Synthesis of 2-Aminoethanesulfonamides of Betulinic and Betulonic Acids

New potentially biologically active sulfonamide derivatives of pentacyclic lupane-type triterpenoids, the sulfonamide group of which was bonded to C-17 of the triterpene skeleton through an amidoethane spacer, were synthesized via conjugation of 2-aminoethanesulfonamides to betulinic and betulonic acids in the presence of Mukaiyama reagent (2-bromo-1-methylpyridinium iodide).

Betulinic and betulonic acids are lupane-type triterpene acids with broad spectra of inherent biological activity, synthetic modification of which led to a series of compounds with activities surpassing those of the natural precursors [1, 2].

In continuation of work on the modification of lupane-type triterpenoids, sulfonamide derivatives of betulinic and betulonic acids were synthesized by us.

The sulfonamide motif was introduced into the structures of drugs [3, 4] and various biologically active compounds with a broad range of activity including antibacterial, anti-inflammatory, antitumor, and antiviral [5, 6] against important viral infections (HIV-1, HCV, SARS-CoV, DENV2) [7,8,9].

Derivatives of lupane triterpenoids containing a sulfonamide in the C-17 side chain or in the C-3 position with antitumor [10] and antiviral activity against HIV-1 virus have been reported [11, 12].

Herein, we report the synthesis of sulfonamides of lupane triterpenoids, the sulfonamide fragment of which is bonded to C-17 of the triterpene skeleton through an amidoethane spacer.

The starting compounds were betulinic (1) and betulonic acids (2) and 2-aminoethanesulfonamides 3a–c, the combination of which in the presence of Mukaiyama reagent (2-bromo-1-methylpyridinium iodide) synthesized the target 2-amidoethanesulfonamides 4a–c and 5a–c.

2-Aminoethanesulfonamides 3a–c (as the hydrochlorides) were prepared by reacting phthalic anhydride with 2-aminoethanesulfonic acid (taurine) in the presence of NaOAc–AcOH by the literature method [13] followed by treatment of the obtained Na-salt of 2-phthalimidoethanesulfonic acid with PCl₅ and combination of the resulting chloride with amines (NHMe₂, piperidine, morpholine). The phthalyl protection was removed using NH₂NH₂ in EtOH to give the corresponding 2-aminoethanesulfonamides [14, 15].

Mukaiyama reagent (2-bromo-1-methylpyridinium iodide) was prepared by treating 2-bromopyridine with MeI [16, 17]. 2-Amidoethanesulfonamides 4a–c and 5a–c were synthesized via conjugation of 2-aminoethanesulfonamides 3a–c (as the hydrochlorides) with acids 1 and 2, the carboxylic group of which was activated by Mukaiyama reagent [18]. The reaction was carried out in refluxing CH₂Cl₂ for 1 h in the presence of Et₃N, Bu₃N, or i-Pr2EtN. Target compounds 4a–c and 5a–c were isolated by chromatography over silica gel in 28–67% yields.

The structures of sulfonamides 4a–c and 5a–c were confirmed by IR, PMR, and ¹³C NMR spectroscopy and mass spectrometry. Resonances in PMR and ¹³C NMR spectra of sulfonamide 4a were completely assigned using 1D and 2D experiments.

The combination of acids 1 and 2 with taurine in the presence of various condensing agents, conversion of the obtained 2-amidoethanesulfonic acids into the chlorides, and reaction of them with amines could serve as an alternative synthesis of sulfonamides 4a–c and 5a–c. Combination of the carboxylic acids with the amines through the action of diethyl phosphorocyanidate (DEPC) in the presence of Et₃N was an effective method for preparing the amides [19]. However, compound 6, the mixed anhydride of 1 and diethyl phosphoric acid that did not subsequently react with taurine, was obtained by us in 85% yield upon reaction of 1 with taurine in DMF in the presence of DEPC–Et₃N. Replacing 1 by sulfonamide 3a under the same conditions led to the formation of anhydride 6 (93%) together with 4a, which was isolated in 5% yield.

The structure of mixed anhydride 6 was confirmed using 1D and 2D PMR, ¹³C NMR, and ³¹P NMR spectra. The PMR spectrum contained characteristic resonances for the ethyls of the diethyl phosphate group as a triplet at 1.35 ppm (6H, J = 7 Hz, 2OCH₂CH₃) and a multiplet at 4.27 ppm (4H, OCH₂CH₃) that correlated with the ³¹P resonance at –7.51 ppm in the 2D ¹H–³¹P HMBC spectrum. The ³¹P resonance in the ³¹P NMR spectrum appeared as a pentet with J_H–P = 7.5 Hz.

Thus, a series of 2-amidoethanesulfonamides, new potentially biologically active derivatives of lupane-type triterpenoids, were synthesized from betulinic and betulonic acids.

Experimental

IR spectra were recorded in Vaseline oil on an IR Prestige-21 spectrophotometer (Shimadzu). PMR, ¹³C NMR, and ³¹P NMR spectra were taken at 295 K on an AMXIII-300 (Bruker, Germany) at operating frequency 300.13 MHz for ¹H and 75.47 MHz for ¹³C or an Avance III-500 spectrometer (Bruker, Germany) at operating frequency 500.13 and 125.47 MHz, respectively, and 202.46 MHz (³¹P). Chemical shifts in ¹³C NMR and PMR spectra were given in ppm vs. CD(H)Cl₃ resonances (δ_H 7.27 ppm, δ_C 77.1 ppm) or TMS internal standard. Chemical shifts of ³¹P resonances were determined vs. phosphoric acid (85%). Mass spectra of positive and negative ions were recorded using APCI or ESI method in an LCMS-2010EV liquid chromatograph (Shimadzu). Rotation angles were measured on a Perkin-Elmer 341C polarimeter. Column chromatography used SiO₂ (L brand, 40/60 μm, Russia); TLC, Sorbfil plates (Imid LLC, Russia). Chromatograms were visualized using anisic detector. Melting points were measured on a Boetius apparatus (Germany).

Preparation of 2-Amidoethanesulfonamides (4a–c, 5a–c). General Method. A suspension of 2-bromo-1-methylpyridinium iodide (0.53 mmol) in CH₂Cl₂ (2 mL) was treated dropwise with acid 1 or 2 (0.44 mmol), the appropriate sulfonamide chloride 3a–c (0.44 mmol), and amine (1.49 mmol) (Et₃N, for 5a and 5c; Bu₃N, for 4a, 4b, and 4c; i-Pr₂EtN, for 5b) in CH₂Cl₂ (10 mL). The mixture was refluxed for 1 h, cooled, diluted with methyl-tert-butylether (MTBE) (50 mL), and washed with HCl solution (5%, 3 × 10 mL). The organic layer was separated, washed with H₂O, dried over Na₂SO₄, and evaporated. The solid was chromatographed over SiO₂ (C₆H₆, C₆H₆–MTBE, 8:1).

N -[2-( N , N -Dimethylsulfamoyl)ethyl]-3β-hydroxylup-20(29)-en-17β-carboxamide (4a). Yield 67%, mp 140–143°C; [α]_D²⁰ –0.3° (c 0.559, CHCl₃). IR spectrum (ν, cm^–1): 1140, 1334, 1517, 1597, 1640, 1686, 3387. ¹H NMR spectrum (500 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.67 (1H, d, J = 9.8, H-5), 0.75, 0.81, 0.93 (3H each, s, CH₃-24, 25, 26), 0.96 (8H, s, CH₃-23, 27, H_a-12, 15), 1.25 (3H, m, H-9, H_a-11, 16), 1.35 (2H, m, 2H-7), 1.38 (1H, m, H_a-21), 1.42 (3H, m, H-13, H_b-11, H_a-6), 1.56 (4H, m, 2H-2, H_b-6, H-18, H_a-22), 1.67 (1H, m, H_b-1), 1.71 (4H, m, H_b-15, CH₃-30), 1.79 (1H, m, H_b-22), 1.92 (1H, m, H_b-21), 1.98 (1H, m, H_b-16), 2.42 (1H, td, J = 13.0, 4.5, H-13), 2.89 (6H, s, N(CH₃)₂), 3.00 (2H, m, NCH₂CH₂SO₃), 3.08 (1H, td, J = 10.7, 4.4, H-19), 3.19 (1H, dd, J = 11.1, 5.6, H-3), 3.73 (2H, m, NCH₂CH₂SO₃), 4.59, 4.74 (1H each, s, both H-29), 6.50 (1H, br.s, NH). ¹³C NMR spectrum (125 MHz, CDCl₃, TMS, δ, ppm): 14.63 (C-27), 15.39 (C-24), 16.15 (C-25), 16.15 (C-26), 18.29 (C-6), 19.42 (C-30), 20.16 (C-11), 25.09 (C-12), 25.57 (C-15), 27.37 (C-2), 27.99 (C-23), 29.40 (C-16), 30.81 (C-21), 33.40 (C-22), 33.45 (NHCH₂), 34.37 (C-7), 37.18 (C-10), 37.44 (NMe₂), 37.72 (C-13), 38.70 (C-1), 38.85 (C-4), 40.75 (C-8), 42.44 (C-14), 46.72 (C-19), 47.17 (CH₂SO₂), 49.96 (C-18), 50.59 (C-9), 55.33 (C-5), 55.73 (C-17), 78.93 (C-3), 109.44 (C-29), 150.84 (C-20), 176.66 (C-28). ESI-MS, m/z 591 [M + H]⁺ (100%) (calcd for C₃₄H₅₈N₂O₄S, 590).

N -[2-(Piperidin-1-ylsulfonyl)ethyl]-3β-hydroxylup-20(29)-en-17β-carboxamide (4b). Yield 40%, mp 218–220°C; [α] +8.5° (c 0.33, CHCl₃). IR spectrum (ν, cm^–1): 1139, 1335, 1514, 1641, 1685, 3422. ¹H NMR spectrum (300 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.68 (1H, d, J = 9.8, H-5), 0.75, 0.80, 0.93, 1.67 (3H each, s, CH₃-24, 25, 26, 30), 0.96 (6H, s, CH₃-23, 27), 1.97 (3H, m, H_b-16, 21, 22), 2.42 (1H, m, H-13), 3.01 (2H, m, NCH₂CH₂SO₃), 3.08 (1H, m, H-19), 3.21 (5H, m, H-3, piperidine: 2H-2, 6), 3.71 (2H, br.s, NCH₂CH₂SO₃), 4.59, 4.73 (1H each, s, both H-29), 6.46 (1H, br.s, NH). ¹³C NMR spectrum (75 MHz, CDCl₃, TMS, δ, ppm): 14.64 (C-27), 15.38 (C-24), 16.13 (C-25), 16.16 (C-26), 18.29 (C-6), 19.42 (CH₃-30), 20.80 (C-11), 23.68 (piperidine: C-4), 25.57 (C-15), 25.57 (C-12), 25.57 (piperidine: C-3, 5), 27.40 (C-2), 27.99 (C-23), 29.43 (C-16), 30.84 (C-21), 33.47 (C-22), 33.47 (NHCH₂CH₂SO₃), 34.40 (C-7), 37.21 (C-10), 37.76 (C-13), 38.72 (C-1), 38.87 (C-4), 40.87 (C-8), 42.47 (C-14), 46.60 (piperidine: C-2, 6), 46.75 (C-19), 48.42 (NHCH₂CH₂SO₃), 49.98 (C-18), 50.61 (C-9), 55.37 (C-5), 55.76 (C-17), 78.98 (C-3), 109.47 (C-29), 150.87 (C-20), 176.61 (C-28). APCI-MS, m/z: 631 [M + H]⁺ (100%), 613 [M + H – H₂O]⁺ (13.3%), 546 [M – NC₅H₁₀]⁺ (2.2%) (calcd for C₃₇H₆₂N₂O₄S, 630).

N -[2-(Morpholinosulfonyl)ethyl]-3β-hydroxylup-20(29)-en-17β-carboxamide (4c). Yield 30%, mp 115–117°C; [α]_D²⁰ +10° (c 0.128, CHCl₃). IR spectrum (ν, cm^–1): 1115, 1152, 1641, 3400. ¹H NMR spectrum (300 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.66 (1H, d, J = 9.4, H-5), 0.73, 0.79, 0.89, 1.66 (3H each, s, CH₃-24, 25, 26, 30), 0.91 (6H, s, CH₃-23, 27), 2.45 (1H, m, H-13), 3.04 (3H, m, NCH₂CH₂SO₃, H-19), 3.15 (1H, dd, J = 10.5, 5.4, H-3), 3.23 (2H, m, morpholine: H-2, 6), 3.75 (6H, m, NCH₂CH₂SO₃, morpholine: 2H-3, 5), 4.57, 4.72 (1H each, s, both H-29), 6.39 (1H, t, J = 5.5, NH). ¹³C NMR spectrum (75 MHz, CDCl₃, TMS, δ, ppm): 14.60 (C-27), 15.40 (C-24), 16.15 (C-25), 16.15 (C-26), 18.30 (C-6), 19.40 (CH₃-30), 20.74 (C-11), 25.10 (C-12), 25.60 (C-15), 27.40 (C-2), 28.00 (C-23), 29.40 (C-16), 30.85 (C-21), 33.38 (C-22), 33.42 (NHCH₂CH₂SO₃), 34.40 (C-7), 37.20 (C-10), 37.73 (C-13), 38.75 (C-1), 38.86 (C-4), 40.78 (C-8), 42.46 (C-14), 45.70 (morpholine: C-2, 6), 46.72 (C-19), 47.90 (NHCH₂CH₂SO₃), 49.98 (C-18), 50.60 (C-9), 55.37 (C-5), 55.73 (C-17), 66.43 (morpholine: C-3, 5), 78.92 (C-3), 109.48 (C-29), 150.75 (C-20), 176.67 (C-28). APCI-MS, m/z: 633 [M + H]⁺ (100%), 615 [M + H – H₂O]⁺ (15.5%), 546 [M – NC₄H₈O]⁺ (5.3%) (calcd for C₃₆H₆₀N₂O₅S, 632).

N -[2-( N , N -Dimethylsulfamoyl)ethyl]-3-oxolup-20(29)-en-17β-carboxamide (5a). Yield 30%, amorphous, [α]_D²⁰ +19.2° (c 0.54, CHCl₃). IR spectrum (ν, cm^–1): 1145, 1333, 1523, 1629, 1700, 3343. ¹H NMR spectrum (300 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.92, 1.01, 1.07, 1.68 (3H each, s, CH₃-25, 24, 23, 30), 0.94 (6H, s, CH₃-26, H-27), 2.47 (3H, m, 2H-2, H-13), 2.89 (6H, s, N(CH₃)₂), 3.07 (3H, m, H-19, NCH₂CH₂SO₃), 3.74 (2H, m, NCH₂CH₂SO₃), 4.60, 4.74 (1H each, s, both H-29), 6.40 (1H, t, J = 5.5, C(O)NH). ¹³C NMR spectrum (75 MHz, CDCl₃, TMS, δ, ppm): 14.54 (C-27), 15.97 (C-25), 15.97 (C-26), 19.44 (C-30), 19.64 (C-6), 21.05 (C-24), 21.46 (C-11), 25.58 (C-12), 26.62 (C-23), 29.38 (C-15), 30.79 (C-16), 33.33 (C-21), 33.51 (NCH₂CH₂SO₃), 33.51 (C-7), 34.13 (C-2), 36.89 (C-10), 36.89 (C-22), 37.42 (NMe₂), 37.78 (C-13), 39.61 (C-1), 40.69 (C-8), 42.49 (C-14), 46.67 (C-19), 47.15 (NCH₂CH₂SO₂), 47.32 (C-4), 49.94 (C-18), 49.94 (C-9), 54.92 (C-5), 55.68 (C-17), 109.51 (C-29), 150.76 (C-20), 176.62 (C-28), 218.22 (C-3). APCI-MS, m/z 589 [M + H]⁺ (100%), 544 [M – N(CH₃)₂]⁺ (6.3%) (calcd for C₃₄H₅₆N₂O₄S, 588).

N -[2-(Piperidin-1-ylsulfonyl)ethyl]-3-oxolup-20(29)-en-17β-carboxamide (5b). Yield 32%, mp 160–161°C; [α]_D²⁰ +24° (c 0.192, CHCl₃). IR spectrum (ν, cm^–1): 1139, 1335, 1507, 1640, 1662, 1705, 3300. ¹H NMR spectrum (300 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.88, 1.02, 1.06, 1.68 (3H each, s, CH₃-25, 24, 23, 30), 0.97 (6H, s, CH₃-26, 27), 2.48 (3H, m, 2H-2, H-13), 3.03 (2H, m, NCH₂CH₂SO₃), 3.09 (1H, m, H-19), 3.21 (4H, m, piperidine: 2H-2, 6), 3.75 (2H, m, NCH₂CH₂SO₃), 4.60, 4.74 (1H each, s, both H-29), 6.42 (1H, t, J = 4.8, C(O)NH). ¹³C NMR spectrum (75 MHz, CDCl₃, TMS, δ, ppm): 14.55 (C-27), 16.00 (C-25), 16.00 (C-26), 19.43 (C-30), 19.64 (C-6), 21.05 (C-24), 21.47 (C-11), 25.68 (piperidine: C-4), 25.57 (piperidine: C-3, 5), 25.57 (C-12), 26.63 (C-23), 29.40 (C-15), 30.80 (C-16), 33.26 (C-21), 33.51 (C-7), 33.70 (NCH₂CH₂SO₃), 34.17 (C-2), 36.92 (C-10), 36.92 (C-22), 37.80 (C-13), 39.62 (C-1), 40.70 (C-8), 42.52 (C-14), 46.62 (C-19), 46.62 (piperidine: C-2, 6), 47.35 (C-4), 48.35 (NCH₂CH₂SO₂), 49.96 (C-18), 50.00 (C-9), 54.95 (C-5), 55.73 (C-17), 109.53 (C-29), 150.81 (C-20), 176.57 (C-28), 218.29 (C-3). APCI-MS, m/z: 629 [M + H]⁺ (100%), 544 [M – NC₅H₁₀]⁺ (2.8%) (calcd for C₃₇H₆₀N₂O₄S, 628).

N -[2-(Morpholinosulfonyl)ethyl]-3-oxolup-20(29)-en-17-carboxamide (5c). Yield 28%, amorphous, [α] +17° (c 0.371, CHCl₃). IR spectrum (ν, cm^–1): 1109, 1157, 1641, 1662, 1703. ¹H NMR spectrum (300 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.89, 0.99, 1.04, 1.66 (3H, s, CH₃-25, 24, 23, 30), 0.95 (6H, s, CH₃-26, 27), 2.45 (2H, m, H-2), 3.07 (3H, m, H-19, NCH₂CH₂SO₃), 3.22 (4H, m, morpholine: 2H-2, 6), 3.74 (6H, m, NCH₂CH₂SO₃, morpholine: 2H-3, 5), 4.57, 4.71 (1H each, s, both H-29), 6.40 (1H, t, J = 5.3, C(O)NH). ¹³C NMR spectrum (75 MHz, CDCl₃, TMS, δ, ppm): 14.54 (C-27), 15.97 (C-25), 15.97 (C-26), 19.42 (C-30), 19.64 (C-6), 21.02 (C-24), 21.45 (C-11), 25.56 (C-12), 26.61 (C-23), 29.37 (C-15), 30.76 (C-16), 33.34 (C-21), 33.49 (C-7), 33.68 (NCH₂CH₂SO₃), 34.13 (C-2), 36.90 (C-10), 36.90 (C-22), 37.76 (C-13), 39.61 (C-1), 40.69 (C-8), 42.49 (C-14), 45.70 (morpholine: C-2, 6), 46.65 (C-19), 47.31 (C-4), 47.85 (NCH₂CH₂SO₂), 49.90 (C-18), 49.94 (C-9), 54.95 (C-5), 55.67 (C-17), 66.43 (morpholine: C-3, 5), 109.55 (C-29), 150.69 (C-20), 176.67 (C-28), 218.16 (C-3). ESI-MS, m/z: 631 [M + H]⁺ (100%), 544 [M – NC₄H₈O]⁺ (3.5%) (calcd for C₃₆H₅₈N₂O₅S, 630).

Reaction of Acid 1 with Taurine. A solution of 1 (0.20 g, 0.44 mmol) in DMF (5 mL) at 0°C was treated sequentially with DEPC (0.1 mL, 0.53 mmol), taurine (0.11 g, 0.88 mmol), and Et₃N (0.73 mL, 5.25 mmol), stirred at 20°C for 10 h, and diluted at 0°C with saturated NaHCO₃ solution (10 mL). The resulting precipitate was filtered off and chromatographed over SiO₂ (C₆H₆–MTBE, 4:1) to afford mixed anhydride 6 (0.22 g, 85%).

Mixed Anhydride of Diethyl Phosphoric and Betulinic Acid (6). Mp 108–110°C; [α]_D²⁰ –5.5° (c 0.35, CHCl₃). IR spectrum (ν, cm^–1): 1019, 1377, 1641, 1763, 3460. ¹H NMR spectrum (500 MHz, CDCl₃, TMS, δ, ppm, J/Hz): 0.67 (1H, d, J = 9.1, H-5), 0.75, 0.82, 0.95, 1.67 (3H each, s, CH₃-24, 25, 23, 30), 0.96 (6H, s, CH₃-26, 27), 1.35 (6H, t, J = 7.0, OCH₂CH₃), 1.97 (2H, m, H_b-21, 22), 2.22 (2H, m, H_b-16, H-13), 2.97 (1H, td, J = 10.7, 4.8, H-19), 3.18 (1H, dd, J = 11.2, 4.8, H-3), 4.27 (4H, m, OCH₂CH₃), 4.61, 4.73 (1H each, s, both H-29). ¹³C NMR spectrum (125 MHz, CDCl₃, TMS, δ, ppm): 14.67 (C-27), 15.36 (OCH₂CH₃), 15.87 (C-26), 16.15 (C-24, 25, OCH₂CH₃), 18.26 (C-6), 19.35 (C-30), 20.83 (C-11), 25.45 (C-12), 27.36 (C-2), 27.97 (C-23), 28.03 (C-13), 29.73 (C-15), 30.16 (C-21), 31.68 (C-16), 34.29 (C-7), 36.16 (C-22), 37.19 (C-10), 38.72 (C-1), 38.84 (C-4), 40.73 (C-8), 42.44 (C-14), 46.45 (C-19), 49.14 (C-18), 50.55 (C-9), 55.35 (C-5), 58.00 (d, ³J_C–P = 5.1, C-17), 64.87 (d, ²J_C–P = 5.2, OCH₂CH₃), 65.17 (d, 2JC–P = 5.2, OCH₂CH₃), 78.93 (C-3), 109.96 (C-29), 149.85 (C-20), 170.17 (d, ²J_C–P = 10.6, C-28). ³¹P NMR spectrum (202 MHz, CDCl₃, TMS, δ, ppm, J/Hz): –7.51 (pentet, ³J_H–P = 7.5). ESI-MS, m/z 563 [M – C₂H₅]^– (calcd for C₃₄H₅₇O₆P, 592).

Reaction of Acid 1 with 3a in the Presence of DEPC. A solution of 1 (0.20 g, 0.438 mmol) in anhydrous DMF (5 mL) at 0°C was treated with 3a (0.153 g, 0.876 mmol), DEPC (0.1 mL, 0.525 mmol), and Et₃N (0.85 mL, 6.132 mmol), stirred for 10 h at 20°C, and diluted with saturated NaHCO₃ solution (3 mL). The resulting precipitate was filtered off, rinsed with H₂O (3 × 5 mL), dried, and chromatographed over SiO₂ (C₆H₆, C₆H₆–MTBE, 8:1) to afford 4a (0.012 g, 5%) and mixed anhydride 6 (0.20 g, 93%).