Synthesis and Characterization of a New Five and Six Membered Selenoheterocyclic Compounds Homologues of Ebselen

The discovery of the antioxidant activity of selenoenzyme glutathione peroxidase (GPx) has attracted growing attention in the biochemistry of selenium. Among molecules which mimic the structure of the active site of the enzyme, N-phenyl-1,2benzisoselenazolin-3-one 1, Ebselen, exhibited useful anti-inflammatory properties. It has been extensively investigated and has undergone clinical trials as an anti-inflammatory agent. Unfortunately, Ebselen exhibits relatively poor catalytic activity, prompting attempts to design more efficacious GPx mimetics that would retain his low toxicity while manifesting improved catalytic properties. In this context, novel 1,2-benzoselenazine and 1,2-benzoselenazols, which are five and six membered homologues of Ebselen were synthesized and characterized. One structure has been proven by single crystal X-ray crystallography.


Introduction
The discovery of selenium as selenocysteine in the active site of the selenoenzyme glutathione peroxidase (GPx) has attracted growing attention in the biochemistry of selenium [1,2].The selenoenzyme acts as an antioxidant and catalyzes the reduction of harmful peroxides by glutathione, thus protecting lipid membranes against oxidative damage [3][4][5][6][7][8][9].The enzyme catalytic site includes a selenocysteine residue in which the selenium undergoes a redox cycle involving the selenol (Enz-SeH) as the active form that reduces hydroperoxides and organic peroxides and the catalytic cycle is shown in Scheme 1.Thus, the selenol (EnzSeH) is first oxidized by the peroxide to the corresponding selenic acid (EnzSeOH), which reacts with GSH to afford a selenenyl sulfide intermediate (EnzSeSG).The latter undergoes further reaction with GSH, thereby regenerating the original selenol and producing oxidized glutathione (GSSG) as a byproduct [10][11][12][13].
Among molecules which mimic the structure of the active site of the enzyme, N-phenyl-1,2-benzisoselenazolin-3-one 1, Ebselen (PZ51), exhibited useful anti-inflammatory properties [14,15].On the other hand, Ebselen contains a selenic moiety stabilized by intramolecular cyclisation in a cyclic N-aroyl selenamide and does not release selenium, as demonstrated by a 75 Se-labeling study, which result in its relatively nontoxic properties [16,17].The discovery of its anti-inflammatory and glutathione-peroxidase (GPx)like activity has initiated numerous biochemical and pharmacological investigations as well as clinical trials as an antioxidant [18][19][20][21].Similarly a study of the mechanism of the (GPx)-like activity of Ebselen has shown the formation in a catalytic cycle of various intermediates constituting different oxidation levels of the selenium atom [22,23].
Several structural modifications of Ebselen including substituent effect and isosteric replacement have been proposed [24,25].For instance, diselenides and non-benzocondensed isoselenazolidinone ring have been reported in the literature [26,27].
In the same context, our group focused on synthesizing new selenoheterocyclic compounds.The new heterocycles should preserve the Se-C aromatic bond to avoid the release of Se atoms and maintain the low toxicity of Ebselen.Secondly, the Se-N bond responsible for the GPx-like activity is retained.

Results and Discussion
The multistep synthesis of the novel benzoselenazine 10 is represented in Scheme 3. The starting molecule was ochlorobenzonitrile 6, which was easily transformed into ketone 7 by reaction with Grignard reagent.The incorporation of selenium was carried out by treatment of ketone 7 with methaneselenol in the presence of potassium carbonate yielding 8 in good yield.The oximination of selenide 8 was carried out using (n-BuNO 2 , HCl) and 2ethoxyethanol as solvent [47].Oxime 9 was cyclized into the novel heterocycle 4H-benzo-1,2-selenazin-4-one 10 via Se-demethylation using trimethylsilyl polyphosphate (PPSE) [48].
The synthesis of the new selenoheterocycle 14 required the use of diketone 12, which was obtained by oxidation of ketone 11 using SeO 2 as an oxidizing agent.The incorporation of selenium was carried out by treatment of diketone 12 with methaneselenol in the presence of potassium carbonate yielding 13.The addition of 2 equivalents of bromine to a saturated solution of diketone 13 produced a yellow-orange precipitate.This precipitate, which is a result of quaternization of selenium by bromine, was treated by ammonia and purified to produce compound 14, as determined by different spectroscopic techniques.
In order to further prove the structure of compound 14, we attempted to independently synthesize the same compound starting from compound 11 according to the reactions outlined in Scheme 4. Thus, compound 14 was obtained by oxidation of novel heterocycle 16, which in turn was prepared by bromination of ketone 15 followed by treatment with ammonia (Scheme 4).
In the same context, we describe a new strategy for the synthesis of a new heterocycle 20 analogue of Ebselen (Scheme 5).
2-Bromo-3-nitrobenzoic acid 17 was transformed via two steps to the hydrazide 18 and the selenium was incorporated as cited previously to give the selenide 19.The treatment of 2-methylseleno-3-nitro-3 -N-phenylbenzohydrazide with excess of bromine produced a yellow-orange precipitate.It allowed unambiguous identification of the molecular formula of the compound 20 (Figure 1).The crystal data structure refinement for compound 20 has been reported previously [49].