Concise and Versatile Synthesis of Sulfoquinovosyl Acyl Glycerol Derivatives for Biological Applications

Abstract

Sulfoquinovosyl acylpropanediol (SQAP), a chemically modified analogue of sulfoquinovosyl acylglycerol (SQAG) that occurs in sea algae, has been reported to show a variety of biological activities, including accumulation in tumor cells and the inhibition of tumor cell growth. We report herein on a new concise and versatile synthesis of SQAP itself and derivatives bearing iodoaryl groups and boronclusters. This method should be useful for the design and synthesis of SQAG/SQAP derivatives for diagnosis and the treatment of cancer and related diseases.

The sulfolipid sulfoquinovosyl diacylglycerol (SQDG) produced by higher plants, and other photosynthetic organisms, is characterized by its unique sulfonic acid head group, 6-deoxy-6-sulfo-glucose (sulfoquinovose) and a 1,2-diacyl-sn-glycerol moiety with a long-chain fatty acid.¹⁾ Due to its strong amphiphilicity, SQDG would have excellent detergent properties. It has recently been reported that some SQDGs show anti-tumor and anti-viral activity (human immunodeficiency virus (HIV) and herpes simplex virus (HSV)) via the inhibition of DNA polymerase.^2–6) Sulfoquinovosyl acylglycerol (SQAG) 1 is one such naturally occurring SQDG derivative that was isolated from sea algae and characterized by Sakaguchi, Sugawara and colleagues⁷⁾ (Chart 1). It has been reported that SQAG and its analogue sulfoquinovosyl acylpropandiol (SQAP) 2 accumulates in tumor cells and has a low cytotoxicity against normal cells.⁸⁾ Furthermore, previous structure–activity relationship studies of SQDG/SQAG derivatives indicate that modification of the long alkyl chain moiety on SQDG derivatives with hydrophobic moieties have a negligible effect on their biological activity, particularly their ability to inhibit DNA polymerase and telomerase.^9,10) Therefore, SQAP derivatives such as 3 and 4 that contain boron and iodine atoms would be expected to be potent lead compounds for use in cancer therapy and diagnosis for boron-neutron capture therapy¹¹⁾ (BNCT) and for the in vivo imaging of tumor sites,¹²⁾ respectively (Chart 1).

Chart 1. Structure of SQAG and SQAP Derivatives

The total synthesis of SQAP was previously reported by Sugawara and colleagues as shown in Chart 2.⁸⁾ α-Allyl glycoside α-5 was prepared from D-glucose via α-selective glycosylation and following protection. The introduction of a SO₃Na group at the 6-position of α-5 and hydroboration of the allyl group gave the key intermediate α-6, to which a long-chain fatty acid was introduced by a condensation reaction to afford 7, followed by the deprotection of 7 to give the desired compound 2 (SQAP). However, this route is not sufficiently general to permit a wide variety of SQAP derivatives to be prepared, especially when the intermediates or target compounds include functions that are labile under the protection and/or deprotection conditions used in Chart 2 (7 to 2) and other reaction conditions. Specifically, SQAP derivatives bearing NO₂, CN, olefin and aryl halide, which would be easily reduced under hydrogenation conditions, would not be synthesized in their synthetic route. As a consequence, the method has only limited use in preparing SQAP analogues.

Chart 2. Synthetic Route of SQAP Reported by Sugawara et al.

In this study, we designed a new synthetic route for preparing SQAP and its analogues, 3 and 4. o-Carborane has been frequently used as a hydrophobic pharmacophore for various pharmacologically active compound such as estrogenic agonists¹³⁾ and vitamin D receptor ligands.¹⁴⁾ In addition, compound 3 would be expected to function as a ¹¹B magnetic resonance imaging (MRI) agent, as suggested by Bradshaw et al.¹⁵⁾ and us.^16,17) SQAP derivatives containing iodine atoms, such as 4, could be utilized as contrast agents for X-ray-computed tomography (CT), which is a well-established tissue imaging technique employed in a variety of research and clinical settings.^12,18)

For this purpose, we envisaged a new synthetic route for the preparation of SQAP and its analogues via the thioglucoside 9 and the mesyl derivative 12 (Chart 3). It was assumed that both α- and β-anomers of 6 would be produced by the glycosylation of 9 with mono(p-methoxybenzyl)-protected 1,3-propanediol 10.¹⁹⁾ The condensation of 6 with a long chain fatty acid part would give 11, the benzyl groups of which could be removed by hydrogenation to give SQAP derivatives. This synthetic route would be ideal for synthesis of SQAP derivatives bearing base-sensitive functional groups such as o-carborane, whose closo-form (R-C₂B₁₀H₁₁) is easily converted to nido-form (R-C₂B₉H₁₀) by the nucleophilic deboronation reaction.²⁰⁾ Alternatively, it was hypothesized that the conversion of a nucleophile (–OH) of α-6 into a leaving group (–OMs), as shown in 12, would permit acyl moieties to be introduced by S_N2 reaction. Namely, we assumed that the mesylation of the primary alcohol of 6 followed by removal of the three benzyl groups would afford 12 and the subsequent S_N2 reaction at the mesylate moiety of 12 should afford 13. This new synthetic route would be useful for the synthesis of a wide variety of analogues of SQAP 13, which could not otherwise be obtained using previously reported procedures.

Chart 3. The Synthetic Route for Preparing SQAP Derivatives in This Study

Results and Discussion

Synthesis of SQAP 2 and Its Derivatives 3 and 4 by the New Synthetic Route

The glycosyl donor 9 was prepared by the glycosylation of commercially available penta-O-acetyl-D-glucopyranose 8, as shown in Chart 4. The β-thioglycoside 14 was prepared by the S-glycosylation of 8 with p-toluenethiol followed by deacetylation using NaOMe in methanol. The OH group at the 6-position of 14 was protected with a trityl group, and the remaining OH groups were protected with benzyl groups to provide 15. The trityl group was then removed by treatment with p-toluenesulfonic acid to give 16,²¹⁾ which was then converted into 17.²²⁾ The 6-OTs group of 17 was substituted with a thioacetyl group to afford 9.

Chart 4. Synthesis of Glycosyl Donor 9

The glycosylation of 9 with 3-(p-methoxybenzyloxy)propanol 10, which was prepared from propanediol,¹⁹⁾ afforded 18 as an anomeric mixture (Chart 5). As listed in Table 1, the effect of the solvent used in the reaction on the chemical yield and stereo-selectivity of 18 was investigated. The glycosylation reaction of 9 was initially conducted in CH₂Cl₂ to afford α-18 in the same yield as β-18 (entry 1). The use of ether-type solvents such as dioxane as a co-solvent improved the α-selectivity, slightly^23,24) (entry 2). The addition of tert-butyl methyl ether (TBME) gave better α-selectivity and a combination of NIS and TfOH in a mixture of CH₂Cl₂ and MeO^tBu gave α-18 in reasonable yields and an acceptable α/β ratio (entries 3–5). Although it has reported that α-selectivity is improved by the increasing temperature, chemical yields of this reaction might be decrease probably due to deprotection of PMB group in the presence of TfOH.²⁵⁾

Chart 5. Glycosylation of Glycosyl Donor 9 with Mono-Protected Propandiol 10 to Afford 18

Table 1. Screening of Glycosylation Conditions for Stereoselectivity

Entry	Solvent	Yield (%)	α/β Ratio
1	CH2Cl2	71	50/50
2	CH2Cl2/1,4-dioxane (1 : 1)	65	57/43
3	CH2Cl2/MeOtBu (1 : 1)	66	67/33
4	CH2Cl2/MeOtBu (1 : 2)	70	79/21
5	CH2Cl2/MeOtBu (1 : 3)	62	80/20

Regarding a o-carborane unit, reaction of decaborane (B₁₀H₁₄) with methyl 10-undecynoate 20,²⁶⁾ which was prepared from 10-undecynoic acid 19, in the presence of 1-butyl-3-methyl imidazolium chloride²⁷⁾ (bmimCl) afforded 21 (Chart 6). Removal of the methyl esters of 21 by hydrolysis with HCl provided the carborane-containing acid 22.²⁸⁾

Chart 6. Synthesis of o-Carborane Derivatives

The thioacetyl group of α-18 at the 6-position was oxidized by treatment with Oxone^® to produce sulfoquinovose derivatives 23 and the p-methoxybenzyl (PMB) group was then removed by DDQ²⁹⁾ to afford the intermediate α-6 (Chart 7). The condensation of the alcohol 6 with stearic acid gave the ester 7, followed by removing the benzyl group by hydrogenation provided 2 (SQAP). In a similar manner, the boron-containing SQAP derivatives 3 was obtained by condensation reaction with 22 followed deprotection of the benzyl groups. However, the use of this route to prepare 4 resulted in the production of deiodinated compounds.

Chart 7. Synthesis of SQAP 2 and Its Derivatives Having o-Carborane 3

Synthesis of the iodo-containing SQAP 4 is illustrated in Chart 8. The condensation of the 2,3,5-triiodobenzoic acid 25 and tert-butyl 6-aminocaproate 26 gave 27, which after the removal of the tert-butyl ester group, provided 28.³⁰⁾ Mesylation of the OH group of 6 then provided the mesylate derivative 29, followed by removal of the benzyl groups by hydrogenation gave the new intermediate 12, to which 28 was introduced by substitution reaction³¹⁾ to give the iodo-containing derivative 4. Using this synthetic route and reaction conditions, it is possible to access various SQAP derivatives that contain labile functional groups.

Chart 8. Synthesis of Triiodobenzoic Acid Conjugated SQAP Derivatives

Conclusion

In this study, we report on new synthetic routes for preparing SQAP itself and its derivatives 3 and 4. The intermediate α-6 was prepared from 8 via the thioglycoside donor 9, whose glycosylation with mono(PMB)-protected propandiol in a mixture of CH₂Cl₂ and MTBE (1 : 3) afforded the O-glycoside 18 in good chemical yield with a high degree of α selectivity. Subsequent condensation reaction and debenzylation provided SQAP and its derivative 3. This synthetic route would be ideal for synthesis of 3, whose o-carborane moieties are decomposed by strong base. Furthermore, the conversion of the nucleophile (–OH) of α-6 into a leaving group (–OMs) enabled various SQAP derivatives that contain labile functional groups, such as 4, to be prepared. These new synthetic routes for preparing SQAP derivatives might be useful for the design and synthesis of chemically modified SQAG/SQAP derivatives, which have the potential for use in the therapy and diagnosis of cancer and related diseases.

Experimental

General Information

IR spectra were recorded on Perkin-Elmer FT-IR spectrophotometer at room temperature. ¹H- (300 MHz) and ¹³C- (75 MHz) NMR spectra were recorded on a JEOL Always 300 spectrometer. ¹H- (400 MHz) and ¹¹B- (128 MHz) NMR spectra were recorded on a JEOL Lambda 400 spectrometer. ¹³C- (150 MHz) NMR spectra were recorded on a JEOL ECA-600 spectrometer. Tetramethylsilane was used as an internal reference for ¹H- and ¹³C-NMR measurements in CDCl₃ and CD₃OD. The sodium salt of 3-(trimethylsilyl)propionic-2,2,3,3-d₄ acid (TSP) was used as an external reference for the ¹H-NMR and 1,4-dioxane for ¹³C-NMR measurements, which were conducted in D₂O. ¹¹B-NMR spectra were measured in a quartz NMR tube using boron trifluoride diethyl ether complex in CDCl₃ as an external reference (0 ppm). Elemental analyses were performed on a Perkin-Elmer CHN 2400 analyzer. MS spectra were recorded on a JEOL JMS-SX102A and Agilent (Varian) 910-MS. Thin-layer (TLC) and silica gel column chromatographies were performed using a Merck 5554 (silica gel) TLC plates and Fuji Silysia Chemical FL-100D, respectively. Oxone^® (potassium peroxymonosulfate, KHSO₅·0.5KHSO₄·0.5K₂SO₄) was purchased from Sigma-Aldrich.

β-1-p-Tolylthio-2,3,4-tri-O-benzyl-6-thioacetyl-D-glucopyranose (9)

Potassium thioacetate (1.51 g, 13 mmol) was added to a suspension of 17²²⁾ (4.70 g, 6.54 mmol) in ethanol (75 mL) at room temperature, and the resulting mixture was stirred at reflux temperature for 5 h. After cooling the reaction solution to room temperature, the mixture was evaporated under reduced pressure, and the resulting residue was dissolved in AcOEt, washed with H₂O and brine, dried over Na₂SO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexanes–AcOEt=4 : 1) and recrystallized from hexanes to give 9 (2.40 g, 3.91 mmol, 60%) as colorless needles; mp 79.5–79.9°C. ¹H-NMR (300 MHz, CDCl₃, TMS): δ=2.36 (s, 6H), 3.01–3.04 (dd, 1H, J=6.9, 13.5 Hz), 3.35–3.48 (m, 3H), 3.53 (dd, 1H, J=2.4, 13.8 Hz), 3.68 (t, 1H, J=8.7 Hz), 4.55 (d, 1H, J=9.9 Hz), 4.64 (d, 1H, J=10.8 Hz), 4.73 (d, 1H, J=10.2 Hz), 4.82–4.93 (m, 4H), 7.132 (d, 2H, J=7.8 Hz), 7.28–7.42 (m, 15H), 7.47 (d, 2H, J=8.1 Hz) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=21.0, 30.4, 30.9, 75.1, 75.3, 75.7, 77.7, 80.1, 80.8, 86.4, 87.6, 127.7, 127.7, 127.9, 128.0, 128.1, 128.3, 128.3, 128.4, 129.4, 129.5, 132.8, 137.6, 137.7, 137.9, 138.1, 194.6 ppm. IR (ATR): 3091, 3065, 3032, 2907, 2869, 1946, 1872, 1811, 1696, 1607, 15589, 1495, 1455, 1398, 1354, 1273, 1252, 1213, 1201, 1132, 1104, 10069, 1029, 992, 968, 909, 847, 805, 797, 738, 695, 657, 626, 578, 559, 496, 487 cm⁻¹. High resolution (HR)-MS (FAB+): Calcd for [M+Na]⁺, C₃₆H₃₈O₅S₂, 637.2051. Found, 637.2051. Elemental analysis Calcd (%) for C₃₆H₃₈O₅S₂: C, 70.33; H, 6.23. Found: C, 70.55; H, 6.13.

1-O-(2,3,4-Tri-O-benzyl-6-thioacetyl-α-D-glucopyranosyl)-3-O-p-methoxybenzylpropane-1,3-diol (18)

NIS (562 mg) was added to a mixture of 9 (307 mg, 0.50 mmol), 3-(4-methoxybenyloxy)-1-propanol 10²¹⁾ (182 mg, 0.93 mmol), molecular sieves (4 Å, 1.0 g) in freshly distilled CH₂Cl₂ (3.3 mL) and tert-butyl methyl ether (6.7 mL) at −40°C. After 15 min, TfOH (cat.) was added, and the reaction mixture was gradually warmed to −30°C. After 2 h, the reaction was quenched by adding Et₃N and the insoluble material was removed by filtration. The filtrate was diluted with CHCl₃ and washed with 10% Na₂S₂O₃ aq., 1 N HCl aq. and brine, dried over Na₂SO₄ and filtered. The filtrate was concentrated under reduced pressure and the remaining residue was purified by silica gel column chromatography (hexanes–AcOEt=10 : 1) and HPLC (Senshu Pak PEGASIL-B 20ϕ×250 nm, MeOH/H₂O (0.1% TFA)=90/10, flaw rate 10 mL/min) to give α-18 (215 mg, 63%) and β-18 (47.8 mg, 14%) as colorless syrups.

α-18: ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.92 (m, 2H), 2.29 (s, 3H), 3.05 (dd, 1H, J=7.7 Hz), 3.28–3.61 (m, 6H), 3.72–3.82 (m, 5H), 3.96 (t, 1H, J=9.2 Hz), 4.42 (s, 2H), 4.60–4.64 (m, 2H), 4.68 (d, 1H, J=3.6 Hz), 4.74 (d, 1H, J=12.0 Hz), 4.80 (d, 1H, J=10.5 Hz), 4.89 (d, 1H, J=10.5 Hz), 4.98 (d, 1H, J=10.5 Hz), 6.84–6.88 (m, 2H), 7.23–7.36 (m, 15H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=29.5, 30.2, 30.7, 54.9, 64.8, 66.6, 69.1, 72.4, 72.8, 75.0, 75.4, 76.6, 77.0, 77.2, 77.4, 79.9, 80.3, 81.5, 96.4, 113.5, 127.4, 127.7, 127.7, 128.0, 129.0, 130.0, 137.7, 137.9, 138.5, 158.9, 194.6 ppm. IR (ATR): 3089, 3063, 3031, 3005, 2911, 2864, 1692, 1613, 1586, 1513, 1498, 1455, 1400, 1357, 1303, 0247, 1209, 1067, 1028, 953, 911, 820, 734, 696, 628 cm⁻¹. HR-MS (FAB+): Calcd for [M+Na]⁺, C₄₀H₄₆O₈S, 709.2811. Found, 709.2814.

β-18: ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.92 (m, 2H), 2.29 (s, 3H), 2.94 (dd, 1H, J=13.6 Hz), 3.32–3.42 (m, 3H), 3.51–3.69 (m, 5H), 3.77 (s, 3H), 3.95–4.00 (m, 1H), 4.33 (d, 1H, J=8.0 Hz), 4.40 (s, 2H), 4.64 (d, 1H, J=12.4 Hz), 4.70 (d, 1H, J=10.0 Hz), 4.76 (d, 1H, J=11.2 Hz), 4.85–4.93 (m, 3H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=30.2, 30.5, 31.0, 55.2, 66.8, 67.0, 72.6, 73.8, 74.7, 75.1, 75.7, 80.3, 82.2, 84.4, 103.4, 113.7, 127.6, 127.9, 127.9, 128.0, 128.2, 128.3, 128.3, 128.4, 129.1, 130.5, 137.7, 138.3, 138.4, 159.1, 194.9 ppm. IR (ATR): 3089, 3063, 3031, 3005, 2911, 2864, 1692, 1613, 1586, 1513, 1498, 1455, 1400, 1357, 1303, 0247, 1209, 1067, 1028, 953, 911, 820, 734, 696, 628 cm⁻¹. HR-MS (FAB+): Calcd for [M+Na]⁺, C₄₀H₄₆O₈S, 709.2811. Found, 709.2813.

Methyl 1-O-1,2-Dicarbadodecaborane-dodecanoate (21)

To a solution of B₁₀H₁₄ (122 mg, 1.00 mmol) in toluene (5.4 mL), 20²⁶⁾ (245 mg, 1.25 mmol, prepared from 19 as indicated in Chart 6 in this work) and 1-butyl-3-methylimidazolium chloride (bmimCl) (59.4 mg, 0.34 mmol) were added. The resulting solution was stirred at reflux condition for 3 h. After evaporation, the resulting residue was purified by silica gel column chromatography (hexanes–AcOEt=10 : 1) to afford 21 as a colorless amorphous (65.8 mg, 21% yield).: ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.25–1.63 (m, 19H), 2.15–2.21 (m, 2H), 2.30 (t, 2H, J=7.5 Hz), 3.49 (s, 3H), 3.56 (br, 1H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=24.7, 28.7, 28.8, 28.9, 28.9, 29.1, 33.9, 38.0, 51.4, 60.9, 75.4 174.1 ppm. ¹¹B{¹H}-NMR (128 MHz, CDCl₃, BF₃·OEt₂): δ=−13.2 (s, 2B), −12.3 (s, 2B), −11.4 (s, 2B), −9.4 (s, 2B), −6.0 (s, 1B), −2.5 (s, 1B) ppm. IR (ATR): 3059, 2956, 2924, 2855, 2605, 2568, 1732, 1467, 1435, 1416, 1383, 1362, 1333, 1297, 1258, 1217, 1189, 1173, 1136, 1106, 1075, 1044, 1017, 998, 975, 930, 888, 855, 799, 721, 660 cm⁻¹. HR-MS (FAB-): Calcd for [M−H], C₁₂H₃₀B₁₀O₂, 316.3176. Found, 316.3188.

1-O-1,2-Dicarbadodecaborane-dodecanoic Acid (22)

A suspension of 21 (65.8 mg, 0.209 mmol) in aqueous solution of 5 N HCl (3.5 mL) was stirred at reflux temperature for 24 h. After cooling the reaction solution to room temperature, the mixture was dissolved in AcOEt, washed with 2 N HCl solution and brine, dried over Na₂SO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexanes–AcOEt=4 : 1) to give 22 (36.9 mg, 0.123 mmol, 59%) as colorless amorphous. ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.26–1.65 (m, 19H), 2.16–2.21 (m, 2H), 2.35 (t, 2H, J=7.5 Hz), 3.56 (br, 1H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=24.5, 28.8, 28.8, 29.0, 29.1, 33.9, 38.1, 60.9, 75.4, 179.5 ppm. ¹¹B{¹H}-NMR (128 MHz, CDCl₃, BF₃·OEt₂): δ=−13.1 (s, 2B), −12.3 (s, 2B), −11.4 (s, 2B), −9.4 (s, 2B), −5.9 (s, 1B), −2.5 (s, 1B) ppm. IR (ATR): 3063, 2934, 2857, 2570, 1697, 1469, 1432, 1411, 1330, 1286, 1248, 1228, 1209, 1124, 1069, 1020 cm⁻¹. HR-MS (FAB-): Calcd for [M−H]⁻, C₁₁H₂₈B₁₀O₂, 302.3020. Found, 302.2978.

Sodium 1-O-(2,3,4-Tri-O-benzyl-6-sulfo-α-D-quinovopyranosyl)-3-O-p-methoxybenzyl-propane-1,3-diol (23)

Oxone^® was added to a suspension of α-18 (161 mg, 0.23 mmol) and sodium acetate (390 mg, 0.48 mmol) in acetic acid (3.4 mL) and the resulting mixture was stirred at room temperature for 24 h. The mixture was then diluted with AcOEt and washed with saturated NaHCO₃ aq., and brine, dried over Na₂SO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=4 : 1) to give 23 (146 mg, 87%) as a colorless amorphous. ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.837 (m, 2H), 3.17–3.31 (m, 2H), 3.37–3.51 (m, 5H), 3.61 (s, 3H), 3.87–3.98 (m, 2H), 4.26 (t, 1H, J=9.0 Hz), 4.32 (s, 2H), 4.52–4.56 (m, 3H), 4.64 (d, 1H, J=11.1 Hz), 4.78 (d, 1H, J=11.1 Hz), 4.87 (d, 1H, J=11.1 Hz), 4.94 (d, 1H, J=3.0 Hz), 5.30 (s, 2H), 6.74 (d, 2H, J=8.7 Hz), 7.13–7.25 (m, 15H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=20.8, 29.3, 52.0, 55.0, 55.1, 65.1, 66.3, 67.4, 72.3, 72.5, 75.0, 75.4, 77.3, 80.2, 80.5, 81.7, 96.4, 113.7, 127.4, 127.7, 127.7, 127.7, 128.2, 128.3, 128.3, 129.5, 129.9, 130.1, 138.1, 138.3, 138.8, 159.1, 175.3 ppm. IR (ATR): 3461, 3064, 3032, 3004, 2933, 2866, 1728, 1612, 1587, 1513, 1498, 1454, 1401, 1363, 1330, 1302, 1246, 1211, 1175, 1086, 1047, 1027, 996, 911, 886, 821, 796, 734, 696, 675, 665, 636, 607, 573, 550, 532, 463 cm⁻¹. HR-MS (FAB-): Calcd for [M−Na]⁻, C₃₈H₄₃O₁₀S, 691.2582. Found, 691.2574.

Sodium 1-O-(2,3,4-Tri-O-benzyl-6-sulfo-α-D-quinovopyranosyl)propane-1,3-diol (6)

DDQ (164 mg, 0.72 mmol) was added to a suspension of 24 (413 mg, 0.60 mmol) in CH₂Cl₂ (30 mL) and H₂O (1.6 mL) and the resulting mixture was stirred at room temperature for 90 min. It was then extracted with CHCl₃ and washed with 2 N HCl aq. and brine, dried over Na₂SO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=8 : 1 to 6 : 1) to give 6 (314 mg, 0.529 mmol, 88%) as a pale brown solid. Spectroscopic data (¹H- and ¹³C-NMR spectra) for 6 were consistent with the previously reported data.³²⁾

Sodium 3-O-(2,3,4-Benzyl-6-sulfo-α-D-quinovopyranosyl)-1-O-1,2-dicarbadodecaborane-1-dodecanoyl-propane-1,3-diol (24)

6 was added to a solution of (EDC·HCl) (49.2 mg, 0.26 mmol), DMAP (2.1 mg, 17 µmol) and 22 (41.7 mg, 0.17 mmol) in CH₂Cl₂ (6.0 mL) and the resulting mixture was stirred at room temperature for 24 h. The mixture was then diluted with CHCl₃, washed with 2 N HCl aq. and brine, dried over Na₂SO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=4 : 1, containing 1% (v/v) AcOH) to give 24 (125.2 mg, 85%) as a colorless amorphous. ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.15 (m, 10H), 2.00–2.23 (m, 19H), 2.55 (t, J=6.6 Hz, 2H), 2.76 (s, 2H), 3.22–3.53 (m, 2H), 3.51 (br s, 2H), 3.95 (t, 2H, J=8.7 Hz), 4.16–4.22 (m, 3H), 4.54–4.74 (m, 4H), 4.83–4.96 (m, 3H) ppm. ¹³C-NMR (150 MHz, CDCl₃, TMS): δ=24.7, 28.4, 28.8, 29.0, 29.0, 29.2, 29.7, 31.9, 34.4, 37.9, 52.2, 52.3, 60.4, 61.0, 63.6, 65.0, 67.0, 73.0, 74.9, 75.3, 75.5, 76.8, 77.0, 77.2, 80.0, 80.2, 81.8, 96.2, 127.6, 127.7, 127.8, 127.9, 128.4, 137.7, 138.0, 138.5, 175.9 ppm. ¹¹B{¹H}-NMR (128 MHz, CDCl₃, BF₃·OEt₂): δ=−11.6 (s, 6B), −9.5 (s, 2B), −5.9 (s, 1B), −2.5 (s, 1B) ppm. IR (ATR): 3434, 3063, 2929, 2858, 2858, 2583, 1728, 1498, 1455, 1360, 1160, 1055, 1029, 734, 697, 633, 543 cm⁻¹. HR-MS (ESI-): Calcd for [M−Na]⁻, C₄₁H₆₁B₁₀O₁₀S⁻, 855.4921. Found, 855.4969.

Sodium 3-O-(6-Sulfo-α-D-quinovopyranosyl)-1-O-1,2-dicarbadodecaborane-1-pentanoyl-propane-1,3-diol (3)

5% Pd/C (820 mg) was added to a solution of 24 (75.3 mg, 0.12 mmol) in MeOH (6.0 mL) under an atmosphere of argon and the mixture was stirred at room temperature for 24 h under an atmosphere of H₂ gas. The resulting solution was then filtered through a pad of celite and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=4 : 1 to 3 : 1) to give 3 (24.9 mg, 48%) as a colorless amorphous. ¹H-NMR (300 MHz, CD₃OD, TMS): δ=1.15 (m, 10H), 2.00–2.23 (m, 19H), 2.55 (t, J=6.6 Hz, 2H), 2.76 (s, 2H), 3.22–3.53 (m, 2H), 3.51 (br s, 2H), 3.95 (t, 2H, J=8.7 Hz), 4.16–4.22 (m, 3H), 4.54–4.74 (m, 4H), 4.83–4.96 (m, 3H) ppm. ¹³C-NMR (150 MHz, CD₃OD, TMS): δ=26.1, 29.8, 30.2, 30.3, 30.4, 30.6, 30.6, 30.7, 31.0, 33.0, 35.1, 54.3, 63.1, 65.4, 69.6, 73.5, 75.0, 75.0, 99.6, 176.0 ppm. ¹¹B{¹H}-NMR (128 MHz, CD₃OD, BF₃·OEt₂): δ=−12.4–10.9 (m, 3B), −9.0 (s, 2B), −5.6 (s, 1B), −2.3 (s, 1B), 32.4 (s, 3B) ppm. IR (ATR): 3351, 2925, 2855, 1715, 1353, 1164, 1030 cm⁻¹. HR-MS (ESI-): Calcd for [M−Na]⁻, C₂₀H₄₃B₁₀O₁₀S, 585.3513. Found, 585.3520.

Sodium 3-O-(2,3,4-Tri-O-benzyl-6-sulfo-α-D-quinovopyranosyl)-1-O-mesylpropane-1,3-diol (29)

To a solution of 6 (100 mg, 0.17 mmol) and DMAP (10.3 mg, 84 µmol) in anhydrous pyridine (2.5 mL), methanesulfonyl chloride (MsCl) (77.0 mg, 0.67 mmol) was added at 0°C, and the mixture was stirred at room temperature. After 5 h, methanol (4.0 mL) was added, and the reaction mixture was extracted with chloroform, washed with 2 N HCl aq. and brine, dried over Na₂SO₄, filtered and evaporated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=25 : 1 containing 1% (v/v) AcOH) to give 29 (71.2 mg, 63%) as a colorless amorphous. mp 164–165. ¹H-NMR (300 MHz, CDCl₃, TMS): δ=1.99 (br s, 2H), 2.85 (s, 3H), 3.16–3.56 (m, 5H), 3.88–3.99 (m, 2H), 4.14 (t, 1H, J=9.6 Hz), 4.29 (br s, 2H), 4.52–4.69 (m, 4H), 4.78 (d, 1H, J=10.2 Hz), 4.90 (d, 1H, J=10.8 Hz), 4.97 (br s, 1H) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=14.1, 28.9, 29.7, 36.7, 52.1, 63.5, 67.4, 68.1, 72.8, 75.0, 75.4, 77.2, 80.0, 80.7, 81.5, 96.2, 127.5, 127.7, 127.8, 128.3, 128.4, 137.9, 138.1, 138.6 ppm. IR (ATR): 3441, 3032, 2931, 1728, 1605, 1562, 1498, 1455, 1406, 1351, 1290, 1214, 1170, 1116, 1087, 1046, 1027, 975, 941, 837, 790, 735, 696, 674, 635, 579, 527, 461 cm⁻¹. HR-MS (FAB-): Calcd for [M]⁻, C₃₁H₃₇O₁₁S₂, 649.1777. Found, 649.1775.

Sodium 3-O-(6-Sulfo-α-D-quinovopyranosyl)-1-O-mesylpropane-1,3-diol (12)

5% Pd/C (575 mg) was added to a solution of 29 (59.3 mg, 88 µmol) in MeOH (6.0 mL) under an atmosphere of argon and the mixture was stirred at room temperature for 16 h under an atmosphere of H₂ gas. It was then filtered through a pad of celite and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=5 : 1 to 4 : 1) to give 12 (24.8 mg, 70%) as a colorless amorphous. ¹H-NMR (300 MHz, CD₃OD, TMS): δ=1.99–2.16 (m, 2H), .2.91 (dd, 1H, J=9.2, 14.3 Hz), 3.04–3.10 (m, 4H), 3.32–3.43 (m, 2H), 3.48–3.67 (m, 2H), 4.03–4.12 (m, 2H), 4.35–4.48 (m, 2H), 4.76 (d, 1H, J=3.6 Hz) ppm. ¹³C-NMR (75 MHz, CD₃OD): δ=30.3, 37.1, 54.3, 64.8, 69.5, 69.7, 73.5, 75.0, 75.1, 99.7 ppm. IR (ATR): 3383, 3021, 2933, 1722, 1639, 1413, 1338, 1165, 1031, 979, 938, 846, 829, 790, 755, 724, 699, 664, 628, 590, 526 cm⁻¹. HR-MS (FAB-): Calcd for [M]⁻, C₁₀H₁₉O₁₁S₂, 379.0369. Found, 379.0368.

Sodium 3-O-(6-Sulfo-α-D-quinovopyranosyl)-1-O-(6-(2,3,5-triiodobenzoyl)-aminohexanoyl)-propane-1,3-diol (4)

To a suspension of 12 (24.8 mg, 62 µmol) and 6-(2,3,5-triiodobenzoyl)aminohexanoic acid 28 (170 mg, 0.28 mmol) in anhydrous DMF (1.5 mL), CsF (93.6 mg, 0.62 mmol) and NaI (36.9 mg, 0.25 mmol) was added at room temperature, and the resulting mixture was stirred at 70°C. After 48 h, methanol (1.0 mL) was added, and the reaction mixture was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (CHCl₃–MeOH=6 : 1 to 4 : 1 to 3 : 1) and HPLC (Senshu Pak PEGASIL-B 20ϕ×250 nm, CH₃CN–H₂O (0.1% TFA)=78 : 22, flaw rate 10 mL/min) to give 4 (24.1 mg, 44%) as a colorless amorphous. mp 169–170°C. ¹H-NMR (300 MHz, CD₃OD, TMS): δ=1.40–1.50 (m, 2H), 1.59–1.72 (m, 4H), 1.93–2.01 (m, 2H), 2.37 (t, 2H, J=7.4 Hz), 2.91 (dd, J=9.2, 14.3 Hz), 3.07 (t, 1H, J=9.3 Hz), 3.35–3.41 (m, 4H), 3.45–3.52 (m, 1H), 3.64 (t, 1H, J=9.3 Hz), 4.02–4.10 (m, 2H), 4.17–4.31 (m, 2H), 4.74 (d, 1H, J=4.6 Hz), 7.54 (d, 1H, J=2.4 Hz), 8.29 (d, 1H, J=1.8 Hz) ppm. ¹³C-NMR (75 MHz, CDCl₃): δ=25.6, 27.6, 29.6, 34.7, 40.7, 66.0, 73.6, 75.1, 94.7, 99.6, 107.2, 112.8, 136.1, 148.5 ppm. IR (ATR): 3260, 2928, 2860, 1725, 1631, 4556, 1520, 1436, 1416, 1392, 1359, 1300, 1152, 1110, 1089, 1050, 1028, 1012, 910, 890, 864, 828, 788, 738, 699, 662, 626, 585, 520, 504 cm⁻¹. HR-MS (FAB-): Calcd for [M−Na]⁻, C₂₂H₂₉I₃NO₁₁S, 895.8595. Found, 895.8597.

Acknowledgments

This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (No. 19659026, 22390005, and 24659085 for S.A. and No. 25893259 for T. T.) and TUS (Tokyo University of Science) fund for strategic research areas. We wish to express our appreciation to Prof. Kengo Sakaguchi (Research Institute of Science and Technology, Tokyo University of Science) and Prof. Fumio Sugawara (Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science) for providing SQAP as a reference. We also wish to express our appreciation for the aid provided by Mrs. Fukiko Hasegawa (Faculty of Pharmaceutical Sciences, Tokyo University of Science) for mass spectral measurements.

Conflict of Interest

The authors declare no conflict of interest.

References

• 1) Benning C., Annu. Rev. Plant Physiol. Plant Mol. Biol., 49, 53–75 (1998).
• 2) Zhang J., Li C., Yu G., Guan H., Mar. Drugs, 12, 3634–3659 (2014).
• 3) Ohta K., Mizushina Y., Hirata N., Takemura M., Sugawara F., Matsukage A., Yoshida S., Sakaguchi K., Chem. Pharm. Bull., 46, 684–686 (1998).
• 4) Ohta K., Mizushina Y., Hirata N., Takemura M., Sugawara F., Matsukage A., Yoshida S., Sakaguchi K., Biol. Pharm. Bull., 22, 111–116 (1999).
• 5) Gustafson K. R., Cardellina J. H. 2nd, Fuller R. W., Weislow O. S., Kiser R. F., Snader K. M., Patterson G. M. L., Boyd M. R., J. Natl. Cancer Inst., 81, 1254–1258 (1989).
• 6) Vo T.-S., Ngo D.-H., Ta Q. V., Kim S.-K., Eur. J. Pharm. Sci., 44, 11–20 (2011).
• 7) Ohta K., Mizushina Y., Yamazaki T., Hanashima S., Sugawara F., Sakaguchi K., Biophys. Res. Commun., 288, 893–900 (2001).
• 8) Ohta, K., Miura, M., Sakaguchi, K., Sugawara, F., Ishima, M., Murata, H., WO Patent, WO2010/082634 A1.
• 9) Hanashima S., Mizushima Y., Ohta K., Yamazaki T., Sugawara F., Sakaguchi K., Jpn. J. Cancer Res., 91, 1073–1083 (2000).
• 10) Brahmi M. M., Portmann C., D’Ambrosio D., Woods T. M., Banfi D., Reichenbach P., Silva L. D., Baudat E., Trucatti G., Lingner J., Gademann K., Chem. Eur. J., 19, 4596–4601 (2013).
• 11) Hosmane N. S., Maguire J. A., Zhu Y., Takagaki M., “Boron and Gadlinium Neutron Capture Therapy for Cancer Treatment,” World Scientific in the Singapore, 2012.
• 12) Lusic H., Grinstaff M. W., Chem. Rev., 113, 1641–1666 (2013).
• 13) Endo Y., Iijima T., Yamakoshi Y., Yamaguchi M., Fukasawa H., Shudo K., J. Med. Chem., 42, 1501–1504 (1999).
• 14) Fujii S., Masuno H., Taoda Y., Kano A., Wongmayura A., Nakabayashi M., Ito N., Shimizu M., Kawachi E., Hirano T., Endo Y., Tanatani A., Kagechika H., J. Am. Chem. Soc., 133, 20933–20941 (2011).
• 15) Bradshaw K. M., Schweizer M. P., Glover G. H., Hadley J. R., Tippets R., Tang P., Davis W. L., Heilbrun M. P., Johnson S., Ghanem T., Magn. Reson. Med., 34, 48–56 (1995).
• 16) Tanaka T., Nishiura Y., Araki R., Saido T., Abe R., Aoki S., Eur. J. Inorg. Chem., 1819–1834 (2016).
• 17) Tanaka T., Araki R., Saido T., Abe R., Aoki S., Eur. J. Inorg. Chem., 3330–3337 (2016).
• 18) Kalender W. A., Phys. Med. Biol., 51, R29–R43 (2006).
• 19) Zhou B., Li Y., Tetrahedron Lett., 53, 502–504 (2012).
• 20) Yoo J., Hwang J.-W., Do Y., Inorg. Chem., 40, 568–570 (2001).
• 21) France R. R., Compton R. G., Davis B. G., Fairbanks A. J., Rees N. V., Wadhawan J. D., Org. Biomol. Chem., 2, 2195–2202 (2004).
• 22) Sun Y., Zhang J., Li C., Guan H., Yu G., Carbohydr. Res., 355, 6–12 (2012).
• 23) Demchenko A., Stauch T., Boons G.-J., Synlett, 7, 818–820 (1997).
• 24) Demchenko A. V., Synlett, 9, 1225–1240 (2003).
• 25) Jung M. E., Koch P., Tetrahedron Lett., 52, 6051–6054 (2011).
• 26) González-Paz R. J., Lligadas G., Ronda J. C., Galià M., Cádiz V., Polym. Chem., 3, 2471–2478 (2012).
• 27) Li Y., Carroll P. J., Sneddon L. G., Inorg. Chem., 47, 9193–9202 (2008).
• 28) Schirrmadher E., Schirrmacher R., Beck C., Mier W., Trautman N., Rosch F., Tetrahedron Lett., 44, 9143–9145 (2003).
• 29) Horita K., Yoshioka T., Tanaka T., Oikawa Y., Yonemitsu O., Tetrahedron, 42, 3021–3028 (1986).
• 30) Chen Q., Gabathuler R., Synth. Commun., 34, 2415–2424 (2004).
• 31) Sato T., Otera J., Nozaki H., J. Org. Chem., 57, 2166–2169 (1992).
• 32) Ohta K., Miura M., Sakaguchi K., Sugawara F., Sato N., Sahara H., Takahashi N., Mori Y., Yamazaki T., Masaki K., Murata H., WO patent, WO 2009/014101 A1.