Aminomethylation of Symmetric Dialkylthioureas with Formaldehyde and Amino Acids

Abstract

Condensation of 1,3-dialkylthiourea, formaldehyde, and terminal amino acids (C₂, C₃, or C₄) taken in the 1 : 2 : 1 molar ratio has afforded the terminally substituted (3,5-dialkyl-4-thioxo-1,3,5-triazinan-1-yl)carboxylic acids.

Well-studied aminomethylation of unsubstituted thiourea with formaldehyde and С₂–С₄ terminal amino acids yields cyclic thioureas—terminally substituted (4-thioxo-1,3,5-triazinan-1-yl)carboxylic acids [1, 2]. These compounds are not formed from unsubstituted urea and the same aminomethylating agents, likely due to faster polycondensation of urea with formaldehyde catalyzed by amino acids, whereas symmetrical dialkylureas readily undergo the aminomethylation reaction with the formation of the corresponding terminally substituted (3,5dialkyl-4-oxo-1,3,5-triazinan-1-yl)carboxylic acids [3].

This study aimed to prepare the thio analogs of those compounds, namely (3,5-dialkyl-4-thioxo-1,3,5-triazinan-1-yl)carboxylic acids, via the condensation of symmetrical dialkylthioureas with formaldehyde and С₂–С₄ terminal amino acids. The reaction was of practical interest in view of the preparation of prodrug forms [4] of active pharmaceuticals containing the thiourea moiety –HNC(=S)NH–.

It was found that, as in the case of the unsubstituted thiourea, 1,3-dimethylthiourea 1 and 1,3-diethylthiourea 2 readily reacted with the simplest amino acids (glycine, β-alanine, and γ-aminobutyric acid) in aqueous formaldehyde to give well-crystallized terminally substituted (3,5-dialkyl-4-thioxo-1,3,5-triazinan-1-yl)carboxylic acids 3a–c and 4a–4c in 76–93% yield (Scheme 1).

Scheme

1.

Structure of the obtained compounds was confirmed by means of IR and NMR spectroscopy as well as mass spectrometry. The ¹Н NMR spectra of compounds 3 and 4 contained a singlet of cyclic СH₂ protons at 4.24–4.31 ppm and signals of the protons of the amino acid moieties and the N-alkyl groups of the respective intensity (Table 1). The signals of the carboxyl group protons were not observed, likely due to fast exchange with the protons of residual water in the solvent. The ¹³С NMR spectra contained the signals of the carbon atoms of the cyclic (66.2–68.8 ppm) and carboxylic (171.5–174.8 ppm) groups as well as the amino acid and N-alkyl moieties (Table 1). The IR spectra revealed the characteristic signals of the thioureia fragment at 1512–1530 cm^–1 and the carboxylic group at 1691–1738 cm^–1. The molecular masses of compounds 3 and 4 determined by means of mass spectrometry corresponded to the expected values.

Table 1. and 4a–4c in DMSO-d₆

Table 2. Crystallographic data for 3a, 4a–4c

The crystal structures of compounds 3a and 4a–4c were confirmed by means of single crystal X-ray diffraction analysis (Table 2). As in the case of the oxo analogs [1], five of the six cycle atoms in compounds 3a, 4a–4c are in the same plane, with the nitrogen atom of the amino acid moiety deviating from it. The ethyl groups and the amino acid fragments are to the same side of the triazinane cycle in the ethyl derivatives 4a–4c. In contrast, the ethyl and amino acid groups are to the opposite sides of the cycle in the oxo analog of compound 4a [1].

The crystallographically unique part of the unit cell of compound 4a contains two molecules, which are not coupled via any symmetry element. The molecules of each type are linked in the infinite chains via the intermolecular H-bonds between the carboxylic groups and the sulfur atoms of the thiocarbamide moiety O^13A–H^13A···S^1A and O¹³–H¹³···S¹ (Fig. 1). Each of the chains forms a spiral column. Hence, the crystal structure consists of two types of the spiral columns. Both spirals are twisted in the same directions, and their axes being aligned with the b axis. The S^1A···H^5A, S^1A···H^11A, and S¹···H^3A, S¹···H^5B van der Waals contacts are formed between the spiral columns.

Fig. 1.

Intermolecular hydrogen bonds in the infinite chains in the structure of compound 4a.

The molecules of the oxo analog of that compound as well as of the oxo analog of the methyl derivative 3a are linked into the similar infinite chains, via the intermolecular H-bonds between the carboxylic groups and the oxygen atoms of the carbamide fragment [1].

The carboxylic H atom is disordered over two crystallographically unique positions (H¹⁵ : H¹⁶ = 0.5 : 0.5) between two carboxylic O atoms with the total site occupancy factor equal to 1.0 in the molecules of compounds 4b and 4c (Fig. 2). Another special feature of the structure of compounds 4b and 4c was the formation of the dimers in the crystal, due to the intermolecular hydrogen bonding (Fig. 2 and Table 3). The dimeric structures were found in the crystal of compound 3a as well. It should be noted that the planes of the carboxylic groups in the structure of compound 3a were coplanar and approximately coplanar with the (131) plane, whereas the carboxylic groups in the structure of compound 4c were approximately coplanar with the (212) and (2-12), the angle between which equaled ~51.8(1)°, likely due to the longer amino acid branch.

Fig. 2.

Intermolecular hydrogen bonds in the dimers in the structure of compound 4c.

Table 3. Parameters of hydrogen bonds in the unit cell of compounds 3a, 4a–4c^a

In summary, symmetrical dialkylthioureas, similarly to their oxo analogs and the unsubstituted thiourea, readily underwent the aminomethylation with formaldehyde and С_2–4 terminal amino acids with the formation of the terminally substituted (3,5-dialkyl-4-thioxo-1,3,5-triazinan-1-yl)carboxylic acids. The studied reaction can serve as a model in the development of the prodrug forms of pharmaceuticals bearing a thioureia moiety—HNC(=S)NH–.

EXPERIMENTAL

IR spectra were recorded using a Shimadzu FTIR-8400S spectrometer (KBr pellets). ¹Н and ¹³С NMR spectra were recorded using a Bruker Avance III-400 spectrometer (400 and 100 MHz) in DMSO-d₆ with the residual protons and ¹³С nuclei of DMSO-d₆ as internal references. Mass spectra were obtained using a MaXis 62 chromato-mass spectrometer (Bruker Daltonik GmbH) equipped with an electrospray ionization source (4.5 eV) and a quadrupole time-of-flight analyzer (ESI-QTOF) in the positive ions detection mode, with methanol as solvent (Resource Center of St. Petersburg State University “Methods of Analysis of Compounds Composition”).

The unit-cell and refinement parameters for 3a, 4a–4c as well are listed in Table 2. The structures were solved via the direct method and refined under anisotropic approximation for the non-hydrogen atoms using the full-matrix least squares method implemented in SHELXL 2018 software package [5]. The XRD measurements have been performed at the X-ray Diffraction Centre of the St. Petersburg State University.

2-(3,5-Dimethyl-4-thioxo-1,3,5-triazinan-1-yl)acetic acid (3a). 1.50 g (0.02 mol) of glycine was added in a single portion to a solution of 2.08 g (0.02 mol) of dimethylthiourea (1) in 3.36 g (0.04 mol) of 36% formalin, and the mixture was stirred until complete dissolution. The mixture was left overnight; the transparent rhombic crystals were filtered off and dried. Yield 3.27 g (80.5 %), mp 170°С (C₂H₅OH). IR spectrum, ν, cm^–1: 1525 s (C=S + C–N), 1711 s (C=O), 2880, 2932 w (CH₂), 2400– 3200 br (ОН). ¹H NMR spectrum, δ, ppm: 3.14 s (6H, NCH₃), 3.44 s (2H, NCHСOOH), 4.31 (4H, CH_2cycle), 12.57 s (1H, OH). ¹³С{¹Н} NMR spectrum, δ_C, ppm: 39.66 (CH₃), 52.31 (NCH₂COO), 68.76 (NCH₂N), 178.85 (C=S), 171.58 (COOH). Mass spectrum, m/z: 204.0801 [M + H]⁺ (calculated for C₇H₁₃N₃O₂S: 204.0801), 226.0624 [M + Na]⁺ (calculated for C₇H₁₃N₃NaO₂S: 226.0621).

3-(3,5-Dimethyl-4-thioxo-1,3,5-triazinan-1-yl)propanoic acid (3b). 1.78 g (0.02 mol) of β-alanine was added in a single portion to a solution of 2.08 g (0.02 mol) of dimethylthiourea (1) in 3.36 g (0.04 mol) of 36% formalin, and the mixture was stirred until complete dissolution. White crystalline precipitate was formed after 20 min. The mixture was left for 2 days; the precipitate was filtered off and dried in a vacuum dessicator. Yield 3.78 g (87.1%), mp 169°С (C₂H₅OH). IR spectrum, ν, cm^–1: 1519 s (C=S + C–N), 1711 s (C=O), 2868, 2932 w (CH₂), 2560–3360 br (ОН). ¹H NMR spectrum, δ, ppm: 2.47 t (2H, NCH₂CH₂СOOH, J = 7.0 Hz), 2.84 t (2H, NCH₂CH₂СOOH, J = 7.0 Hz), 3.15 s (6H, NCH₃), 4.27 s (4H, СH_2cycle). ¹³С{¹Н} NMR spectrum, δ_C, ppm: 39.64 (CH₃), 33.53 (NCH₂CH₂COOH), 47.19 (NCH₂CH₂COOH), 68.49 (NCH₂N), 178.68 (C=S), 173.73 (COOH). Mass spectrum, m/z: 218.0956 [M + H]⁺ (calculated for C₈H₁₅N₃O₂S: 218.0957), 240.0773 [M + Na]⁺ (calculated for C₈H₁₅N₃NaO₂S: 240.0777).

4-(3,5-Dimethyl-4-thioxo-1,3,5-triazinan-1-yl)butanoic acid (3c). 3.40 g (0.033 mol) of γ-aminobutyric acid was added in a single portion to a solution of 3.43 g (0.033 mol) of dimethylthiourea (1) in 5.55 g (0.066 mol) of 36% formalin, and the mixture was stirred until complete dissolution. The reaction mixture was left overnight, transferred on a Petri dish, and evaporated under warm air stream. The crystalline precipitate formed after 2 h was left overnight, filtered off, and dried in vacuum. Yield 5.82 g (76.2%), mp 137–139°С (C₂H₅OH). IR spectrum, ν, cm^–1: 1530 s (C=S + C–N), 1705 s (C=O), 2904, 2935 w (CH₂), 2500–3280 br (ОН). ¹H NMR spectrum, δ, ppm: 1.70 m (2H, CH₂CH₂CH₂), 2.26 t (2H, CH₂COOH, J = 7.2 Hz), 2.60 t (2H, NCH_2, J = 7.2 Hz), 3.14 s (6H, CH₃), 4.26 s (4H, CH_2cycle). ¹³С{¹Н} NMR spectrum, δ_C, ppm: 23.25 (NCH₂CH₂CH₂COOH), 31.70 (NCH₂CH₂CH₂COOH), 39.66 (CH₃), 50.38 (NCH₂CH₂CH₂COOH), 68.42 (NCH₂N), 178.64 (C=S), 174.80 (COO). Mass spectrum, m/z: 232.1114 [M + H]⁺ (calculated for C₉H₁₇N₃O₂S: 232.1114), 254.0934 [M + Na]⁺ (calculated for C₉H₁₇N₃NaO₂S: 254.0934).

2-(3,5-Diethyl-4-thioxo-1,3,5-triazinan-1-yl)acetic acid (4a) was obtained similarly to compound 3a from 1.32 g (0.01 mol) of diethylthiourea 2, 1.68 g (0.02 mol) of 36% formalin, and 0.75 g (0.01 mol) of glycine. Yield 2.15 g (93.0%), mp 129–131°С (acetone). IR spectrum, ν, cm^–1: 1528 s (C=S + C–N), 1706, 1738 s (C=O), 2937, 2967, 2985 w (CH₂), 2480–3360 br (ОН). ¹H NMR spectrum, δ, ppm: 1.05 t (6H, NCH₂CH₃, J = 7.0 Hz), 3.35 s (2H, NCH₂СOOH), 3.68 q (4H, NCH₂CH₃, J = 7.0 Hz), 4.30 s (4H, CH_2cycle). ¹³С{¹H} NMR spectrum, δ_C, ppm: 171.51 (СOOH), 176.44 (С=S), 66.45 (NCH₂N), 51.71 (NCH₂COOH), 45.55 (NCH₂CH₃), 12.54 (NCH₂CH₃). Mass spectrum, m/z: 232.1102 [M + H]⁺ (calculated for C₉H₁₇N₃O₂S: 232.1114), 254.0922 [M + Na]⁺ (calculated for C₉H₁₇N₃NaO₂S: 254.0934).

3-(3,5-Diethyl-4-thioxo-1,3,5-triazinan-1-yl)propanoic acid (4b) was obtained similarly to compound 3b from 1.32 g (0.01 mol) of diethylthiourea 2, 1.68 g (0.02 mol) of 36% formalin, and 0.89 g (0.01 mol) of β-alanine. Yield 2.23 g (90.9%), mp 117–119°С (acetone). IR spectrum, ν, cm^–1: 1512 s (C=S + C–N), 1691, 1714 s (C=O), 2929, 2958, 2985 w (CH₂), 2480–3360 br (ОН). ¹H NMR spectrum, δ, ppm: 1.07 t (6H, NCH₂CH₃, J = 7.0 Hz), 2.47 t (2H, NCH₂CH₂СOOH, J = 7.1 Hz), 2.75 t (2H, NCH₂CH₂СOOH, J = 7.1 Hz), 3.70 q (4H, NCH₂CH₃, J = 7.0 Hz), 4.25 s (4H, CH_2cycle). ¹³С{¹H} NMR spectrum, δ_C, ppm: 173.65 (СOOH), 176.48 (С=S), 66.35 (NCH₂N), 46.82 (NCH₂CH₂COOH), 45.62 (NCH₂CH₃), 33.49 (NCH₂CH₂COOH), 12.71 (NCH₂CH₃). Mass spectrum, m/z: 246.1272 [M + H]⁺ (calculated for C₁₀H₁₉N₃O₂S: 246.1270), 268.1090 [M + Na]⁺ (calculated for C₁₀H₁₉N₃NaO₂S: 268.1090).

4-(3,5-Diethyl-4-thioxo-1,3,5-triazinan-1-yl)butanoic acid (4c) was obtained similarly to compound 3c from 1.32 g (0.01 mol) of diethylthiourea 2, 1.68 g (0.02 mol) of 36% formalin, and 1.03 g (0.01 mol) of γ-aminobutyric acid. Yield 2.22 g (85.6%), mp 81–83°С. IR spectrum, ν, cm^–1: 1517 s (C=S + C–N), 1695, 1710 s (C=O), 2895, 2920, 2980 w (CH₂), 2590–3370 br (ОН). ¹H NMR spectrum, δ, ppm: 1.06 t (6H, NCH₂CH₃, J = 7.0 Hz), 1.71 m (2H, NCH₂CH₂CH₂СOOH), 2.27 t (2H, NCH₂CH₂CH₂СOOH, J = 7.2 Hz), 2.53 t (2H, NCH₂CH₂CH₂СOOH, J = 7.1 Hz), 3.68 q (4H, NCH₂CH₃, J = 7.0 Hz), 4.24 s (4H, CH_2cycle). ¹³С{¹H} NMR spectrum, δ_C, ppm: 174.77 (СOOH), 176.41 (С=S), 66.21 (NCH₂N), 49.98 (NCH₂CH₂CH₂COOH), 45.59 (NCH₂CH₃), 23.21 (NCH₂CH₂CH₂COOH), 31.69 (NCH₂CH₂CH₂COOH), 12.70 (NCH₂CH₃). Mass spectrum, m/z: 282.1237 [M + Na]⁺ (calculated for C₁₁H₂₁N₃NaO₂S: 282.1247).