Isothioureas, Ureas, and Their N-Methyl Amides from 2-Aminobenzothiazole and Chiral Amino Acids

In this investigation, the reaction of 2-dithiomethylcarboimidatebenzothiazole with a series of six chiral amino-acids was studied. The reaction proceeds through the isolable sodium salt of SMe-isothiourea carboxylates as intermediates, whose reaction with methyl iodide in stirring DMF as solvent affords SMe-isothiourea methyl esters. The presence of water in the reaction leads to the corresponding urea carboxylates as isolable intermediates, whose methyl esters were obtained. Finally, the urea N-methyl amide derivatives were isolated when SMe-isothiourea or urea methyl esters were reacted with methylamine in the presence of water. The structures of synthesized compounds were established by 1H and 13C nuclear magnetic resonance and the structures of SMe-isothiourea methyl esters derived from (l)-glycine, (l)-alanine, (l)-phenylglycine, and (l)-leucine, by X-ray diffraction analysis. This methodology allows to functionalize 2-aminobenzothiazole with SMe-isothiourea, urea, and methylamide groups derived from chiral amino acids to get benzothiazole derivatives containing coordination sites and hydrogen bonding groups. Further research on the biological activities of some of these derivatives is ongoing.

A continuous interest in this class of compounds follows nowadays and numerous efforts to synthesize new biologically active heterocyclic compounds derived from the benzothiazole moiety have been made in the last 50 years. Several of these derivatives were found to possess anticonvulsant [27][28][29] and antioxidant [30][31][32] activities. In this context, some results related to molecules containing benzothiazole nuclei in medicinal chemistry have been summarized elsewhere [33,34].
On the other hand, guanidines are an important class of compounds that are found widely throughout nature and as biologically active synthetic compounds, and several uses in organic chemistry are known [35][36][37][38]. For example, the natural amino acid arginine has a guanidine group as In this contribution, we applied this methodology in the reaction of 2 with a series of chiral amino acids to form the corresponding SMe-isothiourea carboxylates 5, which were further transformed into the corresponding isourea carboxylates 6, methyl ester derivatives 8-10 and 12, and isourea amides 11, and 13. This methodology allows the functionalization of 2-aminobenzothiazole to introduce groups such as isothioureas, isoureas ureas, amides, and guanidines derived from chiral amino acids. These functional groups give properties such as coordinating sites, hydrogen bonding interactions, and water solubility, among others, required for the interaction with biomolecules. The reactions were carried out without modification of the chiral center configuration and all compounds were optically pure isolated as shown in the analyzed X-ray structures of compounds 8b,c and 9f.

Reactions and Characterization
Six amino acids Aa-f were tested: glycine (R = H, a), (l)-alanine (R = Me, b), (l)-phenylglycine (R = Ph, c), (l)-phenylalanine (R = Bn, d), (l)-valine (R = i Pr, e), and (l)-leucine (R = i Bu, f), represented as the zwitterions Ba-f. Each amino acid was transformed in situ into the corresponding sodium carboxylate Ca-f by reaction with one molar equivalent of sodium hydroxide in stirring ethanol for 2 h at room temperature, Scheme  The reaction of one molar equivalent of 2-dithiomethylcarboimidatebenzothiazole 2 with the corresponding amino-acid carboxylates Ca-f at refluxing ethanol for eight hours was carried out. In these conditions, the reaction proceeds by nucleophilic attack of the amino group of the aminocarboxylates C to the carbonimidothioate group of compound 2, with elimination of thiomethanol Scheme 1. Dithiomethylcarboimidatebenzothiazole 2, SMe-isothioureas 3 and guanidines 4 starting from 2-aminobenzothiazole 1.
In this contribution, we applied this methodology in the reaction of 2 with a series of chiral amino acids to form the corresponding SMe-isothiourea carboxylates 5, which were further transformed into the corresponding isourea carboxylates 6, methyl ester derivatives 8-10 and 12, and isourea amides 11, and 13. This methodology allows the functionalization of 2-aminobenzothiazole to introduce groups such as isothioureas, isoureas ureas, amides, and guanidines derived from chiral amino acids. These functional groups give properties such as coordinating sites, hydrogen bonding interactions, and water solubility, among others, required for the interaction with biomolecules. The reactions were carried out without modification of the chiral center configuration and all compounds were optically pure isolated as shown in the analyzed X-ray structures of compounds 8b,c and 9f.

Reactions and Characterization
Six amino acids Aa-f were tested: glycine (R = H, a), (l)-alanine (R = Me, b), (l)-phenylglycine (R = Ph, c), (l)-phenylalanine (R = Bn, d), (l)-valine (R = i Pr, e), and (l)-leucine (R = i Bu, f), represented as the zwitterions Ba-f. Each amino acid was transformed in situ into the corresponding sodium carboxylate Ca-f by reaction with one molar equivalent of sodium hydroxide in stirring ethanol for 2 h at room temperature, Scheme 2.
Molecules 2019, 24, x 2 of 15 chemistry are known [35][36][37][38]. For example, the natural amino acid arginine has a guanidine group as side chain, whilst cimetidine, a synthetic guanidine-derived compound, was the first drug used to treat peptic ulcers. Typically, the reaction of amines with thioureas [39] or isothioureas [40][41][42] is the most commonly used method to obtain guanidines. Particularly, the isothiourea group has been bonded to a solid phase as precursor of guanidines [43]. In the last decade, it has been recognized that isothioureas can also serve as remarkably potent inhibitors for a range of enzymatic systems [44][45][46]. On the other hand, inhibition of nitric oxide synthase (NOS) has led to their use in the treatment of a range of life-threatening conditions, including septic shock, acute kidney failure, and rejection after transplantation surgery [47].
In this sense, we report a synthetic way to prepare symmetrical and nonsymmetrical guanidines 4 derived from 2-aminobenzothiazole 1 by the reaction of 2-dithiomethylcarboimidatebenzothiazole 2 with primary amines in refluxing ethanol [48]. The reaction proceeds through the formation of SMeisothioureas 3, as intermediates, and the displacement of two MeSH molecules [49] In this contribution, we applied this methodology in the reaction of 2 with a series of chiral amino acids to form the corresponding SMe-isothiourea carboxylates 5, which were further transformed into the corresponding isourea carboxylates 6, methyl ester derivatives 8-10 and 12, and isourea amides 11, and 13. This methodology allows the functionalization of 2-aminobenzothiazole to introduce groups such as isothioureas, isoureas ureas, amides, and guanidines derived from chiral amino acids. These functional groups give properties such as coordinating sites, hydrogen bonding interactions, and water solubility, among others, required for the interaction with biomolecules. The reactions were carried out without modification of the chiral center configuration and all compounds were optically pure isolated as shown in the analyzed X-ray structures of compounds 8b,c and 9f.

Reactions and Characterization
Six amino acids Aa-f were tested: glycine (R = H, a), (l)-alanine (R = Me, b), (l)-phenylglycine (R = Ph, c), (l)-phenylalanine (R = Bn, d), (l)-valine (R = i Pr, e), and (l)-leucine (R = i Bu, f), represented as the zwitterions Ba-f. Each amino acid was transformed in situ into the corresponding sodium carboxylate Ca-f by reaction with one molar equivalent of sodium hydroxide in stirring ethanol for 2 h at room temperature, Scheme 2. The reaction of one molar equivalent of 2-dithiomethylcarboimidatebenzothiazole 2 with the corresponding amino-acid carboxylates Ca-f at refluxing ethanol for eight hours was carried out. In these conditions, the reaction proceeds by nucleophilic attack of the amino group of the aminocarboxylates C to the carbonimidothioate group of compound 2, with elimination of thiomethanol Scheme 2. Generation of sodium carboxylates C from neutral amino acids A.
The reaction of one molar equivalent of 2-dithiomethylcarboimidatebenzothiazole 2 with the corresponding amino-acid carboxylates Ca-f at refluxing ethanol for eight hours was carried out. In these conditions, the reaction proceeds by nucleophilic attack of the amino group of the amino-carboxylates C to the carbonimidothioate group of compound 2, with elimination of thiomethanol gas to afford the corresponding SMe-isothiourea-carboxylates 5a-f in 40-86% yields. The 1 H NMR spectroscopic data of compounds 5a-f are listed in Table S3. A singlet for the SMe group is found in the 2.35-2.44 ppm range, whose integration area are in a 3:4 proportion in relation to the aromatic hydrogen atoms. The chemical shift of the NH group to high frequency (10.6-11.4 ppm) is explained due to a hydrogen bonding interaction with the nitrogen atom of the benzothiazole moiety, as depicted in Scheme 3. On the other hand, the 13 C NMR data, listed in Table S4, show characteristic signals from 13.9 to 14.1 and 169.7 to 171.6 ppm ranges for the SMe and OC=O groups, respectively. Scheme 3. Synthesis of SMe-isothiourea carboxylates 5a-f, isourea-carboxylates 6a-f, their corresponding methyl esters 8a-f, 9e,f, 10a-f, and 12e,f, isourea-amides 11a-f and urea-amides 13e,f derived from benzothiazole and amino acids.
In the case of the reaction of compound 2 with valine-(Ce, R = i Pr) or leucine-(Cf, R = i Bu) carboxylates, an insoluble yellowish solid appeared in the reaction mixture, which was identified as compound 7. The 1 H NMR spectrum shows two singlets at 2.6 ppm (SMe) and 3.9 (NMe) each in a 3:4 proportion with respect to the aromatic hydrogen atoms. In the 13 C NMR spectrum, the signals for SMe, NMe, and the thiocarbonyl group at 18.6, 33.7, and 208.7 ppm, respectively, appeared. To explain these results, a sigmatropic rearrangement of compound 2 to form compound 7 in 15% yield is proposed, as depicted in Scheme 4, in agreement with 1 H and 13 C NMR data.
In the case of the reaction of compound 2 with valine-(Ce, R = i Pr) or leucine-(Cf, R = i Bu) carboxylates, an insoluble yellowish solid appeared in the reaction mixture, which was identified as compound 7. The 1 H NMR spectrum shows two singlets at 2.6 ppm (SMe) and 3.9 (NMe) each in a 3:4 proportion with respect to the aromatic hydrogen atoms. In the 13 C NMR spectrum, the signals for SMe, NMe, and the thiocarbonyl group at 18.6, 33.7, and 208.7 ppm, respectively, appeared. To explain these results, a sigmatropic rearrangement of compound 2 to form compound 7 in 15% yield is proposed, as depicted in Scheme 4, in agreement with 1 H and 13 C NMR data.
Isorea carboxylate compound 6c was isolated as byproduct in 20% yield, from the remaining mother liquors of 5c. No signal for the SMe group was present in the NMR spectra of compound 6c, but two interchangeable protons with deuterium were observed at 11.8 ppm, corresponding to the isourea OH, and at 8.3 ppm, attributed to the urea NH. 1 H and 13 C NMR spectroscopic data of compound 6c are listed in Tables S5 and S6, respectively. The substitution of the remaining SMe group in compound 5c by one molecule of water afforded compound 6c as one of the two possible tautomers, Table S5.
To improve the yields of the sodium salt of SMe-isotiourea carboxylates 5b-f, the reactions were carried out using anhydrous ethanol and stirring for 4 days at room temperature to avoid hydrolysis. In these conditions, the reaction proceeds more slowly to afford the corresponding SMe-isothiourea-carboxylates 5b-f in 62-95% yields. On the other hand, the complete hydrolysis of SMe-isotiourea carboxylates 5b-f in a refluxing mixture of ethanol:water 1:1 was carried out. In these conditions, the second thiomethanol gas molecule was eliminated to afford the corresponding isourea carboxylates 6b-e as the only products in 65-75% yields. The hydrolysis of compound 5a required more drastic conditions such as refluxing in DMF/H 2 O mixtures to obtain 6a in 40% yield.
In the case of the reaction of compound 2 with valine-(Ce, R = i Pr) or leucine-(Cf, R = i Bu) carboxylates, an insoluble yellowish solid appeared in the reaction mixture, which was identified as compound 7. The 1 H NMR spectrum shows two singlets at 2.6 ppm (SMe) and 3.9 (NMe) each in a 3:4 proportion with respect to the aromatic hydrogen atoms. In the 13 C NMR spectrum, the signals for SMe, NMe, and the thiocarbonyl group at 18.6, 33.7, and 208.7 ppm, respectively, appeared. To explain these results, a sigmatropic rearrangement of compound 2 to form compound 7 in 15% yield is proposed, as depicted in Scheme 4, in agreement with 1    The corresponding SMe-isothiourea carboxylate methyl esters 8a-d were obtained in 56-83% yields after the methylation of carboxylates 5a-d with one molar equivalent of methyl iodide in DMF as solvent, whose 1 H and 13 C NMR chemical shifts are listed in Tables S7 and S8, respectively. In general, their spectra are very similar compared to the corresponding carboxylates 5a-d, except for the OMe group signals which are in the 3.75-3.83 and 45-53 ppm ranges in 1 H and 13 C NMR spectra, respectively. The 13 C NMR data of esters 8c,d in CDCl 3 show broad signals for C2, C9, and C11, suggesting that the usually fast proton exchange between N3 and N12 through tautomeric equilibria becomes slower because of the steric effects of the phenyl and benzyl moieties from the amino-acid residue.
The use of one equivalent of iodomethane in the reaction of SMe-isothiourea-carboxylates 5e or 5f, afforded the respective methyl esters 8e (49% yield) or 8f (51% yield) in mixture with the corresponding N3-Me methyl esters 9e (8% yield) or 9f (25% yield) and their hydroiodides 9e·HI or 9f·HI. Compound 5f was reacted with two molar equivalents of CH 3 I; however, compounds 8f and 9f·HI remained. This last compound precipitated from the reaction mixture and was separated for further analysis. A 1 H and 13 C chemical shifts comparison between compounds 9e, 9f·HI, and 9f is depicted in Figure 1 Isorea carboxylate compound 6c was isolated as byproduct in 20% yield, from the remaining mother liquors of 5c. No signal for the SMe group was present in the NMR spectra of compound 6c, but two interchangeable protons with deuterium were observed at 11.8 ppm, corresponding to the isourea OH, and at 8.3 ppm, attributed to the urea NH. 1 H and 13 C NMR spectroscopic data of compound 6c are listed in Tables S5 and S6, respectively. The substitution of the remaining SMe group in compound 5c by one molecule of water afforded compound 6c as one of the two possible tautomers, Table S5.
To improve the yields of the sodium salt of SMe-isotiourea carboxylates 5b-f, the reactions were carried out using anhydrous ethanol and stirring for 4 days at room temperature to avoid hydrolysis. In these conditions, the reaction proceeds more slowly to afford the corresponding SMe-isothioureacarboxylates 5b-f in 62-95% yields. On the other hand, the complete hydrolysis of SMe-isotiourea carboxylates 5b-f in a refluxing mixture of ethanol:water 1:1 was carried out. In these conditions, the second thiomethanol gas molecule was eliminated to afford the corresponding isourea carboxylates 6b-e as the only products in 65-75% yields. The hydrolysis of compound 5a required more drastic conditions such as refluxing in DMF/H2O mixtures to obtain 6a in 40% yield.
The corresponding SMe-isothiourea carboxylate methyl esters 8a-d were obtained in 56-83% yields after the methylation of carboxylates 5a-d with one molar equivalent of methyl iodide in DMF as solvent, whose 1 H and 13 C NMR chemical shifts are listed in Tables S7 and S8, respectively. In general, their spectra are very similar compared to the corresponding carboxylates 5a-d, except for the OMe group signals which are in the 3.75-3.83 and 45-53 ppm ranges in 1 H and 13 C NMR spectra, respectively. The 13 C NMR data of esters 8c,d in CDCl3 show broad signals for C2, C9, and C11, suggesting that the usually fast proton exchange between N3 and N12 through tautomeric equilibria becomes slower because of the steric effects of the phenyl and benzyl moieties from the amino-acid residue.
The use of one equivalent of iodomethane in the reaction of SMe-isothiourea-carboxylates 5e or 5f, afforded the respective methyl esters 8e (49% yield) or 8f (51% yield) in mixture with the corresponding N3-Me methyl esters 9e (8% yield) or 9f (25% yield) and their hydroiodides 9e•HI or 9f•HI. Compound 5f was reacted with two molar equivalents of CH3I; however, compounds 8f and 9f•HI remained. This last compound precipitated from the reaction mixture and was separated for further analysis. A 1 H and 13 C chemical shifts comparison between compounds 9e, 9f•HI, and 9f is depicted in Figure 1. The characteristic 1 H ( 13 C) NMR signals of 9f•HI, are the SMe group at δ 3.1 (18.3), N-Me at 3.8 (56.5), and N-H at 9.1 ppm. The high frequency shift of the NH suggests a hydrogen bonding interaction with the sulfur atom and/or with the carbonyl oxygen atom, Figure 1. The nitrogen atom of the N-Me group on the benzothiazole produces an electronic effect on C4 of the aromatic ring, shifting it to low frequencies: 114.1 ppm for 9f•HI and ≈ 110 ppm for 9e and 9f. The isourea-carboxylates 6a-d can also be methylated to afford the isourea methyl esters 10a-d in 30-90% yields, Scheme 3. However, the methylation reaction of the sodium salts of isoureacarboxylates 6e or 6f afforded the corresponding methyl esters 10e (66%) or 10f (54%) in mixture with their N-Me esters 12e (24%) or 12f (21%). 1 H and 13 C NMR data of compounds 10a-f are listed in Tables S9 and S10, respectively, and those of urea-methyl esters 12e and 12f are depicted in Figure 2. The isourea-carboxylates 6a-d can also be methylated to afford the isourea methyl esters 10a-d in 30-90% yields, Scheme 3. However, the methylation reaction of the sodium salts of isourea-carboxylates 6e or 6f afforded the corresponding methyl esters 10e (66%) or 10f (54%) in mixture with their N-Me esters 12e (24%) or 12f (21%). 1 H and 13 C NMR data of compounds 10a-f are listed in Tables S9 and S10, respectively, and those of urea-methyl esters 12e and 12f are depicted in Figure 2. Isourea carboxylate methyl esters 10a-f or 12e,f were reacted with methylamine to afford their corresponding isourea amides 11a-f or urea-amides 13e,f in 60-97% yield or 20% and 46% yields, respectively. The 1 H and 13 C NMR spectra of compounds 11a-f are listed in Tables S11 and S12 and those of compounds 13e,f are depicted in Figure 2. The 1 H NMR spectrum of isourea-amides 11a-f show three deuterium labile hydrogen atoms in the 8.4-10.9, 7.0-10.6, and 7.1-8.1 ppm ranges, as well as the characteristic doublet in 2.5-3.0 ppm range for the NHMe group. The C4 NMR frequencies were found at approximately 110 ppm in both NMe esters 12e,f and their amides 13e,f. The 10 ppm shift to low frequencies compared with their NH analogues 10a-f and 11a-f was due to the electronic effect of the NMe group on C4. In compounds 13e and 13f, the urea NH appears as a doublet at 5.9 ( 3 J = 9.3 Hz) and 5.7 ppm ( 3 J = 8.2 Hz); and the amide NH appears at lower frequency as a quartet at 6.6 ( 3 J = 4.4 Hz) and 6.4 ppm ( 3 J = 4.7 Hz), respectively.

S
Isothiourea carboxylate methyl esters 8a-f contain both SMe and OMe groups, which are susceptible to substitution with nucleophiles such as methylamine. The reaction of compounds 8a with one molar equivalent of methylamine produces a complex mixture of several methylated compounds. However, in the presence of an excess of methylamine, the isourea-amide compound 11a precipitated as a white solid. In these conditions, the SMe group was substituted because of the formation of MeNH3OH in aqueous medium. The last procedure was also used with the urea methyl esters 8b-f and 9e,f to obtain compounds 11b-f and 13e,f in 45-60% yields.

Molecular Structure of Compounds 8a-c and 9f
The SMe-isothiourea methyl esters (S)-8a-c and (S)-9f were purified by crystallization from ethanol and suitable crystals for X-ray diffraction analysis were isolated. The molecular structures of compounds 8a and 8c, displayed in Figures 3 and 4, show that the N12H is engaged in intramolecular three-centered hydrogen bonding interaction with the benzothiazole nitrogen and carbonyl oxygen atoms. The distances and angles associated with this N3•••H12•••O14 interaction are N12H•••N3 = 2.01 Å, 132° (8a) and 2.03 Å, 132° (8c); N12H•••O14 = 2.31 Å, 107° (8a) and 2.21 Å, 111° (8c), forming the corresponding adjacent six (S6) and five (S5)-membered rings. This hydrogen bonding interaction fixes the stereochemistry of the imine N10-C11 bond and only the (E) isomer of 8a and 8c was produced. In addition, the lateral side chain is almost in the same plane of the benzothiazole, including the carbon atoms of both OMe and SMe groups. In general, the SMe group is the most deviated from the mean plane [N(  Isourea carboxylate methyl esters 10a-f or 12e,f were reacted with methylamine to afford their corresponding isourea amides 11a-f or urea-amides 13e,f in 60-97% yield or 20% and 46% yields, respectively. The 1 H and 13 C NMR spectra of compounds 11a-f are listed in Tables S11 and S12 and those of compounds 13e,f are depicted in Figure 2. The 1 H NMR spectrum of isourea-amides 11a-f show three deuterium labile hydrogen atoms in the 8.4-10.9, 7.0-10.6, and 7.1-8.1 ppm ranges, as well as the characteristic doublet in 2.5-3.0 ppm range for the NHMe group. The C4 NMR frequencies were found at approximately 110 ppm in both NMe esters 12e,f and their amides 13e,f. The 10 ppm shift to low frequencies compared with their NH analogues 10a-f and 11a-f was due to the electronic effect of the NMe group on C4. In compounds 13e and 13f, the urea NH appears as a doublet at 5.9 ( 3 J = 9.3 Hz) and 5.7 ppm ( 3 J = 8.2 Hz); and the amide NH appears at lower frequency as a quartet at 6.6 ( 3 J = 4.4 Hz) and 6.4 ppm ( 3 J = 4.7 Hz), respectively.
Isothiourea carboxylate methyl esters 8a-f contain both SMe and OMe groups, which are susceptible to substitution with nucleophiles such as methylamine. The reaction of compounds 8a with one molar equivalent of methylamine produces a complex mixture of several methylated compounds. However, in the presence of an excess of methylamine, the isourea-amide compound 11a precipitated as a white solid. In these conditions, the SMe group was substituted because of the formation of MeNH 3 OH in aqueous medium. The last procedure was also used with the urea methyl esters 8b-f and 9e,f to obtain compounds 11b-f and 13e,f in 45-60% yields.

Materials and Methods
Melting points were measured on an IA 9100 apparatus (Electrothermal, Staffordshire, UK) and are uncorrected. IR spectra were recorded using a 3100 FT-IR Excalibur Series spectrophotometer (Varian, Randolph, MA, USA) equipped with an ATR system. Mass spectra were obtained in a 3900-GC Crystals suitable for X-ray analysis of 8a, 8b, 8c, and 9f were obtained after solvent evaporation from saturated ethanol solutions. Single-crystal X-ray diffraction data were recorded on a D8 Quest CMOS (Bruker, Karlsruhe, Germany) or Nonius Kappa (Rotterdam, the Netherlands) area detector diffractometers with Mo K α radiation, λ = 0.71073 Å. A table listing the crystallographic data is provided as Supplementary Material in Table S2. The structures were solved by direct methods using SHELXS97 [50] program of WinGX package [51]. The final refinement was performed by full-matrix least-squares methods on F2 with SHELXL97 [50] program. H atoms on C were geometrically positioned and treated as riding atoms, with C-H = 0.93-0.98 Å, and with Uiso(H) = 1.2Ueq(C). The program Mercury was used for visualization, molecular graphics, and analysis of crystal structures [52]. The software used to prepare material for publication was PLATON [53]. Crystallographic data for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC numbers 1949131 (8a), 1949129 (8b), 1413575 (8c) and 1949130 (9f). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-01-223-336-033 or E-Mail: deposit@ccdc.cam.ac.uk).

General Method for Isothiourea Carboxylates 5a-f
Amino acid (3.94 mmol), NaOH (3.94 mmol), and 15 mL of ethanol were added into a 100 mL round flask and the mixture was stirred for 2 h at room temperature. Then, compound 2 (1.0 g, 3.94 mmol) was added and the mixture was stirred for additional 96 h at room temperature.

Sodium (E)-(3-Benzothiazol-2-Yl-2-Methyl-Isothioureido)-Acetate 5a
As a general method, starting from 0.295 g of glycine, compound 5a precipitated from the reaction mixture, ethanol was eliminated, the resulting mixture was cooled to room temperature and 10 mL of acetone were added, the resulting suspension was filtered and washed with cold acetone, obtaining 5a.3H 2 O as a cream color powder (1.14 g, 82%); mp = 220 • C (dc).
As a general method, starting from 0.594 g of l-phenylglycine, ethanol was eliminated from the reaction mixture and then suspended in acetone (10 mL). The suspension was filtered and the remaining solid washed with cold acetone and dried to obtain a white powder (1.29 g, 86%).
As a general method, starting from 0.650 g of l-phenylalanine, from the same procedure as 5b, compound 5d was obtained as brownish liquid (1.26 g, 81%).

General Method for Isourea Carboxylates 6a-f
Starting from isothiourea carboxylates 5a-f (3.0 mmol), 10 mL of ethanol and 10 mL of water were added into a 100 mL round flask and the mixture was refluxed for 72 h. For isothiourea 5a, DMF was used instead of ethanol. The solvent was evaporated from the reaction mixture and the residue suspended in acetone, the suspension was cooled and filtered, the precipitate was washed with acetone and dried. In the hydrolysis of isothioureas 5e,f, the reaction mixture was filtered and the solvents were evaporated. 5 mL of acetone were added and 50 mL of CHCl 3 were slowly added, the mixture was stirred until a beige solid precipitates, which was filtered and washed with CHCl 3 to get 6e,f.

Sodium (3-Benzothiazol-2-Yl-Isoureido)-Phenyl-Acetate 6c
White powder; 0.75 g, 72% yield; mp = 250 • C (dc). In a 100 mL round flask, 3.0 mmol of the corresponding isothiourea carboxylate 5a-f or urea carboxylate 6a-f were dissolved in DMF (10 mL), methyl iodide (3.5 mmol) were added and the mixture was stirred for 12 h on an ice bath and then 12 h at room temperature. At the end of the reaction, 50 mL of water were added and the corresponding ester was extracted with CHCl 3 . Chloroform was eliminated and the O-methyl compounds were purified by crystallization in ethanol. Compounds 9e or 9f were separated from 8e or 8f by crystallization from their ethanol mixture. Compounds 12e or 12f were separated from 10e or 10f using a chloroform/acetone 10:1 mixture in a silica gel chromatography column. 12e and 12f were precipitated from hexane.

Conclusions
A six sodium salt series of isothiourea-carboxylate benzothiazoles 5a-f, as well as their methyl ester derivatives 8a-f, were obtained in moderate to good yields by the reaction of dimethylcarbonimidate benzothiazole 2 with sodium salts of glycine, (l)-alanine, (l)-phenylglycine, (l)-phenylalanine, (l)-valine, and (l)-leucine in stirring ethanol at room temperature and further methylation under mild conditions. The reaction is stereo selective, only the E-isomer was isolated, the X-ray structure of (R,E)-methyl-2-((benzothiazol-2-ylimino)(methyl-thio)methylamino)-2-phenylacetate 8c confirmed the stereochemistry of the reaction. The structures of 8a and 8c are stabilized by three center hydrogen bonding interactions N3···H12···O14 between the amino N12H12 with the nitrogen atom of benzothiazole ring and the oxygen atom of the carbonyl group, forming two intramolecular adjacent S(6) and S(5) rings, respectively. This finding suggests the stereochemical assistance of the reaction by hydrogen bonding. When the same reactions were carried out in the presence of water, the urea-carboxylate benzotiazoles 6a-f were obtained. Their further methylation produced the corresponding methyl esters 10a-f. In the methylation reaction of sodium isothiourea-carboxylates 5e,f and urea-carboxylates 6e,f, the corresponding N3Me methyl esters 9e,f and 12e,f were produced as byproducts, which were isolated. Methyl esters 8a-f or 10a-f and 9e,f or 12e,f were used as starting materials to produce the corresponding urea carboximides 11a-f and 13e,f by the reaction with methyl amine. Further studies on the synthesis of chiral guanidines from SMe-isothioureas 8 are in progress.