(Z,Z)-Selanediylbis(2-propenamides): Novel Class of Organoselenium Compounds with High Glutathione Peroxidase-Like Activity. Regio- and Stereoselective Reaction of Sodium Selenide with 3-Trimethylsilyl-2-propynamides

The efficient regio- and stereoselective synthesis of (Z,Z)-3,3′-selanediylbis(2-propenamides) in 76–93% yields was developed based on the reaction of sodium selenide with 3-trimethylsilyl-2-propynamides. (Z,Z)-3,3′-Selanediylbis(2-propenamides) are a novel class of organoselenium compounds. To date, not a single representative of 3,3′-selanediylbis(2-propenamides) has been described in the literature. Studying glutathione peroxidase-like properties by a model reaction showed that the activity of the obtained products significantly varies depending on the organic moieties in the amide group. Divinyl selenide, which contains two lipophilic cyclohexyl substituents in the amide group, exhibits very high glutathione peroxidase-like activity and this compound is considerably superior to other products in this respect.

One of the most useful and atom-economic methods is based on addition reactions of selenoles or selenolate anions with acetylenes [8,14,[19][20][21][22]. Examples of these reactions refer mainly to vinyl selenides containing aliphatic or aromatic substituents at the β-carbon atom of the double bond. Examples of the synthesis of vinyl selenides bearing electron-withdrawing groups are scarce in the literature. The synthesis of (Z,Z)-bis (2-acylvinyl) selenides by the addition reaction of sodium selenide with organyl ethynyl ketones was developed [23].
There are no data in the literature about the biological activity of 3-selanylpropenamides. However, it is known that vinyl sulfides bearing the amide group in the β-position exhibit anticancer [28] and antifungal [29] activity ( Figure 1). Containing the 2-amidovinylsulfonyl group methylgerambullone (isolated from Glycosmis angustifolia) acts as the agonist of acetylcholine receptors [30]. Phenoxyquinolines bearing a 2-amidovinylsulfonyl moiety shows the properties of c-Met kinase inhibitors [31] (Figure 1). Taking into account the indicated biological properties of 3sulfanylpropenamides, it can be assumed that selenium analogs of these compounds can also display some kinds of biological activity. Moreover, the vinylamide group, itself, is an important part of some biologically active natural compounds and pharmaceuticals, which exhibit antitumor, antituberculosis, and anticonvulsant activity [32][33][34][35][36].   [28], antifungal [29], agonist of acetylcholine receptors [30], c-Met kinase inhibitor [31]).
To date, considerable effort has been devoted to the discovery of compounds that mimic the action of selenium-containing glutathione peroxidase enzymes [37][38][39][40][41][42][43][44][45][46][47]. The presence of selenium in these enzymes largely determines the glutathione peroxidase activity. In particular, organoselenium compounds bearing amide groups have been shown to be good catalysts for the reduction of peroxides and hydroperoxides with thiols ( Figure 2). Molecules 2020, 25,   Ebselen, which contains the selenenamide function in the cycle, and its analog ethaselen and propylselen show high glutathione peroxidase mimetic properties [37][38][39][40][41]. Additionally, ebselen exhibits anti-inflammatory and neuroprotective activity. These properties combined with glutathione peroxidase-like activity and relatively low toxicity of ebselen has led to therapeutic application of this compound, which has undergone evaluation in clinical trials as an anti-inflammatory agent [42]. This compound is also used for the treatment and prevention of cardiovascular diseases and ischemic stroke [41].
Camphor-derived selenenamide was synthesized by action of bromine on corresponding camphor diselenide, which was obtained based on the reaction of camphor enolate with selenium [42]. The glutathione peroxidase mimetic property of the camphor derived selenenamide was studied using a model reaction of benzenemethanethiol oxidation by tert-butyl hydroperoxide in the presence of the selenenamide as a catalyst (10% mol) in dichloromethane or deuterochloroform at room temperature. A similar model reaction of benzenemethanethiol oxidation by hydrogen peroxide was applied to examine the glutathione peroxidase-like activity of containing hydroxy group divinyl selenides as a catalysts (10% mol) [45]. The progress of the reaction was monitored by 1 H NMR spectroscopy.
To date, the reactions of sodium selenide with neither 2-propynamides nor 3-(triorganylsilyl)-2propynamides have yet been described in the literature. It is known that the introduction of electrondonating triorganylsilyl group at the triple bond changes the reactivity of acetylene derivatives and deactivates the triple bond toward nucleophilic addition [58].
In order to develop the method for preparation of previously unknown divinyl selenides containing amide groups we studied the reaction of sodium selenide with 3-(trimethylsilyl)-2propynamides and found the conditions for regio-and stereoselective addition. The obtained results are described in the present work.

Results and Discussion
Recently we realized the addition of sodium benzeneselenolate to 3-(trimethylsilyl)-2propynamides containing morpholine and phenylamide moieties (Scheme 1) [27]. The reaction was Ebselen, which contains the selenenamide function in the cycle, and its analog ethaselen and propylselen show high glutathione peroxidase mimetic properties [37][38][39][40][41]. Additionally, ebselen exhibits anti-inflammatory and neuroprotective activity. These properties combined with glutathione peroxidase-like activity and relatively low toxicity of ebselen has led to therapeutic application of this compound, which has undergone evaluation in clinical trials as an anti-inflammatory agent [42]. This compound is also used for the treatment and prevention of cardiovascular diseases and ischemic stroke [41].
Camphor-derived selenenamide was synthesized by action of bromine on corresponding camphor diselenide, which was obtained based on the reaction of camphor enolate with selenium [42]. The glutathione peroxidase mimetic property of the camphor derived selenenamide was studied using a model reaction of benzenemethanethiol oxidation by tert-butyl hydroperoxide in the presence of the selenenamide as a catalyst (10% mol) in dichloromethane or deuterochloroform at room temperature. A similar model reaction of benzenemethanethiol oxidation by hydrogen peroxide was applied to examine the glutathione peroxidase-like activity of containing hydroxy group divinyl selenides as a catalysts (10% mol) [45]. The progress of the reaction was monitored by 1 H NMR spectroscopy.
To date, the reactions of sodium selenide with neither 2-propynamides nor 3-(triorganylsilyl)-2-propynamides have yet been described in the literature. It is known that the introduction of electron-donating triorganylsilyl group at the triple bond changes the reactivity of acetylene derivatives and deactivates the triple bond toward nucleophilic addition [58].
In order to develop the method for preparation of previously unknown divinyl selenides containing amide groups we studied the reaction of sodium selenide with 3-(trimethylsilyl)-2-propynamides and found the conditions for regio-and stereoselective addition. The obtained results are described in the present work.

Results and Discussion
Recently we realized the addition of sodium benzeneselenolate to 3-(trimethylsilyl)-2propynamides containing morpholine and phenylamide moieties (Scheme 1) [27]. The reaction was carried out by addition of sodium borohydride to a stirred solution of 3-trimethylsilyl-2-propynamides and diphenyl diselenide in a THF-water (4/1) system at room temperature and accompanied by desilylation. The generation of sodium benzeneselenolate occurred in situ followed by nucleophilic addition of this highly reactive intermediate to the triple bond. carried out by addition of sodium borohydride to a stirred solution of 3-trimethylsilyl-2propynamides and diphenyl diselenide in a THF-water (4/1) system at room temperature and accompanied by desilylation. The generation of sodium benzeneselenolate occurred in situ followed by nucleophilic addition of this highly reactive intermediate to the triple bond. The reaction proceeded in stereo-and regioselective manners affording (Z)-N-phenyl-3-(phenylselanyl)prop-2-enamide (72% yield) and (Z)-1-morpholino-3-(phenylselanyl)prop-2-en-1-one (70% yield), which were isolated as colorless crystalline compounds [27]. To the best of our knowledge, these are first examples of the addition of organylselenolates to 3-silyl-2-propynamides.
The commonly used conditions for generation of organylselenolates from corresponding diselenides and sodium selenide from elemental selenium consist in the application of sodium borohydride as a reducing agent and carrying out the reaction in alcohols [59]. However, when the reactions of sodium benzeneselenolate or sodium selenide with 3-(trimethylsilyl)-2-propynamides proceeded in methanol or ethanol, the formation of 3-alkoxy-2-propenamides as by-products was observed. The possibility of the formation of 3-alkoxy-2-propenamides from 3-(trimethylsilyl)-2propynamides in reactions with alcohols has been previously described [60].
We found that the THF-water system is preferable in addition reactions of selenium-centered nucleophiles with 3-(trimethylsilyl)-2-propynamides compared to commonly used alcohol conditions. The yields of the target products are higher and 3-alkoxy-2-propenamides are not formed as by-products.
The addition reactions of selenide anion with propynamides and 3-silyl-2-propynamides have not yet been described in the literature. In order to obtain previously unknown divinyl selenides containing amide groups we studied the addition of sodium selenide to 3-(trimethylsilyl)-2propynamides bearing various groups (phenyl, alkyl, cyclohexyl, morpholine and piperidine) in the amide moieties (Scheme 2). Sodium selenide was efficiently generated from elemental selenium and sodium borohydride in water and used without isolation in further nucleophilic addition reactions.
The commonly used conditions for generation of organylselenolates from corresponding diselenides and sodium selenide from elemental selenium consist in the application of sodium borohydride as a reducing agent and carrying out the reaction in alcohols [59]. However, when the reactions of sodium benzeneselenolate or sodium selenide with 3-(trimethylsilyl)-2-propynamides proceeded in methanol or ethanol, the formation of 3-alkoxy-2-propenamides as by-products was observed. The possibility of the formation of 3-alkoxy-2-propenamides from 3-(trimethylsilyl)-2propynamides in reactions with alcohols has been previously described [60].
We found that the THF-water system is preferable in addition reactions of selenium-centered nucleophiles with 3-(trimethylsilyl)-2-propynamides compared to commonly used alcohol conditions. The yields of the target products are higher and 3-alkoxy-2-propenamides are not formed as by-products.
The addition reactions of selenide anion with propynamides and 3-silyl-2-propynamides have not yet been described in the literature. In order to obtain previously unknown divinyl selenides containing amide groups we studied the addition of sodium selenide to 3-(trimethylsilyl)-2-propynamides bearing various groups (phenyl, alkyl, cyclohexyl, morpholine and piperidine) in the amide moieties (Scheme 2). Sodium selenide was efficiently generated from elemental selenium and sodium borohydride in water and used without isolation in further nucleophilic addition reactions.
Water is necessary for generating sodium selenide and reacting Na2Se with propynamides 1a-i which are soluble in THF. The ratio of the solvents in the THF-water system was varied from 1/3 (method A) to 3/1(method B). In the method A and B, a solution of silylpropynamides in THF was added to a hot aqueous solution of sodium selenide, which was obtained from elemental selenium and sodium borohydride, and the mixture was refluxed for 10 min. In the case of method C, sodium borohydride was added portionwise to a mixture of propynamides 1b-d,f-i and selenium in the THF-water system (the ratio of the solvents 4/1) and the mixture was stirred at room temperature for 4 h (10 h for 2g).
Best yields of the products were obtained when the reaction mixtures were refluxed for 10 min in the THF-water system. When reaction was carried out at room temperature, the yields dropped Water is necessary for generating sodium selenide and reacting Na 2 Se with propynamides 1a-i which are soluble in THF. The ratio of the solvents in the THF-water system was varied from 1/3 (method A) to 3/1(method B). In the method A and B, a solution of silylpropynamides in THF was added to a hot aqueous solution of sodium selenide, which was obtained from elemental selenium and sodium borohydride, and the mixture was refluxed for 10 min. In the case of method C, sodium borohydride was added portionwise to a mixture of propynamides 1b-d,f-i and selenium in the THF-water system (the ratio of the solvents 4/1) and the mixture was stirred at room temperature for 4 h (10 h for 2g).
Best yields of the products were obtained when the reaction mixtures were refluxed for 10 min in the THF-water system. When reaction was carried out at room temperature, the yields dropped despite increasing the reaction duration. The divinyl selenides 2b-d,f-i were obtained in 50-73% yields with 4 h stirring at room temperature under argon (Scheme 2, method C). Surprisingly, neither unconverted 3-(trimethylsilyl)-2-propynamides 1b-d,f-i nor desilylated propynamides were detected in the reaction mixture in these cases after completion of the reaction (method C). However, the yields of the target products 2b-d,f-i were lower than in the methods A and B.
The reaction of sodium selenide with propynamide 1f bearing two phenyl substituents in the amide group under the conditions of method A led to a mixture containing divinyl selenide 2f in 25% yield, unconverted silylpropynamide 1f (31% conversion) and the desilylated amide, N,N-diphenyl-2-propynamide (1%). The reaction of sodium selenide with silylpropynamide 1g containing two cyclohexyl moieties in the amide group also gave similar poor results. We supposed that the reason of the insufficient yield of selenides 2f,g and low conversion of starting amides 1f,g may be poor solubility of propyneamides 1f,g in the mixture water-THF (3/1, method A) due to lipophilic organic moieties of the amide group. The silylpropynamides 1f,g are insoluble in water but soluble in THF. Indeed, when the method B (THF-water 3/1) was applied, products 2f,g were obtained in 76-77% yields. The method A was found to provide high yields of products 2c-e,h,i (85-93%) derived from silylpropynamides 1c-e,h,i containing monophenyl, dialkyl, morpholine, and piperidine moieties in the amide group.
The possible pathway for the formation of products 2a-i can include both the addition-desilylation processes and the sequential desilylation-addition reactions via the generation of intermediate propynamides 3a-i (Scheme 3). The addition of sodium selenide to the triple bond of silylpropynamides 1a-i is accompanied by the formation of sodium hydroxide, which acts as the catalyst for the desilylation reaction. We suppose that the desilylation process can proceed on different stages of the reaction including various intermediate species (Scheme 3).
despite increasing the reaction duration. The divinyl selenides 2b-d,f-i were obtained in 50-73% yields with 4 h stirring at room temperature under argon (Scheme 2, method C). Surprisingly, neither unconverted 3-(trimethylsilyl)-2-propynamides 1b-d,f-i nor desilylated propynamides were detected in the reaction mixture in these cases after completion of the reaction (method C). However, the yields of the target products 2b-d,f-i were lower than in the methods A and B.
The reaction of sodium selenide with propynamide 1f bearing two phenyl substituents in the amide group under the conditions of method A led to a mixture containing divinyl selenide 2f in 25% yield, unconverted silylpropynamide 1f (31% conversion) and the desilylated amide, N,N-diphenyl-2-propynamide (1%). The reaction of sodium selenide with silylpropynamide 1g containing two cyclohexyl moieties in the amide group also gave similar poor results. We supposed that the reason of the insufficient yield of selenides 2f,g and low conversion of starting amides 1f,g may be poor solubility of propyneamides 1f,g in the mixture water-THF (3/1, method A) due to lipophilic organic moieties of the amide group. The silylpropynamides 1f,g are insoluble in water but soluble in THF. Indeed, when the method B (THF-water 3/1) was applied, products 2f,g were obtained in 76-77% yields. The method A was found to provide high yields of products 2c-e,h,i (85-93%) derived from silylpropynamides 1c-e,h,i containing monophenyl, dialkyl, morpholine, and piperidine moieties in the amide group.
The possible pathway for the formation of products 2a-i can include both the additiondesilylation processes and the sequential desilylation-addition reactions via the generation of intermediate propynamides 3a-i (Scheme 3). The addition of sodium selenide to the triple bond of silylpropynamides 1a-i is accompanied by the formation of sodium hydroxide, which acts as the catalyst for the desilylation reaction. We suppose that the desilylation process can proceed on different stages of the reaction including various intermediate species (Scheme 3).
The formation of intermediate 2-propynamides 3a-i in very small amounts (before the isolation of the reaction products 2a-i) was registered in the reaction mixture by NMR. The NMR data of the intermediate propynamides 3a-i coincide with the spectral characteristics of the previously obtained samples of these compounds, which were synthesized by desilylation of silylpropynamides 1a-i [60].
The formation of propynamides 3a-i in the reaction (Scheme 2) indicates the possibility of the reaction pathway via desilylation of silylpropynamides 1a-i. It was previously established that silylpropynamides 1a-i can be desilylated by the action of various reagents (potassium fluoride, alkali metal hydroxides and other bases) and converted to corresponding propynamides 3a-i [60].  It is worth noting that the application of 3-trimethylsilyl-2-propynamides 1a-i as the initial substrates in the preparation of the target vinyl selenides is preferable compared to 2-propynamides with the terminal triple bond. The latter compounds are hardly available and the price for these chemicals is very high. Their preparation is usually based on toxic and skin-irritating propynoic acid. The silylpropynamides 1a-i were synthesized in the present work by the method depicted in Scheme 4 [61][62][63]. Inexpensive starting propargyl alcohol, good selectivity of these reactions and high yield of the target products allowed to make silylpropynamides 1a-i readily available compounds and to use them in the synthesis of valuable products [64][65][66]. The formation of intermediate 2-propynamides 3a-i in very small amounts (before the isolation of the reaction products 2a-i) was registered in the reaction mixture by NMR. The NMR data of the intermediate propynamides 3a-i coincide with the spectral characteristics of the previously obtained samples of these compounds, which were synthesized by desilylation of silylpropynamides 1a-i [60].
The formation of propynamides 3a-i in the reaction (Scheme 2) indicates the possibility of the reaction pathway via desilylation of silylpropynamides 1a-i. It was previously established that silylpropynamides 1a-i can be desilylated by the action of various reagents (potassium fluoride, alkali metal hydroxides and other bases) and converted to corresponding propynamides 3a-i [60].
It is worth noting that the application of 3-trimethylsilyl-2-propynamides 1a-i as the initial substrates in the preparation of the target vinyl selenides is preferable compared to 2-propynamides with the terminal triple bond. The latter compounds are hardly available and the price for these chemicals is very high. Their preparation is usually based on toxic and skin-irritating propynoic acid. The silylpropynamides 1a-i were synthesized in the present work by the method depicted in Scheme 4 [61][62][63]. Inexpensive starting propargyl alcohol, good selectivity of these reactions and high yield of the target products allowed to make silylpropynamides 1a-i readily available compounds and to use them in the synthesis of valuable products [64][65][66].
Molecules 2020, 25  The obtained selanediylbis(2-propynamides) 2a-i are a novel class of organoselenium compounds. Like ebselen and some organoselenium compounds, which exhibit glutathione peroxidase-like activity (Figure 2), products 2a-i contain the amide function, and their activity deserved to be studied.
We studied glutathione peroxidase-like activity of the obtained products 2a-i using the model reaction of benzenemethanethiol oxidation [42,45] by tert-butyl hydroperoxide (TBHP) in the presence of compounds 2a-i as catalysts and the progress of this reaction was monitored by 1 H NMR spectroscopy. First experiments in the NMR tubes (TBHP, BnSH, 0.1 mmol, deuterochloroform) at room temperature showed that the reactions proceeded too fast when 10% mol of the catalysts were used. In order to realize the 1 H NMR monitoring, the amounts of the catalysts were decreased to 0.5% mol. Diphenyl diselenide was used as the standard compound (this compound is often used as the standard catalyst for in these experiments [42][43][44][45][46][47]).
It was found that the activity of the obtained products 2a-i varies significantly depending on the organic moieties in the amide group. The results of studying the compounds 2d,f,g,h,i, which outperform diphenyl diselenide in the glutathione peroxidase-like activity, are presented in Figure 3 (a 24 h scale) and Figure 4 (a 90 min scale). In the control experiment, under the same reaction conditions but in the absence of the catalyst, the conversion of phenylmethanethiol was only about 4% after 24 h according to 1 H NMR data.
Product 2g containing two lipophilic cyclohexyl substituents in the amide group shows highest glutathione peroxidase-like properties (Figures 4 and 5). This compound is significantly superior to other products in activity. The second most active product is compound 2i (Figure 4) bearing the piperidine moieties in the amide function and the third is product 2d (the activity of which is presented in both Figures 3 and 4). Compounds 2f,h containing the morpholine and phenyl moieties also exhibit higher activity compared to diphenyl diselenide.
The obtained results are very promising. However, the interpretation of the influence of organic moieties on the catalytic activity and discussion on possible intermediates of the catalytic cycle requires additional data and further research. The obtained selanediylbis(2-propynamides) 2a-i are a novel class of organoselenium compounds. Like ebselen and some organoselenium compounds, which exhibit glutathione peroxidase-like activity (Figure 2), products 2a-i contain the amide function, and their activity deserved to be studied.
We studied glutathione peroxidase-like activity of the obtained products 2a-i using the model reaction of benzenemethanethiol oxidation [42,45] by tert-butyl hydroperoxide (TBHP) in the presence of compounds 2a-i as catalysts and the progress of this reaction was monitored by 1 H NMR spectroscopy. First experiments in the NMR tubes (TBHP, BnSH, 0.1 mmol, deuterochloroform) at room temperature showed that the reactions proceeded too fast when 10% mol of the catalysts were used. In order to realize the 1 H NMR monitoring, the amounts of the catalysts were decreased to 0.5% mol. Diphenyl diselenide was used as the standard compound (this compound is often used as the standard catalyst for in these experiments [42][43][44][45][46][47]).
It was found that the activity of the obtained products 2a-i varies significantly depending on the organic moieties in the amide group. The results of studying the compounds 2d,f,g,h,i, which outperform diphenyl diselenide in the glutathione peroxidase-like activity, are presented in Figure 3 (a 24 h scale) and Figure 4 (a 90 min scale). In the control experiment, under the same reaction conditions but in the absence of the catalyst, the conversion of phenylmethanethiol was only about 4% after 24 h according to 1 H NMR data.
Product 2g containing two lipophilic cyclohexyl substituents in the amide group shows highest glutathione peroxidase-like properties (Figures 4 and 5). This compound is significantly superior to other products in activity. The second most active product is compound 2i (Figure 4) bearing the piperidine moieties in the amide function and the third is product 2d (the activity of which is presented in both Figures 3 and 4). Compounds 2f,h containing the morpholine and phenyl moieties also exhibit higher activity compared to diphenyl diselenide.
The obtained results are very promising. However, the interpretation of the influence of organic moieties on the catalytic activity and discussion on possible intermediates of the catalytic cycle requires additional data and further research.  Figure 5. Compounds 2d,f,g,h,i exhibiting higher glutathione peroxidase-like activity compared to Ph 2 Se 2 (the compounds are arranged in the decreasing order of the activity).

Method A (Preparation of Compounds 2c-f,h,i)
A mixture of elemental selenium (19 mg, 0.24 mmol) and degassed water (4.0 mL) was heated on a water bath (90-95 • C) and a solution of NaBH 4 (40 mg, 1.05 mmol) in degassed water (0.5 mL) was added under argon. After dissolution of selenium and the formation of colorless mixture, a solution of 3-trimethylsilyl-2-propynamide (0.48 mmol) in THF (1.5 mL) was added to a hot aqueous solution of the sodium selenide and the mixture was refluxed for 10 min (5 h for 1g) under argon. The mixture was cooled by cold water bath and extracted with CH 2 Cl 2 (3 × 7.0 mL). The organic phase was dried over Na 2 SO 4 and the solvent was removed under reduced pressure. General yields: 85-93%.

Method B (Preparation of Compounds 2a,b,f,g)
A mixture of elemental selenium (19 mg, 0.24 mmol) and degassed water (2.0 mL) was heated on a water bath (90-95 • C) and a solution of NaBH 4 (40 mg, 1.05 mmol) in degassed water (0.4 mL) was added under argon. After dissolution of selenium and the formation of colorless mixture, a solution of 3-trimethylsilyl-2-propynamide (0.48 mmol) in THF (7.0 mL) was added to a hot aqueous solution of the sodium selenide and the mixture was refluxed for 10 min (5 h for 1g) under argon. The mixture was cooled by cold water bath and THF was removed by a rotary evaporator. The residue was extracted with CH 2 Cl 2 (3 × 7.0 mL). The organic phase was dried over Na 2 SO 4 and the solvent was removed under reduced pressure. General yields: 76-91%.

Conclusions
The efficient regio-and stereoselective synthesis of a novel class of organoselenium compounds, (Z,Z)-3,3 -selanediylbis(2-propenamides), based on the reaction of sodium selenide with 3-trimethylsilyl-2-propynamides was developed. Not a single representative of 3,3 -selanediylbis(2-propenamides) has yet been described in the literature. Studying their glutathione peroxidase-like properties by a model reaction showed that compounds 2g,i,d exhibit high activity. It was found that the glutathione peroxidase-like activity of the obtained products varies significantly depending on the organic moieties in the amide group. Containing two lipophilic cyclohexyl substituents in the amide group compound 2g is significantly superior to other products in activity. The second most active product is compound 2i bearing the piperidine moieties in the amide function. Containing the morpholine and diphenyl moieties compounds 2f,h also exhibit higher catalytic activity compared to diphenyl diselenide.