Facile Synthesis of 5-Arylidene Thiohydantoin by Sequential Sulfonylation/Desulfination Reaction

The sequential sulfonylation/desulfination reactions of 5-benzylthiohydantoin with excess arylsulfonyl chlorides in the presence of triethylamine have been developed to afford a wide range of 5-arylidene thiohydantoin derivatives in moderate to excellent yields. A plausible sulfonylation/desulfination mechanism was proposed. The bioassay showed that these compounds exhibit certain fungicidal activities with the 71.9% inhibition rate of 2K against B. cinerea, and 57.6% inhibition rate of 2m against A. solani, respectively.


Results and Discussion
In our laboratory, we needed to prepare 5-aryl derivatives of hydantoin and thiohydantoin as the key intermediate to develop the novel inhibitor of Adenylosuccinate Synthetase (AdSS) [16][17][18], which plays a key role in the two-step conversion of IMP to AMP in the de novo pathway of purine biosynthesis [19][20][21].In order to prepare 5-aryl derivatives of thiohydantoin, 5-(4-hydroxylbenzyl)-thiohydantoin was treated with phenylsulfonyl chloride in acetone at 30 °C in the presence of one equivalent of triethyl amine (entry 1, Table 1).The target product 3a was obtained in 84% yield.In addition, unexpectedly, a new product 2a was isolated in 15% yield.We established the structure of the new product 2a to be that from sequential sulfonylation/desulfination at the 5-position of thiohydantoin, using NMR spectroscopy and X-ray crystallography of the homologous 2k obtained from the reaction of 1 with the p-tert-butyl-phenylsulfonyl chloride 4k (Figure 1, see more in supplementary Figures S1 and S2, Tables S1-S3), and natural occurring compound 5-(4-hydroxy phenylidene)-2-thiohydantoin [22].The reaction provided the trisubstituted exocyclic alkylidene as a single Z isomer [23].Next, we examined the sulfonylation/desulfination reaction in detail.Table 1 presents the results of screening for appropriate reaction conditions for the model reaction between 1 and phenylsulfonyl chloride 4a.When the amount of phenylsulfonyl chloride was increased to 1.2, 1.5, 2 and 3 equation, the corresponding product 2a was obtained in 29%, 69% and 81%, 80% yields, respectively (entries 2-5, Table 1).Therefore, the best quantity of phenylsulfonyl chloride was 2 equivalents for this reaction.When the reaction times were shortened, the yields decreased from 81% to 38% (entry 4,6,7,8, Table 1).As the temperature was decreased to 0 °C, the yield decreased to 53%, while as the temperature was increased to 60 °C, the yield was kept 81% (entry 9,10, Table 1).When the reaction solvents and the bases were canged, the yields decreased greatly (entry 11-15, Table 1).If we added 3-5 equivalents of Et 3 N, the yields also decreased (entry 16-18, Table 1).The following reactions were then performed for 8h in 1:2:2 ratios for 5-(4-hydroxylbenzyl)-thiohydantoin, phenylsulfonyl chloride and Et 3 N in acetone solution at 30 °C.Under the optimized reaction conditions, we examined a range of arylsulfonyl chlorides in the sequential sulfonylation/desulfination of 5-(4-hydroxybenzyl)-2-thioxoimidazolidin-4-one (1).As shown in Table 2, a variety of arylsulfonyl chlorides underwent the sulfonylation/desulfination at the 5-position of thiohydantoin, providing the anticipated 5-arylidene thiohydantoin 2 in moderate to excellent yields (entries 1-14, Table 2).Arylsulfonyl chlorides featuring phenyl groups with either electron-withdrawing or -donating substituents worked well as substrates under the given conditions.Considering that the sulfonyl is a strong electron-withdrawing group, we assumed that 5-aryl-2-thioxoimidazolidin-4-ones having another electron-withdrawing group on the benzene ring will also work.As indicated in Table 3, 5-benzylthiohydantoins were synthesized and treated with phenylsulfonyl chloride under the optimized conditions.We were pleased to observe that the anticipated products 5-phenylidene-thiohydantoin were obtained in 38%-86% yields (entries 1-9, Table 3).In a sharp contrast with these substrates, 5-(4-aminobenzyl)-thiohydantoin having a strong electron-donating NH 2 group on the benzene ring did not carry out the reaction (entries 10, Table 3).In addition, the reaction of 5-aryl derivatives of hydantoin 3a had been explored to give 2a in 91% yield under the given conditions with one equivalent molar of phenylsulfonyl chloride (4a).Interestingly, 5-benzylthiohydantoins undertook a similar reaction using phenylsulfonyl chloride and N,N-diisopropylethylamine (DIPEA) without changing the other reaction conditions (entries 11-13, Table 3).It should be stated that all of the prepared compounds 2 and 6 exhibit similar Z selectivity based on the 1 H NMR analysis of significant methine proton chemical shifts (Figure S1) and comparison with the data in the references [1][2][3][4][5][6].
Based on the above results, we proposed a sequential proton leaving-phenylsulfone formation-phenylsulfinic acid elimination mechanism for the reaction (Figure 2).After the sulfonylation of the starting material 1, the acidity of proton at the 5-postion of thiohydantoin was enhanced.Consequent deprotonation in the presence of base afforded the carbon anion, which was captured by phenylsulfonyl chloride to give intermediate II.Then, the sulfone intermediate II lost one molar of phenylsulfinic acid to give 5-arylidene thiohydantoin.This mechanism was very similar to Ei elimination observed by Jenks in the sulfone chemistry, in which sulfone groups act simultaneously as base and leaving group [25,26], an enone and conjugated olefin preparation via phenylsulfinic acid elimination of β-sulfone developed by Trost and Procopiou groups [27][28][29], and thermal decomposition pathway for polysulfones found by Wudl [30].The data in Table 4 showed that these compounds exhibit certain inhibition rates against Botrytis cinerea, Alternaria solani, Fusarium oxysporum and Dothiorella gregaria at the concentration of 100 µg/mL.In general, the fungicidal activities against B. cinerea and A. solani were better than that of against F. oxysporum and D. gregaria.The inhibition rate of 2k against B. cinerea was 71.9%, and the inhibition rate of 2m against A. solani was 57.6%, respectively.They are much better than that of carbendazim (the positive control) against B. cinerea and A. solani.

General Information
All reactions were performed under an air atmosphere with magnetic stirring.Unless otherwise stated, all reagents were purchased from commercial suppliers and used without further purification.Organic solutions were concentrated under reduced pressure using a rotary evaporator or oil pump.Flash column chromatography was performed using Qingdao Haiyang flash silica gel (200-300 mesh).Meting points were measured on a Yanagimoto apparatus and uncorrected.Infrared spectra were recorded using a Shimadzu IR-435 instrument with KBr plates. 1 H and 13 C NMR spectra were obtained on Bruker DPX 300 spectrometer with DMSO-d 6 as a solvent and TMS as an internal standard.Elemental analysis was performed on a Vario EL instrument (Elementar Vario Micro Cube, Hanau, Germany).X-ray crystallographic data were collected using a Bruker SMART CCD-based diffractometer operated at 293 K. Crystallographic for compound 2K have been deposited at the Cambridge Crystallographic data center, CCDC No. 831811.Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2, 1EZ, UK.

General Procedure for the Synthesis of Compounds 2 and 6
To a stirred solution of 5-(4-hydroxybenzyl)-2-thiohydantoin or 5-substitutedbenzyl-2-thiohydantoin (2 mmol) and Et 3 N or DIPEA(4 mmol) in acetone (15 mL), substituted phenylsulfonyl chloride (2-6 mmol) was added dropwise at 0 °C, then the temperature was allowed to 0, 30, 60 °C.The reaction was monitored by TLC.After 2-8 h, stop the reaction, and filtered.The products were purified by column chromatography on silica gel (petrolem ether-EtOAc, 1.5:1) to give the product 2 and 6.The 1 H and 13 C NMR spectra for new compounds could be found in supplementary Figure S1.

Bioassay of Fungicidal Activity
Fungicidal activities of compounds 2 against B. cinerea, A.solani, F. oxysporurm, and D. gregaria were evaluated using the mycelium growth rate test [33].The culture media with known concentration of the test compounds were obtained by mixing the solution of 2 in acetone with potato dextrose agar (PDA), on which fungus cakes were placed.The blank test was made using acetone and carbendazin was used as positive control.The culture was incubated at 25 ± 0.5 °C.Three replicates were performed.After the mycelium in the blank grew completely, the diameter of the mycelium was measured and the inhibition rate calculated according to the formula in reference [33].In which I is the inhibition rate, P 0 is the average diameter of the mycelium in the blank, and P 1 is the average diameter of the mycelium in the presence of the test samples.Mean measurements were calculated from the three replicates.

Conclusions
In conclusion, we have investigated the sequential sulfonylation/desulfination reaction of 5-(4-hydroxylbenzyl)-thiohydantoin with excess arylsulfonyl chloride in the presence of triethylamine.These reactions are operationally simple and proceed smoothly under very mild reaction conditions, providing a broad range of 5-arylidene thiohydantoin derivatives in moderate to excellent yields.The further applications of the reaction in the synthesis of biologically relevant molecules are ongoing in our laboratory.The bioassay showed that these compounds exhibit certain fungicidal activities with the 71.9% inhibition rate of 2K against B. cinerea, and 57.6% inhibition rate of 2m against A. solani, respectively.

Table 4 .
The fungicidal activities (inhibition rate, %) of compounds 2 against several plant fungi.