Synthesis and Evaluation of Antimicrobial Activity of Nitrones from Derivatives of Aryl-Substituted Dihydroisoquinoline

This paper describes the synthesis of a series of dihydroisoquinoline nitrones by isomerization of the corresponding oxaziridines. Nitrones 4a–c were obtained in excellent yields and high purity by a simple and effective method from the isomerization of oxaziridines. The synthesized compounds were also evaluated for their antibacterial activity against Gram-positive and Gramnegative bacteria and fungus.


Introduction
Nitrones were first used to trap free radicals in chemical systems and then subsequently in biochemical systems.More recently, several nitrones, including alpha-phenyl-tertbutylnitrone (PBN), have been shown to have potent biological activity in many experimental animal models.Many diseases of aging, including stroke, cancer development, Parkinson disease, and Alzheimer disease, are known to have enhanced levels of free radicals and oxidative stress [1].
Nitrones are easily prepared by several methods welldocumented in the primary literature.The most used includes condensation reactions between carbonyl compounds [2,3] and oxidation of amines, imines, or hydroxylamines [4,5].But the most effective method remains isomerization of oxaziridines [6,7] which are interesting heterocyclic compounds containing oxygen, nitrogen, and carbon atoms in a three-member ring.Since their discovery by Emmons in 1956 [8], oxaziridines have gained increasing attention [9].In particular, the oxaziridine is of interest because of an inherently weak N-O bond due to the strained ring that makes the molecule unusually highly reactive.These heterocycles have been shown to be promising reagents with potent antitumour [10,11], antimalarial [12], and antifungal activities [13], and as effective analogues for penicillin [14].
They are also widely used as reagents and intermediates in the preparation of biologically active molecules [9,15,16].
In this paper, we report the synthesis of a series of nitrones by isomerization of the corresponding oxaziridines and their antimicrobial activity.

Experimental
Solvents were purified by standard methods.Melting points (mp) were determined under microscope with a Leitz Wetzlair device and are uncorrected.LC-MS experiments were carried out with an Agilent 1100 LC system consisting of degasser, binary pump, auto sampler, and column heater.The column outlet was coupled to an Agilent MSD ion Trap XCT mass spectrometer equipped with an ESI ion source.High resolutions (HR) were obtained on a GC-HRMS Micromass Autospec (IE).IR spectra were obtained in KBr disks on a UR-20 instrument.NMR spectra were recorded on an AC 300 Bruker spectrometer at 300 MHz for 1 H and 75 MHz for 13 C.Chemical shifts () are given in ppm relative to T.M.S (tetramethylsilane).All reactions were monitored by TLC using commercial silica gel plates and visualization was accomplished by UV light or stained with Dragendorff reagent.[17].To a cooled (0 ∘ C) solution of sulfuric acid H 2 SO 4 (95%) was added dropwise 1.1 mL of CH 3 CN in 20 mL of hexane under magnetic stirring.Then, tertiary alcohol 1 (commercial product) 15.7 g with 10.43 mmol in 15 mL of hexane was added to the solution.After returning to the room temperature, the resulting mixture was stirred at 68 ∘ C for 2.5 h.Then, the solution is cooled at the room temperature and poured on ice-cold water under magnetic stirring.The solution is alkalized with ammonia.The organic layer is extracted with dichloromethane, washed with a solution saturated in sodium chloride, dried over sodium sulfate, and filtered.The solvent is removed in vacuo.

Preparation of Imine 2b.
To a cooled (0 ∘ C) solution (10 mL) of sulfuric acid, H 2 SO 4 (95%) was added dropwise and under magnetic stirring, 1 eq of p-nitrobenzonitrile in 10 mL of cyclohexane.Then, 0.5 g of tertiary alcohol 1 (commercial product) in 10 mL of cyclohexane was added to the solution.
After returning to room temperature, the resulting mixture was stirred under reflux for 2.30 hours.Then, the solution is cooled at the room temperature and versed on icecold water (50 mL) under magnetic stirring.The solution is alkalized with ammonia.The organic layer was extracted with dichloromethane (100 mL), washed with a saturated aqueous NaCl solution, dried over sodium sulfate, and filtered.The solvent was removed in vacuo.mp 116 ∘ C; Yield 83%.

Preparation of Imine 2c.
The cold imine 2a (500 mg, 2.80 mmol) is added dropwise to 2.5 mL of concentrated sulfuric acid.A solution of 380 mg potassium nitrate in 1.4 mL of sulfuric acid is added dropwise by maintaining the temperature at less than 0 ∘ C. The reaction medium was stirred at room temperature for 2 h and then at 60 ∘ C for 4 h.After returning to room temperature, the reaction medium is poured on ice-cold water and alkalized with ammonia.The organic phase is extracted with the dichloromethane, washed with a solution saturated in sodium chloride, dried on sodium sulfate, and filtered.The solvent was removed in vacuo.

Preparation of Oxaziridine 3a
. By portions, a slight excess of m-chloroperbenzoic acid (314 mg, 1.82 mmol) was added to a solution of imine 2a (300 mg, 1.73 mmol) in methanol (9 mL) under magnetic stirring and at 0 ∘ C. The reaction was followed by TLC (dichloromethane : methanol 98 : 2).The solvent was evaporated, and the residue obtained was taken up in dichloromethane.The solution was washed with a solution of sodium bicarbonate and then with a solution saturated with sodium chloride.The organic phase was dried on sodium sulfate, filtered, and concentrated.

Preparation of Oxaziridine 3b.
To a solution of imine, 2b in methanol (9 mL) was added in small portions, with a slight excess of m-chloroperbenzoic acid (1 equiv of active oxygen) under magnetic stirring and at room temperature.The reaction was followed by TLC (CH 2 Cl 2 /MeOH: 98 : 2).The solvent was evaporated and the residue obtained was taken up in dichloromethane (50 mL).The solution was washed with a solution of sodium bicarbonate and then with a saturated aqueous NaCl solution.The organic phase was dried over sodium sulfate, filtered, and concentrated.mp: 90 ∘ C; Yield 91%.

Results and Discussion
3.1.Chemistry.In this study, we have developed a simple and efficient synthetic method of dihydroisoquinoline nitrone from the corresponding oxaziridines.The oxaziridines presented in this paper were synthesized starting from the commercial tertiary alcohol 1 (Scheme 1).
The imines 2a or 2b from step (x) were obtained by acid catalyzed reaction of 2-methyl-1-phenylpropan-2-ol 1 by Ritter-type procedure with acetonitrile or benzonitrile.The nitration of imine 2a under mild conditions leads to imine 2c.The peracid oxidation of imines 2a-c leads quickly to oxaziridines 3a-c in good yields (Scheme 1).
The oxaziridines 3a and 3c have been previously reported to be an excellent agent for the transfer of oxygen on organosulfides, if the oxygen transfer is promoted by an acid [18].

Antimicrobial Activities.
All the synthesized compounds were screened for their antibacterial activities.For this study, microorganisms employed were B. thuringiensis, E. coli, B. subtilis, K. pneumoniae, and E. faecalis.Antimicrobial studies were assessed by minimum inhibition concentration (MIC) method by the serial dilution technique.
The results of antimicrobial testing are reported in (Table 2).
From the antimicrobial activity study, it was found that compounds 4a exhibited an excellent activity against bacteria B. thuringiensis and E. coli. 3 revealed that all compounds showed antimicrobial activities.