Microwave-Assisted Expeditious Synthesis of 2-Alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines Catalyzed by HgCl2 under Solvent-Free Conditions as Potential Anti-HIV-1 Agents

A series of 2-alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines were expeditious prepared under microwave-assisted, catalyzed by HgCl2 and solvent-free conditions. This method has the advantage of low catalyst loading and recovering catalyst, ease reaction and repaid reaction times, easy separation products and excellent yields, and more conducive to the large-scale synthesis products. Furthermore, compounds 3s, 3y, 3a′, 3b′, 3f′, 3i′, 3q′, and 3r′ exhibited more potent anti-HIV-1 activity with EC50 values of 3.35, 6.12, 3.63, 9.54, 1.79, 0.51, 3.00, and 4.01 μg/mL, and TI values of 32.66, >32.68, 31.22, 13.94, 24.27, 39.59, 26.01, and 24.51, respectively. Especially compound 3i′ displayed the highest anti-HIV-1 activity with TI values of 39.59.


Introduction
Acquired immunodeficiency syndrome (AIDS) is mainly caused by human immunodeficiency virus type 1 (HIV-1) infection and has remained one of the most difficult medical barriers for human health since it was first reported in 1981 [1]. The reverse transcriptase (RT) of the HIV-1 plays a significant role in the viral replication process, which makes it a pivotal target for anti-HIV-1 inhibitor discovery [2,3]. Although numerous RT inhibitors, including primarily the nucleoside/nucleotide RT inhibitors (NRTIs) and non-nucleoside RT inhibitors (NNRTIs), have been developed, like other anti-HIV inhibitors, effectiveness of now approved NRTIs and NNRTIs have been hampered because of the fast development of resistance [4][5][6][7]. It is estimated that 36.9 million people (including 2.6 million children) were living with HIV infection in the year 2014 according to UNAIDS-2015 report, and 1.2 million people died due to HIV as well as related diseases [8]. To circumvent this challenge, it is urgent to discover and develop safe, green, efficient, selective, and novel anti-HIV inhibitors having significant potency against drug-resistant RT viral strains as well as less toxicity [8][9][10][11].
Generally, there are three ways to synthesize 1,3,4-oxadiazolines. The first kind is the traditional procedure, which is usually require excess of anhydride, and long reaction time [16][17][18]. The second is the ultrasonic irradiation assisted synthesis, it is with relatively mild reaction conditions and higher yield [19]. The third is the microwave irradiation assisted synthesis, which are under solvent-free conditions with short reaction time [15]. Although many advantages, there need to be further improved on the methods of the ultrasonic irradiation and microwave irradiation assisted synthesis of 1,3,4-oxadiazolines. On the other hand, since the first report of microwave irradiation assisted synthesis in 1986 [20,21], the technique has been accepted as a method for reducing reaction times and increasing yields of product compared to conventional procedure [22][23][24][25][26]. Especially, the use of microwave ovens as tools for synthetic chemistry is a fast growth area [27][28][29][30][31][32], so here we aimed to use this method to synthetize 2-alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines catalyzed by HgCl 2 under solvent-free conditions.

Chemistry
To find out the most compatible reaction conditions for synthesizing 2-alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines, the reaction of N-benzenesulfonyl-3-acetylindole benzoyl hydrazone (1a) with acetic anhydride (2a) under microwave irradiation and solvent-free conditions was investigated, and a wide range of reaction catalysts were also tested. As can be clearly seen in Table 1, the cyclization reaction catalyzed by MgCl 2 . 6H 2 O as a Lewis acid catalyst was found to be sluggish at best condition, providing 3a in 38% yield and recycling 1a in 37% yield after 10 × 3 min following purification by preparative thin-layer chromatography (entry 1). The reaction was accelerated dramatically by the presence of ZnCl 2 providing the product 3a in 48% yield and recycling the raw material 1a in 23% yield after 10 × 3 min (entry 2). Although these experiments were not so efficient, the use of solvent-free reaction conditions does have some intrinsic ecological and chemical value. Inspired by these results, Lewis acid catalyst (entry 3) was found to be greatly enhancing the reaction in AlCl 3 catalyzed, which providing 3a in 53% yield after 10 × 3 min following purification on silica. Similarly, reactions were improved by the presence of SnCl 2 . 2H 2 O (entry 4) and FeCl 3 (entry 5) providing the product 3a in 63% and 68% yield, respectively. However, the optimum conditions for this transformation employed HgCl 2 as a Lewis acid catalyst, providing 3a in 91% yield after 10 × 3 min (entry 6).  In order to further optimize the reaction conditions, a wide range of reaction parameters were tested by altering the amount of 2a and catalyst as well as the reaction time in a test reaction of 1a and 2a ( In order to further optimize the reaction conditions, a wide range of reaction parameters were tested by altering the amount of 2a and catalyst as well as the reaction time in a test reaction of 1a and 2a (Table 2). Table 2. Optimization of the reaction conditions. 5 0.5 1.5 FeCl3 10 × 3 68 18 6 0.5 1.5 HgCl2 10 × 3 91 0 a 10 × 3 means three times 10 min as reaction time, and the progress of the reaction was checked by TLC analysis at the end of each irradiation period. b Isolated yield (%) after preparative thin-layer chromatography.
In order to further optimize the reaction conditions, a wide range of reaction parameters were tested by altering the amount of 2a and catalyst as well as the reaction time in a test reaction of 1a and 2a (Table 2). 0.5 1.5 4 10 × 3 90 0 a 10 × 2 means two times 10 min as reaction time, and the progress of the reaction was checked by TLC analysis at the end of each irradiation period. b Isolated yield (%) after preparative thin-layer chromatography.
As can be clearly seen in Table 2, when 0.5 mmol of 1a and 2.5 mmol of 2a reacted by HgCl2 catalyst at the amount of 5 mol% under microwave irradiation, 3a was obtained in a 97% yield after 10 × 2 min (entry 1). When the amount of 2a reduced to 2.0 mmol, 3a was obtained in a 90% yield for 0.5 1.5 4 10 × 3 90 0 a 10 × 2 means two times 10 min as reaction time, and the progress of the reaction was checked by TLC analysis at the end of each irradiation period. b Isolated yield (%) after preparative thin-layer chromatography.
As can be clearly seen in Table 2, when 0.5 mmol of 1a and 2.5 mmol of 2a reacted by HgCl 2 catalyst at the amount of 5 mol% under microwave irradiation, 3a was obtained in a 97% yield after 10 × 2 min (entry 1). When the amount of 2a reduced to 2.0 mmol, 3a was obtained in a 90% yield for 10 × 2 min (entry 2). Moreover, when the amount of 2a decreased to 1.5 mmol, 3a was obtained in a 91% yield if the reaction time was prolonged to 10 × 3 min (entry 3). Nevertheless, if we keep reducing the amount of 2a to 1.0 mmol, only providing 3a in 41% yield and recycling 1a in 55% yield after 10 × 4 min following purification by preparative thin-layer chromatography (entry 4). However, the cyclization reaction was terminated by the absence of HgCl 2 even if the reaction time was prolonged to 10 × 12 min and the amount of 2a was increased to 2.5 mmol (entry 5). Inspired by these phenomena, we realized that the amount of HgCl 2 as a Lewis acid catalyst was significant for enhancing the cyclization reaction. For example, when 0.5 mmol of 1a and 2.5 mmol of 2a reacted by HgCl 2 catalyst at the amount of 2.5 mol% under microwave irradiation, 3a was synthesized in a 69% yield after 10 × 5 min (entry 6). When the amount of HgCl 2 increased to 3.0 mol%, 3a was prepared in a 78% yield for 10 × 3 min (entry 7). To our delight, when the amount of HgCl 2 increased to 4.0 mol%, the cyclization reaction was accelerated dramatically providing the product 3a in 91% yield after 10 × 3 min (entry 8). We concluded that when 0.5 mmol of 1a and 1.5 mmol of 2a reacted by HgCl 2 catalyst at the amount of 4 mol% under microwave irradiation, 3a was smoothly synthesized in a 90% yield after 10 × 3 min (entry 9).
We have previously developed an efficient method for the synthesis of 2-monosubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines 3e -r under ultrasonic irradiation [19], however, under the above reaction conditions, 2,2-disubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines were not obtained at all even if the reaction time was prolonged. So we have previously described another a convenient, rapid, and high-yielding reaction for the synthesis of 2,2-disubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines 3a-w under microwave irradiation and solvent-free conditions [15]. On the other hand, compared with the previous procedure, the present methodology has the advantages of low catalyst loading and recovering catalyst, short reaction and repaid reaction times, easy separation products, and increased reaction yields (except 3i, 3j, 3o, 3r, 3t, 3u, and 3w), and more conducive to the large-scale synthesis products. = H or m-Me) with anhydrides (2, R 5 = Me or Et) was investigated to explore the scope of the reaction. As outlined in Table 3, 2-alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines (3a-d′) were prepared in 80-95% yields for 10 × 3-10 × 4 min (Of course, the reaction can also be carried out directly in one go for 30 or 40 min, with little impact on yield). The steric and electronic effects of substituents of 1 to the reaction were not very obvious.
We have previously developed an efficient method for the synthesis of 2-monosubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines 3e′-r′ under ultrasonic irradiation [19], however, under the above reaction conditions, 2,2-disubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines were not obtained at all even if the reaction time was prolonged. So we have previously described another a convenient, rapid, and high-yielding reaction for the synthesis of 2,2-disubstituted 3-N-acyl-5-phenyl-1,3,4-oxadiazolines 3a-w under microwave irradiation and solvent-free conditions [15]. On the other hand, compared with the previous procedure, the present methodology has the advantages of low catalyst loading and recovering catalyst, short reaction and repaid reaction times, easy separation products, and increased reaction yields (except 3i, 3j, 3o, 3r, 3t,  3u, and 3w), and more conducive to the large-scale synthesis products. Table 3. Synthesis of 2-alkyl-2-(N-arylsulfonylindol-3-yl)-3-N-acyl-5-aryl-1,3,4-oxadiazolines (3a-d′) a . In order to further study the cyclization reaction, we have done the following study. As illustrated in Table 4, when the cyclization reaction of 1a (3 mmol) with 2a (9 mmol) was complete, the reaction mixture was cooled to room temperature. Then the mixture was filtered and the filter residue was washed with dichloromethane (3 × 10 mL). The HgCl 2 catalyst was reused directly in the next reaction. For examples, when 3 mmol of 1a and 9 mmol of 2a reacted by HgCl 2 catalyst at the amount of 4 mol% under microwave irradiation, 3a was smoothly synthesized in 98% yield after 5 min for the first time (entry 1). When the reaction time prolonged to 6 min, 3a was obtained in 98% yield for recovering HgCl 2 catalyst was reused directly in the reaction (entry 2). Entries 3 and 4, 3a was obtained in 97% and 98% yield, respectively.
To our delight, in the case of reaction amplification, it was found that HgCl 2 had better catalytic effect on the reaction. It may be that HgCl 2 induces the cyclization reaction in which the turnover frequency is accelerated, and the yield of compound 3a increased significantly.

3d′
H H Et H Me 10 × 3 82 a A mixture of 1 (0.5 mmol), 2 (1.5 mmol), and HgCl2 (0.02 mmol) was reacted under microwave heating at 700 W for 30-40 min. b 10 × 3 means three times 10 min as reaction time, and the progress of the reaction was checked by TLC analysis at the end of each irradiation period. c Isolated yield (%) after preparative thin-layer chromatography.
In order to further study the cyclization reaction, we have done the following study. As illustrated in Table 4, when the cyclization reaction of 1a (3 mmol) with 2a (9 mmol) was complete, the reaction mixture was cooled to room temperature. Then the mixture was filtered and the filter residue was washed with dichloromethane (3 × 10 mL). The HgCl2 catalyst was reused directly in the next reaction. For examples, when 3 mmol of 1a and 9 mmol of 2a reacted by HgCl2 catalyst at the amount of 4 mol% under microwave irradiation, 3a was smoothly synthesized in 98% yield after 5 min for the first time (entry 1). When the reaction time prolonged to 6 min, 3a was obtained in 98% yield for recovering HgCl2 catalyst was reused directly in the reaction (entry 2). Entries 3 and 4, 3a was obtained in 97% and 98% yield, respectively.
To our delight, in the case of reaction amplification, it was found that HgCl2 had better catalytic effect on the reaction. It may be that HgCl2 induces the cyclization reaction in which the turnover frequency is accelerated, and the yield of compound 3a increased significantly.
Supplementary Materials: The following are available online. The NMRs and MS of the new compounds (3s-d ).
Author Contributions: Designed the experiments, synthesized the compounds, and analyzed the data: Z.C. Wrote the paper: Z.C., Y.T., S.L., J.J., M.H., and G.C. All authors approved the final manuscript.
Funding: The present research was supported by the National Natural Science Foundation of China (grant no. U1604105).