Chain Extension of Piperazine in Ethanol: Synthesis of 2-(4-(2-(Phenylthio)ethyl)piperazinyl)acetonitriles and ACAT-1 Inhibitors

A base-induced synthesis of 2-(4-(2-(phenylthio)ethyl)piperazinyl) acetonitriles by reaction of disulfides, 1-(chloromethyl)-4-aza-1-azonia bicyclo[2.2.2]octane chloride and trimethylsilyl cyanide is reported. The scope of the method is demonstrated with 30 examples. The reaction mechanism research indicates that the three-component reaction would be a SN2 reaction. The products exhibit good activities towards advanced synthesis of aqueous soluble acyl-CoA: cholesterol O-acyltransferase-1 (ACAT-1) inhibitors. Our work is superior as it uses less-odor disulfides as carbon sources and EtOH as solvent in a water and dioxygen insensitive reaction system, followed by a simple purification process.

Sulfur-containing ethyl piperazine compounds were originally obtained by introducing thiol groups on the side chain of the 4-substituted 1-(2-chloroethyl)piperazine or 1-(2-hydroxyethyl) piperazine via nucleophilic substitution reactions [8,[19][20][21][22] (Scheme 1).A ring-opening method was developed using cyclic tertiary amines as the ethyl piperazine source enabled by the incorporation of thiolates through facile C-N bond cleavage.Reaction of aromatic halogenated compounds with triethylenediamine (DABCO) in the presence of Na 2 S as the sulfur source at 120 • C afforded the products 1-(2-(pyridin-2-yl)ethyl)-4-(pyridin-2-yl)piperazines in considerable yields [23] (Scheme 1b).Similar products were obtained by reaction of either o-silyl aryl triflates or pyridine-N-oxides with thiolates in the presence of CsF or trifluoroacetic anhydride as the activating agent, respectively [24,25].The method was extended using 1-alkyl group bound 4-aza-1-azoniabicyclo[2.2.2]octane as the ethyl piperazine source under alkaline conditions [26][27][28][29].Although significant advances have been made, the synthesis of sulfur-containing ethyl piperazine compounds is of interest to scientists.Up to now, almost all the works concerning the building of the sulfur-containing ethyl piperazine skeleton have said that the C-N bond cleavage of the cyclic tertiary amines was achieved in two steps: quaternization first, ring-opening second.Synthesis of the sulfur-containing ethyl piperazine compound by a SN2 reaction was rarely reported.

Results and Discussion
Reaction of diphenyl disulfide (1a, 0.1 mmol), CAABC (0.2 mmol), TMSCN (0.22 mmol) and Cs2CO3 (0.6 mmol) under air atmosphere in EtOH (1 mL) for 3 h at 100 °C provided the product 2-(4-(2-(phenylthio)ethyl)piperazinyl)acetonitrile (2a) in 90% yield (based on diphenyl disulfide, Table 1, entry 1).Other alkali salts, such as K2CO3, Na2CO3, KOH, and t BuOK, afforded the product in lower yields (entries 2-5).The addition of a trace of water had little effect on production, but a greater amount of water (7:3) would lead to the generation of 2a in a lower yield (entry 6).Only a trace of product could be found when the reaction was performed in a clear aqueous solution (entry 7).The use of MeOH as the solvent provided 2a in 75% yield (entry 8).With other polar solvents, such as DMF and DMSO, no corresponding product could be obtained (entries 9 and 10).Higher temperature would not help to improve the yield, while a lower temperature would decrease the reaction (entries 11 and 12).A period of 3 h would be enough for the Scheme 2. Different strategies (route 1 [8]; route 2 [20,[30][31][32]) to synthesize the ACAT-1 inhibitor [K-604].

Results and Discussion
Reaction of diphenyl disulfide (1a, 0.1 mmol), CAABC (0.2 mmol), TMSCN (0.22 mmol) and Cs 2 CO 3 (0.6 mmol) under air atmosphere in EtOH (1 mL) for 3 h at 100 • C provided the product 2-(4-(2-(phenylthio)ethyl)piperazinyl)acetonitrile (2a) in 90% yield (based on diphenyl disulfide, Table 1, entry 1).Other alkali salts, such as K 2 CO 3 , Na 2 CO 3 , KOH, and t BuOK, afforded the product in lower yields (entries 2-5).The addition of a trace of water had little effect on production, but a greater amount of water (7:3) would lead to the generation of 2a in a lower yield (entry 6).Only a trace of product could be found when the reaction was performed in a clear aqueous solution (entry 7).The use of MeOH as the solvent provided 2a in 75% yield (entry 8).With other polar solvents, such as DMF and DMSO, no corresponding product could be obtained (entries 9 and 10).Higher temperature would not help to improve the yield, while a lower temperature would decrease the reaction (entries 11 and 12).A period of 3 h would be enough for the completeness of the reaction.A longer or shorter time is of no advantage to the reaction (entries 13 and 14).At the optimized conditions, the scope of the substrates was investigated for the production of 2 (Figure 2).Thirty compounds were prepared in terms of the electronic effect and steric effect of the functional groups on the substrates.It was found that both the electron-donating groups and the electron-withdrawing groups on the benzene ring of diphenyl sulfides would lead to the reduction of yields.By contrast, the electron-donating groups (2b-2d) might have a larger effect than the electron-withdrawing groups (2e-2g), except for the strong electron-withdrawing groups CF3 by which the yield decreased dramatically, down to 61%.This inference was consistent with the experimental results obtained from the comparison of compounds 2i and 2j with 2k-2m, and 2n and 2o with 2p-2r.The influence of steric hindrance on the reaction was studied by the employment of methoxyl-(2b, 2i and 2n), methyl-(2d, 2j and 2o), Br-(2e, 2k and 2p), Cl-(2f, 2l and 2q), and F-(2g, 2m and 2r) groups at the para, meta, and ortho positions of the benzene rings.The results showed that the steric effect had little impact on the production of the target products.It was in accordance with the reaction of the disubstituted diphenyl disulfide under the same conditions (2s-2u, 74-78%).Our reaction exhibited good compatibility with other cyclic thiol sources, such as 2-naphthalenethiol (2v), 2-mercaptopyridine (2w), thiophenethiol (2x), 2-methyl-3-furanthiol (2y), 2-benzothiazolethiol (2z), 2-benzoxazolethiol (2aa), and 2-mercaptobenzimidazole (2ab).The corresponding products were obtained in yields of 57-84%.In addition, the reaction of n-hexyl disulfide (2ac) or diphenyl diselenide (2ad) under the standard conditions also produced the desired products in yields of 51% and 80%, respectively.These results implicate a relatively broad range of substrates in our reaction.In addition, the synthetic utility of reaction was checked by performing the experiments on the gram scale.The a Reaction conditions: TMSCN (0.22 mmol), base (0.6 mmol), solvent (1 mL), air atmosphere.b Yields were determined by 1 H NMR analysis of the crude product using 1,3,5-trimethoxybenzene as the internal standard; Temp = temperature; En = entry.
At the optimized conditions, the scope of the substrates was investigated for the production of 2 (Figure 2).Thirty compounds were prepared in terms of the electronic effect and steric effect of the functional groups on the substrates.It was found that both the electron-donating groups and the electron-withdrawing groups on the benzene ring of diphenyl sulfides would lead to the reduction of yields.By contrast, the electron-donating groups (2b-2d) might have a larger effect than the electron-withdrawing groups (2e-2g), except for the strong electron-withdrawing groups CF 3 by which the yield decreased dramatically, down to 61%.This inference was consistent with the experimental results obtained from the comparison of compounds 2i and 2j with 2k-2m, and 2n and 2o with 2p-2r.The influence of steric hindrance on the reaction was studied by the employment of methoxyl-(2b, 2i and 2n), methyl-(2d, 2j and 2o), Br-(2e, 2k and 2p), Cl-(2f, 2l and 2q), and F-(2g, 2m and 2r) groups at the para, meta, and ortho positions of the benzene rings.The results showed that the steric effect had little impact on the production of the target products.It was in accordance with the reaction of the disubstituted diphenyl disulfide under the same conditions (2s-2u, 74-78%).Our reaction exhibited good compatibility with other cyclic thiol sources, such as 2-naphthalenethiol (2v), 2-mercaptopyridine (2w), thiophenethiol (2x), 2-methyl-3-furanthiol (2y), 2-benzothiazolethiol (2z), 2-benzoxazolethiol (2aa), and 2-mercaptobenzimidazole (2ab).The corresponding products were obtained in yields of 57-84%.In addition, the reaction of n-hexyl disulfide (2ac) or diphenyl diselenide (2ad) under the standard conditions also produced the desired products in yields of 51% and 80%, respectively.These results implicate a relatively broad range of substrates in our reaction.In addition, the synthetic utility of reaction was checked by performing the experiments on the gram scale.The reaction of diphenyl disulfide (1a) and CAABC on a 1.5 g scale produced compound 2a in 90% yield (Supporting Information, Section S2.4), which implies that the amount of the starting material did not directly influence the quality of reaction, and our reaction was suitable for the production of sulfur-containing ethyl piperazine compounds for further synthesis.
reaction of diphenyl disulfide (1a) and CAABC on a 1.5 g scale produced compoun in 90% yield (Supporting Information, Section S2.4), which implies that the amoun the starting material did not directly influence the quality of reaction, and our reac was suitable for the production of sulfur-containing ethyl piperazine compounds further synthesis.With the progress of research on compounds 2, a number of experiments w conducted to study the reaction mechanism.The reaction of diphenyl disulfide (1a) CAABC, TMSCN and Cs2CO3 under N2 atmosphere in EtOH afforded the product 2 91% yield (Scheme 3a).The reaction of thiophenol with CAABC and TMSCN unde standard conditions also provided the product 2a in similar yield (Scheme 3b).It m that the reaction was independent of dioxygen and the thiophenol could be an inte diate of reaction.Compound 3a was obtained when the reaction was repeated in the sence of TMSCN.The result was checked using TMSCF3 and Et4NCN as the nu philes.The reaction of 1a, CAABC and TMSCF3 under the standard conditions affo 3a in a similar yield.With the replacement of TMSCF3 by Et4NCN, compound 2a obtained in 60% yield.It indicated that cyanide anion was the right nucleophile fo substitution of the chloride group, and the chloromethyl group might work as a lea With the progress of research on compounds 2, a number of experiments were conducted to study the reaction mechanism.The reaction of diphenyl disulfide (1a) with CAABC, TMSCN and Cs 2 CO 3 under N 2 atmosphere in EtOH afforded the product 2a in 91% yield (Scheme 3a).The reaction of thiophenol with CAABC and TMSCN under the standard conditions also provided the product 2a in similar yield (Scheme 3b).It meant that the reaction was independent of dioxygen and the thiophenol could be an intermediate of reaction.Compound 3a was obtained when the reaction was repeated in the absence of TMSCN.The result was checked using TMSCF 3 and Et 4 NCN as the nucleophiles.The reaction of 1a, CAABC and TMSCF 3 under the standard conditions afforded 3a in a similar yield.With the replacement of TMSCF 3 by Et 4 NCN, compound 2a was obtained in 60% yield.It indicated that cyanide anion was the right nucleophile for the substitution of the chloride group, and the chloromethyl group might work as a leaving group without the presence of CN − .Meanwhile, the reaction of 1a with 1-(cyanomethyl)-4-aza-1-azonia bicyclo[2.2.2]octane chloride (CYAABC) produced a piperazine amide compound 4a rather than the desired compound 2a.They seemed to be SN2 reactions with the attack of sulfide (obtained by the reduction of diphenyl disulfide) on the ethylene group of the DABCO ring on one side, and the attack of the hydroxyl ion (generated by the alkalization of H 2 O in EtOH) on the cyanomethyl groups of the other side.The speculation was supported by the reaction of 1a and TMSCN with triethylenediamine (DABCO) or 1-ethyl-4-aza-1-azonia bicyclo[2.2.2]octane bromide (EAABB), from which no desired product could be observed (Scheme 3e,f).At last, there was no reaction between CAABC and TMSCN under the standard conditions.
Molecules 2024, 29, x FOR PEER REVIEW 6 group without the presence of CN − .Meanwhile, the reaction of 1a 1-(cyanomethyl)-4-aza-1-azonia bicyclo[2.2.2]octane chloride (CYAABC) produced perazine amide compound 4a rather than the desired compound 2a.They seemed t SN2 reactions with the attack of sulfide (obtained by the reduction of diphenyl disul on the ethylene group of the DABCO ring on one side, and the attack of the hydroxy (generated by the alkalization of H2O in EtOH) on the cyanomethyl groups of the o side.The speculation was supported by the reaction of 1a and TMSCN with triethy diamine (DABCO) or 1-ethyl-4-aza-1-azonia bicyclo[2.2.2]octane bromide (EAA from which no desired product could be observed (Scheme 3e,f).At last, there wa reaction between CAABC and TMSCN under the standard conditions.
Based on the above results, a possible reaction mechanism was proposed in Scheme 4. Diphenyl disulfide is reduced to thiophenolate in the presence of Cs 2 CO 3 in EtOH when heated.The PhS − anion attacks CAABC on the ethylene with the attack of CN − on the chloromethyl group to yield the desired compound 2a.
Our reaction exhibited good tolerance towards the transformation of aromatic thiols and/or disulfides to ACAT-1 inhibitors.Two routes were discovered in this work, one by acylation of compounds 2, the other by cutting the chloromethyl group off from the piperazine (Scheme 5).The stirring of compound 2ab in the presence of KOH under air atmosphere in t BuOH for 3 h at 110 • C afforded compound 4b in 75% yield.Reaction of 4b with 2,6-diisopropylaniline and 6-methyl-2,4-bis(methylthio)pyridin-3-amine provided the desired products 5a and 5b in yields of 10 and 12%, respectively.The poor solubility of 4b in MeCN restricted its application.Other polar solvents, such as MeOH, H 2 O and DMF, would cause the increase in by-products.Although the method had its disadvantage, it offered a chance to improve the rate of production.The reaction of 2mercaptobenzimidazole (1ab) with CAABC in the absence of TMSCN under the standard conditions produced compound 3b in 80% yield, which was further reacted with 2-bromo-N-(2,6-diisopropylphenyl)acetamide and 2-bromo-N-(6-methyl-2,4-bis(methylthio)pyridin-3-yl)acetamide to yield compounds 5a and 5b in yields of 60% and 75%.In comparison to what has been described in the literatures, our method is superior in offering fewer reaction steps with a similar yield in EtOH (first step).The structures of compounds were determined by X-ray crystallography and are shown in Figure 3

Conclusions
In summary, we have described a simple and eco-friendly method for the synthesis of 2-(4-(2-(phenylthio)ethyl)piperazinyl)acetonitriles (2) in one-pot.Further reactions produced aqueous soluble acyl-CoA:cholesterol O-acyltransferase-1 (ACAT-1) inhibitors such as [K-604].The advantage of this method lies in green solvent, water and dioxygen insensitivity, less-odor disulfide source, and easy purification.Gram level of reaction with purity over 90% makes this method more practical.The methodology would prove very useful in the area of medicinal chemistry.

Molecules 2024 , 1 (
29,  x FOR PEER REVIEW 2 of DABCO) in the presence of Na2S as the sulfur source at 120 °C afforded the products 1-(2-(pyridin-2-yl)ethyl)-4-(pyridin-2-yl)piperazines in considerable yields[23] (Scheme 1b).Similar products were obtained by reaction of either o-silyl aryl triflates or pyri dine-N-oxides with thiolates in the presence of CsF or trifluoroacetic anhydride as the activating agent, respectively[24,25].The method was extended using 1-alkyl group bound 4-aza-1-azoniabicyclo[2.2.2]octane as the ethyl piperazine source under alkaline conditions[26][27][28][29].Although significant advances have been made, the synthesis of sul fur-containing ethyl piperazine compounds is of interest to scientists.Up to now, almos all the works concerning the building of the sulfur-containing ethyl piperazine skeleton have said that the C-N bond cleavage of the cyclic tertiary amines was achieved in two steps: quaternization first, ring-opening second.Synthesis of the sulfur-containing ethy piperazine compound by a SN2 reaction was rarely reported.

Table 1 .
Optimization of the formation of compound 2a a .

Table 1 .
Optimization of the formation of compound 2a a .