Total Synthesis of Thailandepsin B, a Potent HDAC Inhibitor Isolated from a Microorganism

Abstract

Thailandepsin B, a bicyclic depsipeptide histone deacetylase inhibitor, was efficiently synthesized in 51% overall yield in eight steps, starting from commercially available D-norleucine methyl ester and known (S,E)-3-(4-methoxybenzyloxy)-7-(tritylthio)hept-4-enoic acid. The method features a convergent approach in which the corresponding seco-acid, a key precursor in macrolactonization, is directly assembled through the condensation of a D-allo-isoleucine-D-cysteine-containing segment with a D-norleucine-containing segment.

In 2011, Cheng and colleagues reported the isolation and structural elucidation of the two bicyclic depsipeptide histone deacetylase (HDAC) inhibitors, thailandepsins A (1) and B (2)¹⁾ (Chart 1). These compounds were identified through the systematic overexpression of transcription factors associated with natural product gene clusters encoded within Burkholdenia thailandensis E264.¹⁾ Almost simultaneously, Brady and colleagues independently reported the identification of the two bicyclic depsipeptide HDAC inhibitors, burkholdacs A (3) and B (1), using the same bacterial strain and gene clusters but a different technical approach.²⁾ Coincidentally, burkholdac B was identical to thailandepsin A.^1,2) Thailandepsins A and B exhibit comparable HDAC inhibitory activity to that of FK228 (4; romidepsin),¹⁾ which is an anticancer agent that has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of cutaneous T-cell lymphoma.^3,4) Note that isozyme selectivity of thailandepsins is superior to that of 4¹⁾; isozyme selectivity is critically important for developing new HDAC inhibitors as anticancer agents with reduced undesirable side effects.^4,5) Thailandepsins, therefore, can be considered as intriguing and valuable targets for total synthesis. To date, the total synthesis of thailandepsin A (1; burkholdac B) has been reported by Ganesan and colleagues⁶⁾ and our group.⁷⁾ However, the total synthesis of thailandepsin B (2) has only been reported once in the patent literature.⁸⁾

Chart 1. Structures of Thailandepsins A (1, Burkholdac B), B (2), Burkholdac A (3), FK228 (4; Romidepsin), and Spiruchostatins A–D (5–8)

i-Pr=isopropyl, s-Bu=sec-butyl, and i-Bu=isobutyl.

During our efforts synthesizing and biologically evaluating bicyclic depsipeptide HDAC inhibitors including burkholdacs A (3),⁷⁾ B (1, thailandepsin A),⁷⁾ FK228 (4, romidepsin)⁹⁾ and spiruchostatins A (5),^9,10) B (6),^9,11) C (7)¹²⁾ and D (8),¹²⁾ we became interested in the synthesis of thailandepsin B (2) and its analogues with the aim of identifying novel mechanism-based anticancer agents. In this study, we describe our total synthesis of 2 using a synthetic strategy developed previously in our laboratory.^7,9–12)

Results and Discussion

Retrosynthetic analysis of thailandepsin B (2) is illustrated in Chart 2. We envisioned that target molecule 2 could be synthesized via the macrolactonization of the corresponding seco-acid 9 followed by internal disulfide bond formation. The key macrolactonization precursor 9 could be prepared through the direct condensation of the D-allo-isoleucine- and D-cysteine-containing segment 10 with the D-norleucine-containing segment 11. Segment 11 could be formed by the condensation of commercially available D-norleucine methyl ester hydrochloride (12) with the known carboxylic acid 13 (previously prepared from L-malic acid in our laboratory^7,9–12)). Segment 10 was obtained as previously described in our total synthesis of 1 and 6.^7,9)

Chart 2. Retrosynthetic Analysis of Thailandepsin B (2)

TBS=tert-butyldimethylsilyl; Tr=triphenylmethyl; PMB=4-methoxybenzyl.

We initially pursued the synthesis of the key segment 11, which is the coupling partner of 10, as shown in Chart 3. The condensation of 12 with 13^7,9–12) afforded the desired coupling product 14 in excellent yield (98%). Subsequent saponification of the methyl ester moiety in 14 furnished the requisite segment 11 in 90% yield.

Chart 3. Synthesis of Segment 14

a) PyBOP, i-Pr₂NEt, MeCN, rt, 98%; b) LiOH, MeOH, rt, 90%. PyBOP=(benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate.

After obtaining the key segment 11, we directed our attention to the synthesis of the target thailandepsin B (2), as shown in Chart 4. The crucial condensation of 10 with 11 proceeded well under the mild conditions [O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) (1.3 equiv), 1-hydroxy-7-azabenzotriazole (HOAt) (1.3 equiv), i-Pr₂NEt (2.6 equiv), CH₂Cl₂, −30°C, 5 h] explored in our previous studies.^7,9–12) The desired condensation product 15 was obtained in 90% yield without appreciable epimerization at the C2 stereogenic center (D-norleucine part in 15). In this condensation reaction, the use of a combination of HATU and HOAt at low temperature was critical to effectively suppress unfavorable epimerization event.⁹⁾ The condensation product 15 was then converted to the requisite seco-acid 9 with an overall yield of 82%, using the alcohol 16 by successive removal of both the 4-methoxybenzyl (PMB) and allyl-protecting groups. The subsequent macrolactonization of 9 was achieved by employing the Shiina method^13–17) [2-methyl-6-nitrobenzoic anhydride (MNBA) (1.3 equiv), 4-dimethylaminopyridine (DMAP) (3.0 equiv), CH₂Cl₂ (1.0 mM), rt, 12 h], which resulted in the desired cyclization product 17 in 85% yield. Finally, simultaneous S-Tr deprotection and internal disulfide bond formation in 17 (95%), followed by the deprotection of the TBS group in the resulting disulfide 18 (90%) furnished thailandepsin B (2) with [α]_D²⁵ −28.4 (c=1.0, MeCN) {lit.¹⁾ [α]_D²⁵ −22.8 (c=1.0, MeCN)}. The spectroscopic properties (infrared (IR), ¹H- and ¹³C-NMR, and MS) of the synthetic sample 2 were identical to those reported for natural 2.¹⁾

Chart 4. Synthesis of Thailandepsin B (2)

(a) HATU, HOAt, i-Pr₂NEt, CH₂Cl₂, −30°C, 90%; (b) DDQ, CH₂Cl₂/H₂O, rt, 89%; c) Pd(PPh₃)₄, morpholine, THF, rt, 92%; (d) MNBA, DMAP, CH₂Cl₂, rt, 85%; (e) I₂, CH₂Cl₂–MeOH, rt, 95%; (f) HF·pyridine, pyridine, rt, 90%. HATU=O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate; HOAt=1-hydroxy-7-azabenzotriazole; DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone; MNBA=2-methyl-6-nitrobenzoic anhydride; DMAP=4-dimethylaminopyridine.

Conclusion

We have achieved the total synthesis of thailandepsin B (2) with 51% overall yield in eight steps, starting from commercially available D-norleucine methyl ester hydrochloride (12) and the known carboxylic acid 13. The method is in highly convergent manner and features the condensation of a D-allo-isoleucine-D-cysteine-containing segment 10 with a D-norleucine-containing segment 11 to directly construct the requisite peptide framework 15 (10+11→15, Chart 4). The present study should be useful for producing thailandepsin analogues that possess various alkyl side chains. Further investigations concerning the synthesis and biological evaluation of thailandepsin analogues are currently under way and will be reported in due course.

Experimental

General Techniques

All reactions involving air- and moisture-sensitive reagents were carried out using oven dried glassware and standard syringe-septum cap techniques. Routine monitorings of reaction were carried out using glass-supported Merck silica gel 60 F254 TLC plates. Flash column chromatography was performed on Kanto Chemical Silica Gel 60N (spherical, neutral 40–50 nm) with the solvents indicated.

All solvents and reagents were used as supplied with following exceptions. Tetrahydrofuran (THF) was freshly distilled from Na metal/benzophenone under argon. CH₂Cl₂, MeCN, pyridine and i-Pr₂NEt were distilled from calcium hydride under argon.

Measurements of optical rotations were performed with a Anton Paar MCP-100 automatic digital polarimeter. ¹H- and ¹³C-NMR spectra were measured with a JEOL AL-400 (400 MHz) and JEOL LA-600 (600 MHz) spectrometers. Chemical shifts were expressed in ppm using Me₄Si (δ=0) as an internal standard. The following abbreviations are used: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br). IR spectral measurements were carried out with a JASCO FT/IR-4100 spectrometer. Low- and high-resolution mass spectra (LR-HR-MS) were measured on a JEOL JMS-700 high resolution mass spectrometer.

(R)-Methyl 2-[(S,E)-3-(4-Methoxybenzyloxy)-7-(tritylthio)hept-4-enamido]hexanoate (14)

i-Pr₂NEt (0.33 mL, 1.9 mmol) was added dropwise to a stirred solution of D-norleucine methyl ester hydrochloride (12) (88.0 mg, 0.48 mmol)¹⁸⁾ and (S,E)-3-(4-methoxybenzyloxy)-7-(tritylthio)hept-4-enoic acid (13)^7,9–12) (174 mg, 0.32 mmol) in MeCN (6.5 mL) containing (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP) (252 mg, 0.48 mmol) at 0°C under argon, and stirring was continued for 2 h at room temperature. The reaction mixture was diluted with EtOAc (30 mL), and the organic layer was washed successively with 10% aqueous HCl (2×10 mL), saturated aqueous NaHCO₃ (2×10 mL) and brine (2×10 mL), then dried over Na₂SO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (hexane–EtOAc 2 : 1) to give 14 (211 mg, 98%) as a colorless viscous oil. [α]_D²⁵ −6.5 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3309, 3057, 2953, 2929, 2860, 1743, 1652, 1613, 1514, 1443, 1362, 1248, 1080, 821, 744, 700; ¹H-NMR (400 MHz, CDCl₃) δ: 0.83 (3H, t, J=7.1 Hz), 1.14–1.28 (4H, m), 1.53–1.60 (1H, m), 1.70–1.77 (1H, m), 2.04–2.25 (4H, m), 2.38 (1H, dd, J=15.4, 3.2 Hz), 2.51 (1H, dd, J=15.4, 8.8 Hz), 3.71 (3H, s), 3.78 (3H, s), 4.10 (1H, td, J=8.3, 3.4 Hz), 4.31 (1H, d, J=10.7 Hz), 4.51 (1H, d, J=10.7 Hz), 4.55 (1H, td, J=7.7, 5.2 Hz), 5.32 (1H, dd, J=15.6, 8.3 Hz), 5.59 (1H, dt, J=15.1, 6.6 Hz), 6.81–6.84 (2H, m), 6.95 (1H, d, J=8.3 Hz), 7.19–7.29 (11H, m), 7.41–7.45 (6H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: 13.8, 22.2, 27.4, 31.2, 31.4, 32.0, 42.7, 52.0, 52.1, 55.2, 66.6, 70.1, 76.8, 113.8 (2C), 126.6 (3C), 127.9 (6C), 129.5 (6C), 129.7 (2C), 130.0, 130.3, 132.9, 144.9 (3C), 159.2, 170.5, 173.0; HR-FAB-MS m/z: 688.3087 (Calcd for C₄₁H₄₇NO₅SNa [M+Na]⁺: 688.3073).

(R)-2-[(S,E)-3-(4-Methoxybenzyloxy)-7-(tritylthio)hept-4-enamido]hexanoic Acid (11)

One mol LiOH (1.30 mL, 1.3 mmol) was added dropwise to a stirred solution of 14 (211 mg, 0.32 mmol) in MeOH (6.3 mL) at room temperature. After 3 h, 10% aqueous HCl was added to the mixture at 0°C until pH was 6. The resulting mixture was extracted with EtOAc (3×30 mL), and the combined extracts were washed with saturated aqueous NaHCO₃ (2×20 mL) and brine (2×20 mL), then dried over Na₂SO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (CHCl₃–MeOH 15 : 1) to give 11 (186 mg, 90%) as a colorless amorphous solid. [α]_D²⁵ −2.4 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3340, 3057, 2955, 2930, 2862, 1732, 1614, 1514, 1449, 1443, 1248, 1079, 1034, 974, 822, 745, 701; ¹H-NMR (400 MHz, CDCl₃) δ: 0.82 (3H, t, J=6.8 Hz), 1.21–1.27 (4H, m), 1.53–1.62 (1H, m), 1.75–1.84 (1H, m), 2.04–2.24 (4H, m), 2.41 (1H, dd, J=15.4, 3.2 Hz), 2.52 (1H, dd, J=15.1, 8.8 Hz), 3.76 (3H, s), 4.10 (1H, td, J=8.2, 3.4 Hz), 4.27 (1H, d, J=10.7 Hz), 4.46–4.50 (2H, m), 5.30 (1H, dd, J=15.4, 8.0 Hz), 5.58 (1H, dt, J=15.1, 6.8 Hz), 6.79–6.83 (2H, m), 7.12 (1H, d, J=7.3 Hz), 7.18–7.29 (11H, m), 7.39–7.42 (6H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: 13.8, 22.2, 27.4, 31.2, 31.37, 31.40, 42.4, 52.4, 55.2, 66.6, 70.1, 76.6, 113.9 (2C), 126.6 (3C), 127.9 (6C), 129.5 (6C), 129.7, 129.8 (2C), 130.1, 133.1, 144.9 (3C), 159.3, 171.6, 175.8; HR-FAB-MS m/z: 652.3094 (Calcd for C₄₀H₄₆NO₅S [M+H]⁺ 652.3097).

(7S,11R,14S,17R,18S,E)-Allyl 17-[(S)-sec-Butyl]-11-butyl-18-(tert-butyldimethylsilyloxy)-7-(4-methoxybenzyloxy)-9,12,15-trioxo-1,1,1-triphenyl-14-(tritylthiomethyl)-2-thia-10,13,16-triazaicos-5-en-20-oate (15)

i-Pr₂NEt (0.12 mL, 0.72 mmol) was added dropwise to a stirred solution of (3S,4R,5S)-allyl-4-[(S)-2-amino-3-(tritylthio)propanamido]-3-(tert-butyldimethylsilyloxy)-5-methylheptanoate (10)^7,9) (187 mg, 0.28 mmol) and 11 (186 mg, 0.28 mmol) in CH₂Cl₂ (5.6 mL) containing HATU (137 mg, 0.36 mmol) and HOAt (49.1 mg, 0.36 mmol) at −30°C under argon. After 5 h, the reaction mixture was diluted with CHCl₃ (50 mL). The organic layer was washed successively with 10% aqueous HCl (2×20 mL), saturated aqueous NaHCO₃ (2×20 mL) and brine (2×20 mL), then dried over MgSO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (hexane/EtOAc 1 : 1) to give 15 (328 mg, 90%) as a colorless viscous liquid. [α]_D²⁵ +13.1 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3285, 3060, 2956, 2930, 2857, 1734, 1637, 1541, 1515, 1490, 1458, 1444, 1387, 1249, 1173, 1085, 1036, 938, 830, 744, 700; ¹H-NMR (400 MHz, CDCl₃) δ: 0.00 (3H, s), 0.07 (3H, s), 0.77–0.81 (6H, m,), 0.83–0.88 (12H, m), 1.05–1.28 (6H, m), 1.36–1.41 (1H, m), 1.64–1.69 (1H, m), 1.75–1.81 (1H, m), 2.07–2.17 (2H, m), 2.20–2.25 (2H, m), 2.34 (1H, dd, J=15.6, 3.4 Hz), 2.40–2.58 (4H, m), 2.77 (1H, dd, J=12.7, 7.8 Hz), 3.77 (3H, s), 3.91–3.96 (2H, m), 4.01 (1H, td, J=8.0, 3.3 Hz), 4.10–4.20 (3H, m), 4.38 (1H, d, J=10.7 Hz), 4.44–4.55 (2H, m), 5.17–5.31 (3H, m), 5.45 (1H, dt, J=15.3, 6.6 Hz), 5.82–5.91 (1H, m), 6.10 (1H, d, J=10.2 Hz), 6.49 (1H, d, J=7.8 Hz), 6.80–6.84 (2H, m), 7.00 (1H, d, J=7.3 Hz), 7.14–7.45 (32H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: −4.8, −4.5, 11.7, 13.6, 13.7, 17.9, 22.3, 25.8 (3C), 27.2, 27.6, 31.1, 31.2, 31.4, 32.9, 34.1, 40.0, 42.4, 52.4, 53.2, 55.2, 56.0, 65.1, 66.6, 67.0, 69.5, 70.1, 76.5, 113.9 (2C), 118.2, 126.6 (3C), 126.8 (3C), 127.8 (6C), 128.0 (6C), 129.48 (6C), 129.50 (6C), 129.66, 129.73 (2C), 130.0, 132.1, 133.0, 144.3 (3C), 144.8 (3C), 159.3, 169.7, 171.2, 171.5, 171.8; HR-FAB-MS m/z: 1308.6550 (Calcd for C₇₉H₉₈N₃O₈S₂S [M+H]⁺ 1308.6565).

(7S,11R,14S,17R,18S,E)-Allyl 17-[(S)-sec-Butyl]-11-butyl-18-(tert-butyldimethylsilyloxy)-7-hydroxy-9,12,15-trioxo-1,1,1-triphenyl-14-(tritylthiomethyl)-2-thia-10,13,16-triazaicos-5-en-20-oate (16)

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (112 mg, 0.50 mmol) was added in small portions to a stirred solution of 15 (324 mg, 0.25 mmol) in CH₂Cl₂–H₂O 9 : 1 (12 mL) at room temperature. After 3 h, the mixture was diluted with CHCl₃ (60 mL), and the organic layer was washed with saturated aqueous NaHCO₃ (2×20 mL) and brine (2×20 mL), then dried over MgSO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (hexane–EtOAc, 2 : 1) to give 16 (262 mg, 89%) as a colorless viscous liquid. [α]_D²⁵ +11.6 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3272, 3060, 2956, 2929, 2856, 1737, 1672, 1634, 1542, 1490, 1444, 1387, 1254, 1181, 1084, 1035, 985, 938, 836, 743, 699; ¹H-NMR (400 MHz, CDCl₃) δ: 0.00 (3H, s), 0.06 (3H, s), 0.77–0.87 (19H, m), 1.06–1.12 (1H, m), 1.18–1.33 (5H, m), 1.48–1.57 (1H, m), 1.73–1.81 (2H, m), 2.04–2.09 (2H, m), 2.17–2.22 (3H, m), 2.31 (1H, dd, J=13.7, 2.4 Hz), 2.41 (1H, dd, J=16.1, 6.8 Hz), 2.54–2.63 (2H, m), 3.14 (1H, d, J=3.4 Hz), 3.88–3.98 (2H, m), 4.14 (1H, td J=7.4, 3.4 Hz), 4.27 (1H, td, J=8.2, 4.7 Hz), 4.32–4.36 (1H, m), 4.42 (1H, ddt, J=8.4, 4.5, 1.9 Hz), 4.50 (1H, ddt, J=8.4, 4.4, 2.0 Hz), 5.17–5.20 (1H, m), 5.27 (1H, dq, J=17.1, 1.5 Hz), 5.35 (1H, dd, J=15.4, 6.1 Hz), 5.48 (1H, dt, J=15.3, 6.7 Hz), 5.79–5.89 (1H, m), 5.95 (1H, d, J=10.2 Hz), 6.02 (1H, d, J=7.8 Hz), 6.92 (1H, d, J=8.3 Hz), 7.15–7.43 (30H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: −4.8, −4.5, 11.7, 13.7, 13.8, 18.0, 22.3, 25.8 (3C), 27.1, 27.7, 31.3, 31.4 (2C), 33.1, 34.1, 40.0, 44.0, 52.5, 53.7, 56.1, 65.2, 66.6, 67.0, 69.4, 69.7, 118.3, 126.6 (3C), 126.8 (3C), 127.8 (6C), 128.1 (6C), 129.5 (6C), 129.6 (6C), 129.9, 132.1, 132.6, 144.3 (3C), 144.9 (3C), 170.1, 171.5, 171.59, 171.61; HR-FAB-MS m/z: 1210.5809 (Calcd for C₇₁H₈₉N₃O₇S₂SiNa [M+Na]⁺ 1210.5809).

(7S,11R,14S,17R,18S,E)-17-[(S)-sec-Butyl]-11-butyl-18-(tert-butyldimethylsilyloxy)-7-hydroxy-9,12,15-trioxo-1,1,1-triphenyl-14-(tritylthiomethyl)-2-thia-10,13,16-triazaicos-5-en-20-oic Acid (9)

Morpholine (38.7 µL, 0.44 mmol) was added dropwise to a stirred solution of 16 (262 mg, 0.22 mmol) in THF (11 mL) containing Pd(PPh₃)₄ (25.5 mg, 22 µmol) at room temperature under argon. After 1 h, the reaction mixture was diluted with EtOAc (30 mL), and the organic layer was washed with 10% aqueous HCl (2×10 mL) and brine (2×10 mL), then dried over Na₂SO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (CHCl₃–MeOH 40 : 1) to give 9 (234 mg, 92%) as a colorless amorphous solid. [α]_D²⁵ +0.6 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3284, 3059, 3018, 2957, 2929, 2857, 1714, 1635, 1541, 1490, 1444, 1254, 1217, 1185, 1094, 1034, 951, 836, 753, 700; ¹H-NMR (400 MHz, CDCl₃) δ: 0.05 (6H, s), 0.80 (3H, d, J=6.8 Hz), 0.83–0.88 (15H, m), 1.06–1.14 (1H, m), 1.19–1.28 (5H, m), 1.54–1.59 (1H, m), 1.78–1.84 (2H, m), 2.03–2.08 (2H, m), 2.17–2.26 (3H, m), 2.32 (1H, dd, J=13.7, 2.9 Hz), 2.39 (1H, dd, J=16.1, 3.9 Hz), 2.45 (1H, dd, J=12.4, 5.6 Hz), 2.54 (1H, dd, J=16.3, 6.6 Hz), 2.75 (1H, dd, J=12.0, 7.6 Hz), 3.92 (1H, td, J=13.2, 4.8 Hz), 4.06–4.14 (2H, m), 4.30–4.35 (2H, m), 5.34 (1H, dd, J=15.4, 6.1 Hz), 5.46 (1H, dt, J=15.3, 6.6 Hz), 6.36 (2H, d, J=9.8 Hz), 7.02 (1H, br s), 7.15–7.42 (30H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: −4.9, −4.3, 11.8, 13.4, 13.8, 17.9, 22.3, 25.7 (3C), 27.2, 27.8, 29.7, 31.3, 31.4, 33.1, 34.2, 40.3, 44.1, 53.1, 53.8, 57.0, 66.6, 67.0, 68.9, 69.7, 126.6 (3C), 126.9 (3C), 127.9 (6C), 128.1 (6C), 129.5 (6C), 129.6 (6C), 130.0, 132.3, 144.3 (3C), 144.9 (3C), 170.3, 171.9 (2C), 172.3; HR-FAB-MS m/z: 1170.5470 (Calcd for C₆₈H₈₅N₃O₇S₂SiNa [M+Na]⁺ 1170.5496).

(2S,6R,9S,12R,13S)-12-[(S)-sec-Butyl]-6-butyl-13-(tert-butyldimethylsilyloxy)-2-[(E)-4-(tritylthio)but-1-en-1-yl]-9-tritylthiomethyl-1-oxa-5,8,11-triazacyclopentadecane-4,7,10,15-tetraone (17)

A solution of 9 (191 mg, 0.17 mmol) in CH₂Cl₂ (17 mL) was added very slowly to a stirred solution of MNBA (74.4 mg, 0.22 mmol) in CH₂Cl₂ (170 mL, 1.0 mM concentration) containing DMAP (61.0 mg, 0.50 mmol) at toom temperature over 14 h. After 1 h, the mixture was diluted with CH₂Cl₂ (200 mL), and the organic layer was washed successively with saturated aqueous NaHCO₃ (2×80 mL), water (2×80 mL) and brine (2×80 mL), then dried over MgSO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (hexane–EtOAc, 2 : 1) to give 17 (160 mg, 85%) as a colorless amorphous solid. [α]_D²⁵ −6.4 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3286, 3059, 3031, 2956, 2928, 2857, 1733, 1652, 1539, 1490, 1444, 1258, 1198, 1098, 1034, 990, 834, 743, 700 cm⁻¹; ¹H-NMR (400 MHz, CDCl₃) δ: −0.10 (3H, s), 0.00 (3H, s), 0.76 (3H, d, J=6.8 Hz), 0.79–0.86 (15H, m), 0.98–1.05 (1H, m), 1.12–1.19 (1H, m), 1.25–1.38 (4H, m), 1.58–1.66 (1H, m), 1.75–1.86 (2H, m), 2.00–2.07 (2H, m), 2.16–2.20 (2H, m), 2.30 (1H, dd, J=16.6, 5.9 Hz), 2.35–2.43 (2H, m), 2.48 (1H, dd, J=13.9, 3.2 Hz), 2.55–2.62 (1H, m), 3.24–3.28 (2H, m), 3.70 (1H, td, J=10.2, 2.0 Hz), 3.99–4.04 (1H, m), 4.10–4.15 (1H, m), 5.34 (1H, dd, J=15.6, 7.3 Hz), 5.51–5.56 (1H, m), 5.66 (1H, dt, J=15.3, 6.7 Hz), 6.13 (1H, d, J=7.8 Hz), 6.82 (1H, d, J=5.9 Hz), 7.19–7.43 ppm (31H, m); ¹³C-NMR (100 MHz, CDCl₃) δ: −4.9, −3.9, 12.0, 13.0, 13.8, 17.9, 22.4, 25.8 (3C), 27.4, 27.9, 30.9, 31.0, 31.3, 31.9, 34.0, 41.8, 41.9, 54.0, 56.6, 58.1, 66.6, 66.8, 68.7, 71.8, 126.6 (3C), 126.8 (3C), 127.9 (6C), 128.0 (6C), 128.1, 129.48 (6C), 129.53 (6C), 133.4, 144.4 (3C), 144.8 (3C), 169.5, 170.2, 170.4, 173.0; HR-FAB-MS m/z: 1152.5363 (Calcd for C₆₈H₈₃N₃O₆S₂SiNa [M+Na]⁺ 1152.5390).

(1S,5S,6R,9S,20R,E)-6-[(S)-sec-Butyl]-20-butyl-5-(tert-butyldimethylsilyloxy)-2-oxa-11,12-dithia-7,19,22-triazabicyclo[7.7.6]docos-15-ene-3,8,18,21-tetraone (18)

A solution of 17 (156 mg, 0.14 mmol) in CH₂Cl₂–MeOH 9 : 1 (35 mL) was added dropwise to a vigorously stirred solution of I₂ (350 mg, 1.4 mmol) in CH₂Cl₂–MeOH 9 : 1 (280 mL, 0.5 mM concentration) over 10 min at room temperature. After 10 min, the reaction was quenched with 10% aqueous Na₂S₂O₃ (60 mL) at room temperature. The resulting mixture was diluted with CH₂Cl₂ (150 mL), and the organic layer was washed with saturated aqueous NaHCO₃ (2×60 mL) and brine (2×60 mL), then dried over MgSO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (CHCl₃–MeOH 40 : 1) to give 18 (84.2 mg, 95%) as a colorless amorphous solid. [α]_D²⁵ +5.7 (c=1.0, CHCl₃); IR (neat) cm⁻¹: 3339, 2957, 2930, 2858, 1747, 1661, 1542, 1463, 1433, 1362, 1257, 1158, 1138, 1080, 1027, 989, 949, 837, 810, 775, 755 cm⁻¹; ¹H-NMR (400 MHz, CDCl₃) δ: 0.09 (3H, s), 0.16 (3H, s), 0.88 (3H, d, J=6.8 Hz), 0.90–0.95 (15H, m), 1.21–1.28 (1H, m), 1.35–1.49 (5H, m), 1.64–1.72 (1H, m), 1.85–1.98 (2H, m), 2.58–2.62 (3H, m), 2.73–2.76 (3H, m), 2.92–3.13 (4H, m), 3.68 (1H, br s), 4.20–4.24 (1H, m), 4.86 (1H, dt, J=10.2, 3.4 Hz), 5.00–5.05 (1H, m), 5.65 (1H, d, J=15.6 Hz), 5.70–5.72 (1H, m), 5.81 (1H, d, J=3.9 Hz), 6.48 (1H, br s), 6.69 (1H, d, J=9.8 Hz), 7.09 ppm (1H, d, J=6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃) δ: −4.9, −4.3, 11.9, 13.8, 14.5, 17.9, 22.2, 25.7 (3C), 27.8, 28.5, 30.8, 34.9, 36.3, 39.6, 40.5, 41.2, 43.5, 53.4, 56.4, 61.1, 67.1, 68.7, 128.7, 133.0, 169.0 (2C), 170.5, 170.9; HR-FAB-MS m/z: 644.3228 (Calcd for C₃₀H₅₄N₃O₆S₂Si [M+H]⁺ 644.3223).

Thailandepsin B (2)

HF·pyridine (1.3 mL) was added to a stirred solution of 18 (83.2 mg, 0.13 mmol) in pyridine (2.6 mL) at room temperature. After 22 h, the reaction mixture was diluted with EtOAc (60 mL), and the organic layer was washed successively with 3% aqueous HCl (3×15 mL), saturated aqueous NaHCO₃ (2×15 mL) and brine (2×15 mL), then dried over Na₂SO₄. Concentration of the solvent in vacuo afforded a residue, which was purified by column chromatography (CHCl₃–MeOH 10 : 1) to give 2 (thailandepsin B) (61.2 mg, 90%) as a white amorphous solid. [α]_D²⁵ −28.4 (c=1.0, MeCN), {lit.¹⁾ [α]_D²⁵ −22.8 (c=1.0, MeCN)}. The IR, ¹H- and ¹³C-NMR, and MS spectra were identical to those of natural thailandepsin B¹⁾; IR (neat) cm⁻¹: 3330, 2960, 2931, 2873, 1733, 1660, 1540, 1457, 1434, 1405, 1375, 1277, 1162, 1049, 1023, 981, 910, 731; ¹H-NMR (600 MHz, CDCl₃) δ: 0.91–0.95 (9H, m), 1.21–1.28 (1H, m), 1.36–1.49 (4H, m), 1.51–1.56 (1H, m), 1.60–1.74 (1H, m), 1.91–1.97 (1H, m), 2.08–2.12 (1H, m), 2.46–2.51 (1H, m), 2.59 (1H, dd, J=13.2, 1.1 Hz), 2.70–2.75 (4H, m), 2.86 (1H, d, J=10.6 Hz), 2.91–2.95 (1H, m), 3.08–3.12 (1H, m), 3.15–3.22 (1H, m), 3.36 (1H, dd, J=13.4, 7.1 Hz), 3.41–3.49 (1H, m), 4.17–4.20 (1H, m), 4.63–4.67 (1H, m), 4.94 (1H, td, J=8.6, 3.3 Hz), 5.50–5.51 (1H, m), 5.67 (1H, d, J=15.4 Hz), 5.90 (1H, d, J=3.7 Hz), 6.47 (1H, br s), 6.72 (1H, d, J=9.5 Hz), 7.25 (1H, d, J=7.0 Hz); ¹³C-NMR (150 MHz, CDCl₃) δ: 11.6, 13.8, 15.3, 22.2, 27.1, 28.4, 30.6, 33.7, 36.3, 39.5, 40.5, 40.8, 41.9, 54.2, 56.6, 61.9, 68.3, 70.4, 128.3, 133.7, 169.1, 170.5, 170.7, 171.9; HR-FAB-MS m/z: 530.2360 (Calcd for C₂₄H₄₀N₃O₆S₂ [M+H]⁺ 530.2359).

Acknowledgments

This study was supported by a Grant-in-Aid for the Strategic Research Foundation Program at Private Universities (No. S1511001L, 2015–2019) and a Grant-in-Aid for Scientific Research (C) (No. 15K07865, 2015–2017) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

The online version of this article contains supplementary materials.

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