Synthesis of Isothiocyanates Using DMT/NMM/TsO− as a New Desulfurization Reagent

Thirty-three alkyl and aryl isothiocyanates, as well as isothiocyanate derivatives from esters of coded amino acids and from esters of unnatural amino acids (6-aminocaproic, 4-(aminomethyl)benzoic, and tranexamic acids), were synthesized with satisfactory or very good yields (25–97%). Synthesis was performed in a “one-pot”, two-step procedure, in the presence of organic base (Et3N, DBU or NMM), and carbon disulfide via dithiocarbamates, with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4-sulfonate (DMT/NMM/TsO−) as a desulfurization reagent. For the synthesis of aliphatic and aromatic isothiocyanates, reactions were carried out in a microwave reactor, and selected alkyl isothiocyanates were also synthesized in aqueous medium with high yields (72–96%). Isothiocyanate derivatives of L- and D-amino acid methyl esters were synthesized, under conditions without microwave radiation assistance, with low racemization (er 99 > 1), and their absolute configuration was confirmed by circular dichroism. Isothiocyanate derivatives of natural and unnatural amino acids were evaluated for antibacterial activity on E. coli and S. aureus bacterial strains, where the most active was ITC 9e.

In recent years, several new methods have been developed for the synthesis of ITCs, using fluorine-containing reagents such as Langlois reagent (F 3 CSO 2 Na) [39], Ph 3 P + CF 3 CO 2 (PDFA)/S 8 [40], (Me 4 N)SCF 3 [41], CF 3 SiMe 3 /S 8 or AgSCF 3 [42], and BrCF 2 CO 2 Na/S 8 [43]. However, three methods still predominate. The most common method is the Staudinger/aza-Wittig tandem reaction, in which the starting azides react with triphenylphosphine and are converted into iminophosphorane intermediates. Then, in the presence of carbon disulfide, the iminophosphate intermediates are transformed into the final ITCs ( Figure 1, route a) [44][45][46][47]. The second method involves the use of a primary amine and thiophosgene as a reagent for the transfer of the thiocarbonyl moiety in the presence of a base (Figure 1, route b) [48][49][50][51]. Due to the toxicity of thiophosgene and the sensitivity of certain functional groups, this reagent is increasingly replaced by surrogates such as di-(2-pyridyl) thionocarbamate [52], 1,1 -thiocarbonyldiimidazole [53], or 1,1 -thiocarbonyldi-2-(1H)-pyridone [54]. The third method is a two-step one-pot procedure. Primary amines are transformed in the presence of a base and carbon disulfide into intermediate dithiocarbamates, which after treatment with the desulfurating agent are transformed into the target ITCs (Figure 1, route c). Many desulfurating agents are currently available. Their choice depends on the reaction conditions and the presence of functional groups in the reagents. The most often used desulfurating agents include tosyl chloride [55], sodium persulfate [56], iodine [57], mesyl chloride [58], ethyl chloroformate [59], di-tert-butyl dicarbonate [60], cyanuric chloride [61], and others [62]. Microwave radiation has been found to support the conversion of dithiocarbamates into ITCs [63]. Coupling reagents used in peptide chemistry, such as HBTU and PyBOP [64], DCC [65], and T3P ® [66], can also be used as desulfurating reagents. However, despite the availability of numerous reagents, new compounds are continually sought to provide for the efficient synthesis of structurally diverse ITCs from dithiocarbamates, supported by microwave radiation.
Here, we describe the microwave-assisted synthesis of structurally diverse aliphatic and aromatic isothiocyanates, as well as normal synthesis of isothiocyanate derivatives of natural and unnatural amino acids, using 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4-sulfonate (DMT/NMM/TsO − , 1) as a new, efficient desulfurating agent for the indirect formation of dithiocarbamates from primary amines or hydrochlorides ( Figure 2). DMT/NMM/TsO − [67,68] (1) is an effective, and environmentally friendly coupling reagent used for the synthesis of amides, esters, and peptides in solution or solid phase, as well as under microwave-assisted conditions [69]. It enables the synthesis of peptides using Z-, Boc-, and Fmoc-protected substrates as well as unnatural amino acids. Using DMT/NMM/TsO − , synthesis of peptides occurs rapidly, with high yields, without racemization, and often with sufficiently high purity to avoid the necessity of additional chromatographic purification. Use of DMT/NMM/TsO − in the synthesis of structurally diverse isothiocyanates characterized by a wide range of biological activity could allow for the efficient synthesis of peptide conjugates with isothiocyanates, requiring only a single coupling reagent. Reducing the numbers of compounds utilized in the synthesis of biologically active compounds should reduce the number and variety of by-products, which is particularly important for the impurity profile.

Optimizing the Synthesis of Aliphatic Isothiocyanates
In the first stage of the research, it was checked whether DMT/NMM/TsO − (1) acts as a desulfurating agent, and experiments to optimize the reaction conditions were carried out. Phenethylamine (2a) was used as a model amine in the optimization tests. Use of carbon disulfide (CS 2 ) (3 equiv.) in the presence of triethylamine (Et 3 N) (4 equiv.) in DCM as the solvent enabled formation of intermediate dithiocarbamate 3a in just 5 min at room temperature (rt) [63]. The obtained dithiocarbamate 3a was subjected to a microwaveassisted reaction with 1 equiv. of DMT/NMM/TsO − (1) as a desulfurating agent, producing the target isothiocyanate 4a with 90% yield in 3 min at 90 • C (initial 200 W power) ( Table 1, entry 1).  Table 1. Standard mode (initial power 200 W); b -yield of 4a after flash chromatography (hexane 100%); c -elongation time to 5 and 10 min resulted the same yield (90%); d -increasing the amount of 1 to 1.6 equiv. resulted in the same yield (92%); e -increasing the amount of 1 to 1.6 equiv. resulted in the same yield (89%); f -normal reaction at rt.; g -prolonging the time to 60 min resulted in the same yield (82%), shortening the time to 10 min resulted in a lower yield (79%); h -reaction in sealed tube (30 min, 90 • C) resulted in the same yield (82%).
Extending the reaction time to 5 min and 10 min did not further increase the yield (Table 1, entry 1, footnote c). Next, we investigated the influence of the base on the yield.
Replacing triethylamine with N-methylmorpholine (NMM) or 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) resulted in lowering of the yield to 78% and 69%, respectively (Table 1, entry  2 and 3), whereas the reaction without base resulted in final isothiocyanate 4a with only 51% yield (Table 1, entry 4). Decreasing the amount of Et 3 N to 3 equiv. resulted in a slight increase the yield, to 92% (Table 1, entry 5). Increasing the amount of coupling reagent 1 to 1.3 equiv. (Table 1, entry 6) and to the 1.6 equiv. (Table 1, entry 6, footnote d) did not affect the yield, which remained at 92% in both cases. However, the reduction of DMT/NMM/TsO − (1) to 0.7 equiv. reduced the yield to 86% (Table 1, entry 7). The reaction without desulfurating agent 1 produced 4a with only 55% yield ( Table 1, entry 8). In the next step, we investigated the possibility of performing the reaction in aqueous media, using optimized conditions: 3 min, 90 • C, CS 2 (3 equiv.), Et 3 N (3 equiv.) as a base, with DMT/NMM/TsO − (1 equiv.). In these conditions, ITC 4a was obtained in a satisfactory yield 73% (Table 1, entry 9). It has been found that increasing the amount of agent 1 to 1.3 equiv. allowed us to obtain isothiocyanate 4a with a very high yield of 89% (Table 1, Entry 10). Further increasing the amount of 1 to 1.6 equiv. did not affect the yield of the reaction (Table 1, entry 10, footnote e). We also show that this reaction occurs in normal conditions at rt, although in lower yield. Performing the reaction in 30 min at rt resulted in obtaining the product 4a with 82% yield (Table 1, Entry 11). Prolonging the time to 60 min did not affect the results, but shortening the time to 10 min resulted in a decrease in the yield to 79% (Table 1, entry 11, footnote g). Additionally, performing the reaction in the sealed tube for 30 min at 90 • C (Table 1, entry 11, footnote h) did not affect the yield of reaction, which was still 82%. The results of experiments in normal conditions at room and at high temperature undoubtedly indicate that the use of microwave radiation assistance leads to higher yields of the ITC 4a, and also that it is possible to significantly shorten the reaction time.
A library of isothiocyanates 4a-j, 7a-j, 9a-j, and 11a-c using structurally various aliphatic amines 2a-j (Figure 3), aromatic amines 5a-j ( Figure 4), hydrochlorides of methyl and the ethyl esters of L-and D-amino acids 8a-j ( Figure 5), as well as hydrochlorides of the ethyl esters of unnatural amino acids 10a-c ( Figure 6), has been synthesized. All the compounds were isolated with good or very good yields (97-25%) after purification on a short pad of silica gel.

Synthesis of Aliphatic Isothiocyanates 4a-j
Aliphatic isothiocyanates were synthesized in a microwave reactor using two methods with different solvents. In Method A, DCM was used as solvent, whereas in Method B, the synthesis of selected ITCs was performed in H 2 O ( Figure 3).
Comparing the yields of aliphatic ITCs obtained in optimal conditions in Method A with previous results using only microwave radiation to synthesis of aliphatic ITCs [63], it can be concluded that application of DMT/NMM/TsO − (1) as a desulfurating agent allows us to receive the final isothiocyanates 4a-i with higher yield. The biggest yield difference was found for ITC 4f (72% vs. 50% [63]), and the smallest for ITC 4i (83% vs. 82% [63]). For other ITCs, except 4j, which was not previous obtained, the yields were higher, from 5 to 14%, than in previous publication [66].

Synthesis of Aromatic Isothiocyanates 7a-j
In the next stage of the research, attempts were made to check the usefulness of DMT/NMM/TsO − (1) as a desulfurating agent in reactions with aromatic amines. Performing the reaction under optimized conditions (Table 1, entry 5 and Figure 3 Method A) with aniline (5a) resulted in a low yield of final phenyl isothiocyanate (7a) at 30% ( Table 2, entry 1). For this reason, the synthesis of aromatic ITCs required optimization ( Table 2). Table 2. Optimization of reaction conditions of synthesis of isothiocyanate 7a, derived from model amine 5a a .
Introducing the methyl group to the aromatic ring in the para position increased the yield of ITC 7b to 87%, whereas introducing two methyl groups in positions 2 and 6 resulted in a slight increase of the yield of 7c to 92%. Replacing the methyl with a methoxy group in the para position gave ITC 7d with 87%. However, the methoxy group in the metha position as well as the presence of two dimethoxy groups in positions 3 and 5 in the aromatic ring diminished the yield of ITCs 7e and 7f to 66% and 67%, respectively. Application of this protocol to halogenoanilins 5g-i enabled ITCs 7g-i to be obtained with 61-53% yields. Aromatic isothiocyanate with two -NCS group -1,3-diisothiocyanatobenzene (7j) were also obtained, although with a moderate yield of 40%. The synthesis of aromatic isothiocyanates in aqueous medium has failed. Model phenyl isothiocyanate (7a) was obtained with a low yield of 21% (Table 2, entry 6), and the synthesis of other aromatic isothiocyanates in H 2 O was not carried out. Comparing the yields of obtained aromatic isothiocyanates 7a, 7c-e, and 7g-j in microwave-assisted synthesis with desulfurating agent 1 and without it [63], we can conclude that in almost all cases, the lack of desulfurating agent resulted in better yields of final isothiocyanates. The yields of ITCs 7a, 7c, 7e, and 7g-j in reaction without desulfurating agent were from 6 to 42% higher than yields with DMT/NMM/TsO − . The biggest difference was for ITC 7i (53% vs. 95% [63]). Only ITC 7d was obtained with higher yield in reaction with agent 1 than without it (87% vs. 75% [63]). It can conclude that in contrast to aliphatic isothiocyanates, aromatic iothiocyanates do not need addition of desulfurating agent, however its presence does not cause a significant drop of yield.

Synthesis of Isothiocyanate Derivatives of Natural 9a-j and Unnatural 11a-c Amino Acids
In next stage of the research, we attempted to synthesize ITCs using hydrochlorides of methyl and ethyl esters of L-and D-amino acids 8a-j as substrates. As Et 3 N causes partial racemization of products [66], it was replaced with NMM. Performing the reaction on model L-alanine methyl ester hydrochloride (8a) under optimal conditions (3 min, 90 • C) in a microwave reactor produced 9a with a yield of 59% and concomitant high racemization (er 65:35). To prevent the loss of enantiomeric homogeneity, the reaction was performed under normal conditions (30 min, rt). The product (S)-methyl 2-isothiocyanatopropanoate (9a) was obtained with 50% yield and low racemization (er 99 > 1) ( Figure 5). Using these optimal conditions, a second enantiomer (R)-methyl 2-isothiocyanatopropanoate (9b) was produced with 51% yield and low racemization (er 99 > 1), using D-alanine methyl ester hydrochloride (8b) as a substrate. In addition to hydrochlorides of methyl esters of alanine, L-alanine benzyl ester hydrochloride (8c), and tert-butyl ester of L-alanine hydrochloride (8d), hydrochlorides of methyl esters of L-valine (8e), L-leucine (8f), L-isoleucine (8g), L-phenylalanine (8h), and L-lysine (8i), as well as the achiral hydrochloride of the ethyl ester of glycine (8j), were used as starting materials. Figure 5 presents the results of the synthesis of isothiocyanate derivatives of amino acids 9a-j.
Isothiocyanates derived from the benzyl ester 9c and tert-butyl ester 9d of the L-alanine group were synthesized with lower yields of 38% and 35%, respectively. This may have been due to the presence of a larger ester group and thus greater steric hindrance. Isothiocyanate analogs of L-valine 9e, L-leucine 9f, L-isoleucine 9g, and L-phenylalanine 9h with isopropyl, isobutyl, sec-butyl, and the benzyl moiety, respectively, were synthesized with satisfactory yields (30-63%). During the synthesis of these compounds, it was observed that the yield depended on the effect of steric hindrance (steric hindrance 9h>9g>9f>9e) by the substituent on the amino acid side chain (yield of 9e>9f>9g >9h). In the case of the achiral ethyl ester of the isothiocyanates of glycine 9j, the yield of the reaction was 48%. As in the case of the aliphatic and aromatic isothiocyanates with two -NCS groups also an isothiocyanate derivative from L-lysine 9i with two -NCS groups was obtained, although with a low yield (25%). ITCs 9a, 9b, and 9h were previous synthesized using as a desulfurating agent T3P ® also in normal conditions [66]. ITCs 9a and 9b were obtained with 10% better yield than with DMT/NMM/TsO − , however, ITC 9h was synthesized with definitely higher yield -72% [66]. Others ITCs were not synthesized using T3P ® .
In addition to methyl and ethyl esters of natural amino acids, hydrochlorides of ethyl esters of unnatural amino acids with aliphatic, aromatic, and cyclohexyl linkers were used as substrates for the synthesis ITCs. Hydrochlorides of ethyl esters of 6-aminocaproic acid (10a), 4-(aminomethyl)benzoic acid (10b) and trans-4-(aminomethyl)cyclohexanecarboxylic acid (Tranexamic acid) (10c) were converted into target ITCs 11a-c in microwave-assisted synthesis under optimal conditions (3 min, 90 • C), with DMT/NMM/TsO − (1) as the desulfurating agent. All ITCs 11a-c were isolated after flash chromatography with a very good yields and high purity ( Figure 6). In the case of compound 11c which is trans-isomer the synthesis occurred without racemization, due to the lack of signals of cis-isomer on 1 H NMR spectrum.

Determination of the Absolute Configuration Using Circular Dichroism
Inspired by Michalski and Cież [75], we attempted to determine the absolute configuration of chiral ITCs 4c-d and 9a-i using circular dichroism (CD). All samples were dissolved in methanol. The concentrations of the samples were between 0.52 mg/mL and 0.30 mg/mL. For all S enantiomers of the ITCs analogs of L-amino acids 9a and 9c-i, the Cotton effect (CE) was positive. For the R enantiomer of the ITC analog of D-alanine 9b the CE was negative. These results support the literature data [75,76]. The ITC 9a with the S configuration at C-2 exhibited a strong positive CE at 205 nm, and the second enantiomer ITC 9b with an R configuration at C-2 exhibited a strong negative CE, also at 205 nm. The CE for ITCs 9a and 9b were similar but reversed, confirming the presence of two different enantiomers (Figure 7). For the other two ITC analogs of L-alanine with a benzyl 9c or tert-butyl 9d ester moiety, strong positive CE were observed at 205 nm and 210 nm, respectively (see Supporting Material Figures S77 and S78). A strong positive CE for ITCs 9e-g was also observed at 210 nm (see Supporting Material Figures S79-S81). However, for ITC 9h, a strong positive CE was observed at 225 nm (see Supporting Material Figure S82), which could be due to the presence of an aromatic ring. For ITC 9i with two -NCS groups, a strong positive CE was observed at 210 nm (see Supporting Material Figure S83).
As in the case of the chiral ITCs analogs of L-and D-amino acids 9a-i, the absolute configuration of the chiral ITCs with a benzyl moiety 4c and 4d was also determined using CD (Figure 8). For both enantiomers 4c and 4d, a strong CE was observed at 215 nm. For ITC 4d, the CE was positive; consequently, the absolute configuration was S. In the case of ITC 4c, the CE was negative; consequently, the absolute configuration was R. The presence of two different enantiomers was confirmed.
The absolute configurations of all chiral ITCs 4c-d and 9a-i, as well as their optical rotations, are presented in Table 3.

Proposed Mechanism of Synthesis of ITC Using DMT/NMM/TsO −
We postulate the following mechanism for the synthesis of ITCs using DMT/NMM/TsO − (Figure 9). Dithiocarbamate 3b formed in a reaction amine 2b with Et 3 N and CS 2 reacts with DMT/NMM/TsO − (1), affording active ester 12. Next, under the influence of Et 3 N, leaving group 13 is eliminated with the simultaneous formation of ITC 4b. Forming the intermediate product which is a derivative of dithiocarbamate with desulfurating agent 12, and then formation the final isothiocyanate in the presence a base is characteristic stage for synthesis of isothiocyanates using various desulfurating agents and it was described in many publication [57,60,64,66]. Next, the anion 13 can undergo two reactions. The first possible reaction is protonation of anion 13 by protonated triethylamine and formation if thiol 15 which is in equilibrium with its thiocarbonyl form 16. However, due to the rapid tautomerism identification was impossible. The second is possible methylation, leading to preparation of 2,4-dimethoxy-6-methylthio-1,3,5-triazine (14), which was isolated after flash chromatography with low yield and confirmed by 1 H and 13 C NMR. The yield of the isolated compound 14 was low, indicating a slight share this reaction.
The above results show that effectiveness of all desulfurating agents in microwaveconditions is very good (yield of 4b is between 92 and 75%); however, DMT/NMM/TsO − turned out to be the best desulfurating agent (yields of 4b 92%) ( Table 4, Entry 1). The TCT as well as I 2 were also very efficient reagents; however, the yields of ITC 4b were slightly lower, respectively, 87% and 86% (Table 4, Entries 2 and 3). Application of Boc 2 O with DMAP, T3P ® , and TsCl also caused the yields of 4b to be satisfactory (84-81%) ( Table 4, Entries 4-6). The lower yields below 80% obtained using ethyl chloroformate and 30% H 2 O 2 (Table 4, Entries 7 and 8) show that these compounds were definitely less effective than DMT/NMM/TsO − . Although, some of the reagents (TsCl and iodine) are cheaper than DMT/NMM/TsO − , the DMT/NMM/TsO − allows us to obtain the product with higher yield, which is undoubtedly its greatest advantage.

Antibacterial Activity
Isothiocyanates derivatives of natural 9a-j and unnatural amino acids 11a-c were evaluated for antibacterial activity on Gram positive bacteria strain S. aureus and Gram negative bacteria strain E. coli, using chloramphenicol as positive control. The antibacterial activity of ITCs 9a-j and 11a-c based on the well diffusion method on trypticase soy agar (TSA) is presented in Table 5, and on the pictures of Petri dishes in the (Supporting Material Figure S87). ITCs 9a-j and 11a-c at micromolar concentration (16-27 µM) in DMSO or mixture DMSO and water (4 mg/mL) [78,79] were used in the test. The inhibition zones were reported in millimeter (mm). Based on results in Table 5 more of the tested ITCs exhibited antibacterial activity. For E. coli the most active ITCs were 9d and 9e (φ > 36 mm). Additionally, good activity characterized ITCs 9i and 9j (φ about 30 mm). Others ITCs 9a-c, 9f-h, 11a-11c also had antibacterial activity, although slightly lower (φ < 25 mm). Additionally ITCs 9e, 9j and 11a exhibited very high antibacterial activity (φ > 32 mm) against S. aureus. ITCs 9d, 9f, 9h, and 9i had weaker, but also good activity (30 > φ > 25 mm). The other ITCs 9a-c, 9g, 11b-c showed moderate antibacterial activity (φ < 25 mm).
For all tested compounds, the most active for both bacterial strains was ITC 9e derived from methyl ester of L-valine. Probably the short and branched alkyl chain of isopropyl moiety was responsible for the highest activity. Despite the satisfactory antibacterial activity of tested ITCs 9a-j and 11a-c (16-27 µM), the activity of chloramphenicol used as a positive control was definitely higher (0.062 µM). The study on the mechanism of antimicrobial action and toxicity will be continued.

General Information
NMR spectra were measured on a Bruker Avance II Plus (Bruker Corporation, Billerica, MA, USA) spectrometer (700 MHz for 1 H NMR and 176 MHz for 13 C NMR) in CDCl 3 solution. 1 H and 13 C NMR spectra were referenced according to the residual peak of the solvent, based on literature data. Chemical shifts (δ) were reported in ppm and coupling constants (J) in Hz. 13 C NMR spectra were proton-decoupled. A monomode microwave reactor (CEM Discover) (CEM, Matthews, NC, USA) equipped with an Intelli Vent pressure control system was used. The standard method was applied and the maximum pressure was set to 250 psi. The temperatures of the reaction mixtures were measured with an external infrared sensor. Flash chromatography was performed using a glass column packed with Baker silica gel (30-60 µm). For TLC, silica gel with a 254 nm indicator on Al foils (Sigma−Aldrich, St. Louis, MO, USA) was used. All reagents and solvents were purchased from Sigma-Aldrich (Poland) and used as obtained. Optical rotations were measured at 25 • C on a PolaAAr 3001 Polarimeter (A. KRÜSS Optronic GmbH, Hamburg, Germany) at λ = 589 nm. Optical rotations were reported as follows: [α] D 25 (c = g/100 mL solvent). The enantiomeric ratio (er) of 4c and 4d was determined by chiral stationary phase HPLC, using a Daicel Chiralpak ID column (hexane) and a Daicel Chiralpak IC column for compounds 9a-b (hexane/i-PrOH, 98:2), with a column temperature of 30 • C and a flow rate of 1.0 mL/min. Melting points were obtained using a Büchi SMP-20 apparatus. CD studies were performed using a Jasco J-1500 spectrometer Far-UV (ABLE JASCO Polska, Krakow, Poland) and a rectangular quartz cuvette (1 mm path length, Hellma, Müllheim, Germany). The samples were prepared in methanol at a concentration of 0.52-0.30 mg/mL. All studies were carried out at rt. CD spectra were measured in the range of 190-300 nm. The experimental parameters were as follows: data pitch, 5 nm; scanning mode, continuous; scanning speed, 100nm/min; bandwidth, 4 nm; integration time, 1 s. Mass spectrometry analysis was performed on a Bruker microOTOF-QIII (Bruker Corporation, Billerica, MA, USA) equipped with electrospray ionization mode and a time of flight detector (TOF). IR spectra were measured on an FT-IR Alpha Bruker (ATR) instrument in cm −1 .

Study of Antimicrobial Activity
Antibacterial activities of solutions of tested compounds were evaluated using well diffusion method on trypticase soy agar TSA (Merck). The inhibition zones were reported in millimeter (mm). The 24 h single colonies on agar plates were used to prepare the bacterial suspensions with the turbidity of 0.5 McFarland degree (~1.5 × 10 8 CFU/mL). Turbidity of the bacterial suspensions were measured using a DEN−1 densitometer (Merck). Staphylococcus aureus (ATCC 6538) and Escherichia coli (ATCC 8739) were used as references for the antibacterial assay. TSA agar plates were inoculated with bacterial strain under aseptic conditions and wells (diameter = 12mm) were filled with 250 µL of the test samples and incubated at 37 • C for 24 h. After the incubation period, the diameter of the growth inhibition zones was measured.
The sterile distilled water was used as a negative control while chloramphenicol (Sigma-Aldrich) at the concentration (20 µg/mL) 0.062 µM in DMSO was used as a positive standard. All tests were performed in triplicate.

Conclusions
To conclude, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4sulfonate (DMT/NMM/TsO − ) can be recognized as a new and efficient desulfurating agent suitable for microwave-assisted, one-pot synthesis of aliphatic and aromatic isothiocyanates and classical synthesis (normal reaction). The synthetic procedures presented in this study enabled the rapid and simple acquisition of a library of 20 structurally diverse isothiocyanates with good or very good yields (40-97%), high purity, and in the case of optically active compounds, low racemization (er 99 > 1). The synthesis of aliphatic isothiocyanates also took place in aqueous medium in the presence of organic base, with only slightly lower yields compared to those obtained in DCM.
DMT/NMM/TsO − was also used to synthesis of isothiocyanate derivatives from methyl, ethyl, benzyl, and tert-butyl esters of L-and D-natural amino acids (alanine, valine, leucine, isoleucine, phenylalanine, and lysine) as well as achiral glycine, with satisfactory yields (25-63%), high purity, and low racemization (er 99 > 1). Due to the observed racemization under microwave-assisted syntheses, these reactions were performed under normal conditions. The absolute configuration of chiral ITCs was determined by CD. The Cotton Effect was strongly positive for ITCs with the S configuration, and for ITCs with the R configuration it was strong negative. The optical rotation of all chiral ITCs was also measured. As well as isothiocyanate derivatives of natural amino acids, isothiocyanate derivatives of ethyl esters of 6-aminocaproic acid, 4-(aminomethyl)benzoic acid, and tranexamic acid were synthesized with DMT/NMM/TsO − in a microwave reactor, rapidly and with very high yields (75-92%), and in the case of trans ITC 11c without racemization.
Antibacterial activity against E. coli and S. aureus bacterial strains showed that the most of tested isothiocyanates derivatives of natural and unnatural amino acids are active; however, their activity was worse than activity of positive control-chloramphenicol. The most active for both strains was ITC 9e. Further research is ongoing to determine the biological properties of ITCs derived from both natural and unnatural amino acids.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.