols, 13 cyclodehydration of hydroxy thioamides 14 and heterocyclic inter conversions from oxazolines 15 or oxazolidines. 16However, lanthanide amino alkoxide complexes as reagents for the preparation of oxazolines and thiazolines are sparsely reported. 17Herein we wish to report our results pertaining to the development of bis heterocycles from aroylethenesulfonylacetic acid methyl ester by exploiting ester moiety using lanthanide 2aminoalkoxide complexes to get 2-oxazolines and 2-thiazolines and olefin moiety using 1,3dipolar cycloaddition of TosMIC to develop pyrrole unit.

Results and Discussion
The synthetic intermediate aroylethenesulfonylacetic acid methyl ester ( 4) is prepared by the condensation of 1-aroyl-2-chloroethene (1) with mercaptoacetic acid followed by oxidation and esterification (Scheme 1) Treatment of 4 with 2-aminoethanol in the presence of n-butyllithium complexed with a suspension of 5-10% mol.equiv. of anhydrous SmCl 3 in toluene produced 2-(aroylethenesulfonylmethyl)-4,5-dihydrooxazole (5) (Scheme1).Adopting similar methodology, the reaction of 4 is carried out with 2-aminoethanethiol and n-butyllithium in the presence of anhydrous SmCl 3 in toluene.The compound obtained is found to be 2-(aroylethenesulfonylmethyl)-4,5-dihydrothiazole (6) (Scheme 1).The 1 H-NMR spectra of 5a and 6a displayed two triplets for C 4 -H and C 5 -H of 2-oxazoline and 2-thiazoline rings at δ 3.78, 4.88 and 3.91, 3.38.Besides, two doublets and a singlet are observed at δ 7.96, 7.62 and 4.29 in 5a and at δ 7.89, 7.61 and 4.23 in 6a which are assigned to olefin protons H A , H B and methylene protons, respectively.The J value (J = 14.4,14.4 Hz) indicates that H A and H B possess trans geometry.The 13  The olefin moiety present in 5 and 6 is utilized to develop pyrrole ring.The reaction of 5 with tosylmethyl isocyanide in the presence of sodium hydride in a mixture of ether and dimethyl sulfoxide gave a solid which is identified as 2-(4′-aroyl-1′H-pyrrol-3′-sulfonylmethyl)-4,5-dihydrooxazole (7).Similarly treatment of 6 with tosylmethyl isocyanide in the presence of sodium hydride furnished 2-(4′-aroyl-1′H-pyrrol-3′-sulfonylmethyl)-4,5-dihydrothiazole (8) (see Scheme 1).The 1 H-NMR spectra of 7a and 8a exhibited two singlets at δ 6.48, 6.56 and 6.96, 6.78 which are assigned to C 2 ′-H and C 5 ′-H of pyrrole ring.Apart from these two triplets are observed at δ 3.73, 4.94 (7a) and at 3.95, 3.36 (8a) due to C 4 -H and C 5 -H of oxazoline and thiazoline rings.A sharp singlet is observed at 4.21 (7a) and 4.17 (8a) for methylene protons.Besides a broad singlet is appeared at 9.21 (7a) and 9.13 (8a) for NH which disappeared on deuteration.The 13  Biological evaluation -Antimicrobial activity.The compounds 5-8 were tested for antimicrobial activity at two different concentrations: 100 and 200µg/mL.The antibacterial activity was screened against Staphylococcus aureus, Bacillus subtilis (Gram-positive bacteria) and Escherichia coli, Klebsiella pneumoniae (Gram-negative bacteria) on nutrient agar plates at 37 ºC for 24 h using chloramphenicol (25 µg per disc) as reference drug.The compounds were also evaluated for their antifungal activity against Fusarium solani, Curvularia lunata and Aspergillus niger using ketoconazole (25 µg per disc) as standard drug.Fungi cultures were grown on potato dextrose agar medium (PDA) at 25 ºC for 3 days.The spore suspension was adjusted to 10 6 pores ml -1 at an mg ml -1 concentration by the Vincent and Vincent method. 18he results of the compounds of preliminary antibacterial testing shown in Table 1 revealed that, in general, the inhibitory activity against the Gram-positive bacteria was higher than that of the Gram-negative bacteria.The compounds 6c, 8a and 8c showed excellent activity against Grampositive bacteria (inhibitory zone > 25 mm) and good activity against Gram-negative bacteria (inhibitory zone > 18 mm).The compounds 5 and 7 displayed least activity against both bacteria.All the test compounds exhibited moderate (5a-c and 7a-c) to high (6a-c and 8a-c) inhibitory effect towards tested fungi (Table 2).The MIC values were determined as the lowest concentration that completely inhibited visible growth of the microorganisms (Table 3).The structure-antimicrobial activity relationship of the synthesized compounds revealed that the compounds having pyrrole in combination with oxazoline moiety exhibited least activity when compared with compounds having pyrrole with thiazoline moiety.Among the substituents on the aryl group, 4-chloro phenyl derivatives were the most active.The maximum activity was observed with compound 8c.

2-(Aroylethenesulfonylmethyl)-4,5-dihydrooxazole (5) / 2-(Aroylethenesulfonyl-methyl)-4,5-dihydrothiazole (6)
. Typical procedure.To a flask charged with anhydrous samarium chloride (0.1 mmol), dry toluene (10 mL) and 2 mmols of aminoethanol / aminoethanethiol were added followed by 2.2 mmol of n-butyllithium at 0 o C. The reaction mixture was stirred at 0 o C for 30 min.Then the contents were refluxed to 120 o C and 1mmol of aroylethenesulfonylacetic acid methyl ester (4) was added and continued the refluxion for an additional period of 12-14 h.The suspension was cooled to room temperature and filtered.The filtrate was extracted with chloroform, washed with water followed by brine solution.The solvent was removed in vacuo.

Antibacterial and antifungal assays
Preliminary antimicrobial activities of compounds 5-8 were tested by agar disc-diffusion method.Sterile filter paper discs (6 mm diameter) moistened with the test compound solution in DMSO of specific concentration 100 µg and 200 µg /disc were carefully placed on the agar culture plates that had been previously inoculated separately with the microorganisms.The plates were incubated at 37 ºC and the diameter of the growth inhibition zones were measured after 24 h. in case of bacteria and after 48 h. in case of fungi.The MICs of the compound assays were carried out using microdilution susceptibility method.Chloramphenicol was used as reference antibacterial agent.Ketoconazole was used as reference antifungal agent.The test compounds, Chloramphenicol and Ketoconazole were dissolved in DMSO at concentration of 800 µg/mL.The twofold dilution of the solution was prepared (400, 200, 100, …, 6.25 µg/mL).The microorganism suspensions were incubated at 36ºC for 24 and 48 h. for bacteria and fungi, respectively.The minimum inhibitory concentrations of the compounds were recorded as the lowest concentration of each chemical compounds in the tubes with no turbidity (i.e.no growth) of inoculated bacteria/fungi.

, 8a and 8c
19lting points were determined in open capillaries on a Mel-Temp apparatus and are uncorrected.The purity of the compounds was checked by TLC (silica gel H, BDH, ethyl acetate-hexane, 1:4).The IR spectra were recorded on a Thermo Nicolet IR 200 FT-IR spectrometer as KBr pellets and the wave numbers were given in cm -1 .The 1 H NMR spectra were recorded in CDCl 3 / DMSO-d 6 on a Jeol JNM λ-300 MHz.The 13 C NMR spectra were recorded in CDCl 3 / DMSO-d 6 on a Jeol JNM spectrometer operating at 75.5 MHz.All chemical shifts were reported in δ (ppm) using TMS as an internal standard.The microanalyses were performed on Perkin-Elmer 240C elemental analyzer.The starting compound 1-aroyl-2-chloroethene (1) was prepared by passing vinyl chloride gas into a solution of aroyl chloride in chloroform in the presence of anhydrous AlCl 3 at 10-15 o C.19To a solution of sodium hydroxide (20 mmol) in methanol (10 mL), mercaptoacetic acid (10 mmol) was added dropwise.To this, compound 1 (10 mmol) was added in portions and the reaction mixture was stirred at 0 o C for 3 h.The contents were poured onto crushed ice and neutralized with dil.HCl.The aqueous layer was extracted with ethyl acetate.The solvent was removed under reduced pressure and the solid obtained was recrystallized from methanol to get 8a.